American Political Science Review Vol. 99, No. 2 May 2005

Are Political Orientations Genetically Transmitted? JOHN R. ALFORD Rice University CAROLYN L. FUNK Virginia Commonwealth University JOHN R. HIBBING University of Nebraska etestthepossibilitythatpoliticalattitudesandbehaviorsaretheresultofbothenviron- mental and genetic factors. Employing standard methodological approaches in behavioral W genetics—–specifically, comparisons of the differential correlations of the attitudes of monozy- gotic twins and dizygotic twins—–we analyze data drawn from a large sample of twins in the United States, supplemented with findings from twins in Australia. The results indicate that genetics plays an important role in shaping political attitudes and ideologies but a more modest role in forming party identification; as such, they call for finer distinctions in theorizing about the sources of political attitudes. We conclude by urging political scientists to incorporate genetic influences, specifically interactions between genetic heritability and social environment, into models of political attitude formation.

hy do people think and act politically in the the world and over the decades is difficult for behavio- manner they do? Despite the foundational ralists to explain. But if there is a genetic component to Wnature of this question, answers are unfortu- political ideologies, if the constraints on belief systems nately incomplete and unnecessarily tentative, largely come not just from intellectualization or indoctrination because political scientists do not take seriously the but from something deeper, the concept of ideology possibility of nonenvironmental influences. The sug- takes on greater meaning and the commonality of ide- gestion that people could be born with political pre- ology becomes easier to understand. dispositions strikes many as far-fetched, odd, even perverse. However, researchers in other disciplines—– ATTITUDE FORMATION notably behavioral genetics—–have uncovered a sub- stantial heritable component for many social attitudes Debates concerning the source of political attitudes re- and behaviors and it seems unlikely that political atti- volve primarily around the question of whether early tudes and behaviors are completely immune from such childhood factors have lasting relevance or whether forces. In this article, we combine relevant findings in these factors tend to be overwhelmed by more proxi- behavioral genetics with our own analysis of data on a mate events. Survey responses to political items pre- large sample of twins to test the hypothesis that, con- sumably reflect attitudes and are thought to be a trary to the assumptions embedded in political science combination of longstanding “predispositions” and research, political attitudes have genetic as well as en- more recent “off-the-top-of-the-head” considerations vironmental causes.1 (Zaller 1992, chaps 1–3; also see Converse 1964). Al- Testing this hypothesis is important for two reasons. ternatively, an “on-line” pattern of processing could First and most broadly, as behavioral scientists we need allow new incidents to ratchet affect one way or an- to analyze all possible shapers of behavior, not just a other from previously existing summary locations (see select few. Second, a more complete understanding of Lodge, McGraw, and Stroh 1989). Regardless, proxi- the sources of attitudes and behaviors will help us to mate forces include recent conversations and experi- sort through existing puzzles of considerable interest to ences, question-wording, priming from previous ques- political scientists. One example is political ideology. tions, and a variety of similar factors. Predispositions, Why is a reasonably standard left–right spectrum so on the other hand, are thought to be a “distillation widely applicable cross-culturally and over time? The of a person’s lifetime experiences, including childhood universal left–right elements of belief systems around socialization and direct involvement with the raw in- gredients of policy issues” (Zaller 1992, 23).2 Great interest exists in determining the relative clout of the John R. Alford is Associate Professor, Department of Political Sci- early as opposed to the late environment but no interest ence, Rice University, Houston, TX 77251 ([email protected]). has been displayed in determining the relative clout of Carolyn L. Funk is Associate Professor, L. Douglas Wilder School of Government and Public Affairs, Virginia Commonwealth Univer- environmental as opposed to genetic variables. sity, Richmond, VA 23298 ([email protected]). Aparallelconclusionappliestoresearchonindivid- John R. Hibbing is Foundation Regents Professor, Department ual attitudes rather than survey responses generally. of Political Science, University of Nebraska—–Lincoln, Lincoln, NE For example, the consensus among those who study 68588 ([email protected]). tolerance is that the extent to which individuals are The authors gratefully acknowledge the assistance of Professor Lindon Eaves and colleagues, particularly for granting us access to tolerant hinges on a combination of “antecedent con- data from the sample of twins known as Virginia 30K. Any errors ditions and contemporary information” (Marcus et al. in the analysis or interpretation of the data are, of course, solely our 1995). Antecedent conditions, in turn, are believed to own responsibility. 1 Evidence consistent with an evolutionary theory of political behav- ior is found in Brewer 2000, Hibbing and Alford 2004, and Orbell 2 To his credit, Zaller (1992) goes on to acknowledge a possible role et al. 2004. for “inherited” traits in shaping predispositions (23).

153 Are Political Orientations Genetically Transmitted? May 2005 be shaped by “personal circumstances” such as “family, tein has a chemical sequence that then interacts with neighborhood, region ...and early group experiences” other chemicals in the body, sometimes reacting di- (Marcus et al. 1995, 5; for more on the importance of rectly with these other chemicals but often serving as long established proclivities, or antecedent conditions, enzymes that facilitate but are not themselves altered see Stouffer 1955). Typically, no role for genetically- by chemical reactions. If a gene coding for a particular induced tendencies is considered (for an exception, see enzyme is absent, the chemical reaction it is meant to Monroe 2004, chap 6). enhance will occur with much less efficiency. For exam- More broadly, the literature on political socializa- ple, a gene for the enzyme tryptophan hydroxylase-2 tion has long revolved around the question of the ef- (Tph2) facilitates production of the neurotransmitter fects of early as opposed to late environmental forces. serotonin in the brain, but a certain form of this gene Early political socialization researchers (e.g., Easton (which varies from the standard form by a single amino and Dennis 1969, Greenstein 1960, Jennings and Niemi acid) produces about 80% less serotonin and people 1968, and Searing, Schwartz, and Lind 1973) and the with this mutant allele appear to be significantly more authors of The American Voter (Campbell et al. 1960) likely to suffer from unipolar depression (Zhang et al. presented arguments and evidence supporting the pri- 2005). macy of early events. Later researchers, however, ques- Still, the connection is rarely so simple that a given tioned the value of early childhood socialization and genetic allele can be seen as causing a certain behavior. provided evidence that judgments about more recent More typically, findings in modern behavioral genetics conditions and occurrences can dramatically alter pref- reveal the effect of genes to be interactive rather than erences we might have held as children and adoles- direct, let alone determinative. To provide one illus- cents (see, e.g., Fiorina 1980; for good summaries of the tration, in humans there is a gene on chromosome 17 debate over the relative importance of early and late involved with serotonin reuptake (5-HTT). As is of- environmental events, see Cook 1985; Merelman 1986, ten the case with genes, 5-HTT has a long allele and and Sears 1989). In the last 50–60 years, the empha- a short allele. Mice have a parallel gene, and in that sis in the literature has gone from personality studies species the short form had previously been connected (Adorno et al. 1950; Eysenck 1954; Laswell 1930), to to listless, depressive behavior. Scientists were eager to ideological and childhood socialization studies, to the determine if such a correlation between the short form effects of media frames, perceptions of current condi- of 5-HTT and depression was present in humans. In a tions, and other types of contemporary information. In long-term study of the health records of nearly 1,000 fact, for the past couple of decades research on polit- New Zealanders whose 5-HTT alleles were known, it ical socialization has been suffering through a “bear was found that major episodes of depressive behavior market” (Cook 1985), and studies of personality, while were not much more prevalent among those with the experiencing a remarkable comeback in psychology short form. But then the researchers combined genetics (for an introduction, see Wiggins and Trapnell 1997), and the environment; specifically, they interacted each have been largely absent from political science since subject’s 5-HTT allele with the number of high-stress McCloskey’s (1958) work in the 1950s on the con- events (romantic calamities, bankruptcies, deaths of servative personality. Thus, political science debates loved ones, etc.) experienced in that individual’s life. concerning the source of political attitudes and be- They found that those who had a high number of such haviors have been over timing, over whether attitudes events and who had the short form of 5-HTT were and behaviors are primarily shaped early in life or by significantly more likely to display behaviors associated more proximate occurrences. Conspicuously absent is with depression compared to either those experiencing consideration of the possibility that certain attitudes few high stress events or those with the long form who and behaviors may be at least partially attributable to suffered through a comparably large number of high- genetic factors. stress events (see Caspi et al. 2003). In this particular case, genotype did not make peo- ple behave a certain way; rather, it influenced the ex- MODERN BEHAVIORAL GENETICS tent to which their behavior was contingent on the environment—–and this pattern likely will apply to all But what is the physical process by which a genetic sorts of other human activities. Whether the behavior allele could shape a political attitude? If there is any of interest is depression, cooperation, fear response, or connection at all, is it not that the effect is so small susceptibility to drug addiction, some people are more that it can be safely ignored? And even if this is not sensitive than others to particular features of their en- the case, in light of potentially troubling normative vironment, and genetics, far from determining behav- implications such as biological determinism, is it not ior, influences its sensitivity. Genetics makes the mood best to ignore relationships between genes and social of some people far more dependent on the extent to behavior? It is difficult for many outside the biological which their lives have been beset with difficulties and it sciences to understand how it is even possible for genes likely makes some people’s political attitudes far more to influence behavior, so a brief discussion is in order. contextually dependent than others. In other words, Genes provide instructions for the production of pro- the connection between genes and attitudes may not teins, which are built and identified by a specific com- involve specific attitudes as much as the flexibility of bination of amino acids (which in turn are constructed those attitudes (Is abortion always wrong, or does it from complex organic molecules). As such, each pro- depend?). The issue is not nature versus nurture but

154 American Political Science Review Vol. 99, No. 2 the manner in which nature interacts with nurture (see by their parents to be MZ twins) and the differential Marcus 2004 and Ridley 2003). correlation persists in these instances of miscategoriza- tion. In other words, the degree of correspondence be- tween MZ twins surpasses that of DZ twins even in the MONOZYGOTIC AND DIZYGOTIC TWINS large subpopulation of twins thought by their parents to be MZ twins (Bouchard and McGue 2003; Bouchard The process of identifying in the laboratory the pre- et al. 1990; Plomin 1990). The contention that MZ cise genes responsible for given human behaviors twins have closer or more frequent contact than DZ (especially those behaviors that do not have corollar- twins turns out to be at best irrelevant. The correla- ies in lab-friendly animals such as mice) is extremely tion between the frequency of contact between twins challenging. Fortunately, even without identifying the and the similarity between twins on all attitudinal and genes responsible, it is possible to compile informa- behavioral variables tested, including conservatism, is tion on the matter of most concern to social scien- slight and actually negative (Martin et al. 1986). In tists: the extent to which attitudes and behaviors have other words, twins in greater contact with their cotwins ageneticcomponent.Therelevantprocedurescenter are not more likely to share the same attitudes and on comparisons of monozygotic (MZ; frequently but behaviors, so even if MZ twins have more contact than erroneously called identical) twins and dizygotic (DZ; DZ twins, this contact is not the cause of any elevated fraternal) twins. correlations. But the most powerful refutation of both MZ twins develop from a single egg, fertilized by of these criticisms comes in recent studies utilizing MZ asinglesperm,andshareanidenticalgeneticinheri- and DZ twins raised apart. These studies uniformly val- tance. DZ twins develop from two separate eggs, fer- idate MZ and DZ differences found in earlier studies tilized by two separate sperm, and are in effect simply of twins raised together. Arguments about the relative two siblings that happen to be born simultaneously. As degree of shared environmental effects between MZ such, DZ twins share the same average of 50% of ge- and DZ twins simply offer no credible explanation if netic material as do any two biological siblings. It is this the twins in question have been raised apart (Bouchard fixed ratio (two to one) of genetic similarity between 1998; Bouchard et al. 1990). In effect, this naturally MZ and DZ twins, and the contrasting average equiv- occurring, if uncommon, condition provides precisely alence of environment influence, that provides most of the sort of laboratory control that we would want in an the power of twin designs. It is important to appreciate experimental setting.3 that the assumption of environmental equivalence is Other evidence against the exclusive environmental one of equivalence across types of twins, not across argument is that the empirical results suggest MZ twins pairs of twins or across twins within a given pair. For reared together are often less likely to share behavioral example, there is undoubtedly at least some variability traits with their twins than are MZ twins reared apart, in parental socialization across siblings, even those of presumably because of extra efforts to establish distinct identical age, but across multiple twin pairs the assump- identities when the twins live together. In addition, as tion is that this variability is essentially equal for the adult MZ twins living apart age, they tend to become MZ and the DZ pairs. more, not less, similar (Bouchard and McGue 2003), a This assertion that the effect of genetics is measur- finding that is difficult to reconcile with the belief that ably distinct for MZ and DZ twins, while the effect of only the environment matters. Interestingly, this pre- the environment is either equivalent or at least ran- cise effect is predicted in an early landmark criticism of domly distributed around equivalence, is crucial to ev- behaviorism and the conditioned response research on erything that follows from twin research. It is important animal behavior that formed its empirical core. Over therefore to raise and consider the criticisms of this time, substantial anomalies began to accumulate in this fundamental assumption. The arguments come in two research pointing toward a primacy for some nonen- essential varieties. The first is that MZ twins, genetics vironmental behaviors. Breland and Breland (1961) aside, experience a more similar environment because summarized this tendency with the phrase “learned they are treated more similarly than are DZ twins. This behavior drifts toward instinctive behavior” (684). would seem particularly telling for childhood socializa- Given the genetic differences and environmental tion, where, for example, parents might show less of a similarities of the two types of twins, for any trait tendency to treat MZ twins as individuals compared that is partly heritable the tendency for MZ twins to to DZ twins. The second is that MZ twins, genetics share that characteristic should be stronger than the aside, interact with each other more throughout life tendency for DZ twins to share that characteristic. than do DZ twins. This would seem to be of particular In contrast, characteristics that arise purely from the importance for adult socialization, where closer adult environment, whether shared by the twins, as would contact between MZ twins might lead us to expect a typically be the case for parental socialization, or not greater degree of environmentally induced similarity than we would see for the more distant DZ twins. Both caveats have been subject to sustained and var- 3 To explain this finding, opponents would need to argue that adop- ied investigation and neither has been found to hold up tion agencies are more likely to place MZ twins in similar homes than they are to place DZ twins in similar homes. In fact, information under empirical scrutiny. The argument of more similar on twin zygosity is typically unavailable to those making placement treatment fails on several fronts. Parents frequently decisions, and even if it were available, it seems highly unlikely that miscategorize their twins (DZ twins are often believed it would factor into their decisions.

155 Are Political Orientations Genetically Transmitted? May 2005 shared by the twins, as would be the case for many leading to trait dissimilarity across individuals. While adult experiences, should not generate any significantly much of the early childhood environment, for example, different patterns when we contrast MZ and DZ twins is similar across siblings, much is nonetheless variable. (see Eaves, Eysenck, and Martin 1989 and Plomin et al. Siblings differ in diet, disease exposure, peer influences, 2001 for a thorough discussion of the relevant statistical and a host of other unique experiences. Even twins, techniques). whose childhood environment is made increasingly The procedures involved with the twin methodol- similar by virtue of their identical age, are exposed to ogy are standard fare in behavioral genetics but are substantial unique external influences. With the shift to not familiar to most political scientists, so it is appro- adult life, the share of unique influences on siblings in- priate that we explain the basic terminology, theory, creases sharply, as peer, workplace, family, and physical and technique in some detail. Influences on an individ- settings typically diverge. ual trait, whether it is a political attitude or a physi- In the classic political science studies of socializa- cal characteristic, are typically divided into two broad tion (see, especially, Jennings and Niemi 1968, 1991 groups—–heredity (H) and environment (E). The total and Tedin 1974), the focus has been on the correlation variation in a trait can thus be represented as the sum between the attitudes of parents and their children. In H E. Heredity is the impact of genetic inheritance on terms of the three sources of trait variability outlined trait+ variation. In the case of a physical characteristic above, as informative as it is, this design does not allow such as adult height, this would be the proportion of the for an unambiguous estimation of any of the three cat- total variation in height across individuals due to the egories. The correlation between a parent and a child variation across individuals in the multiple genes that arises from a combination of shared genes, shared en- control ultimate physical height. For any one individ- vironment, and parental socialization (an indirect form ual, the source of this genetic influence is relatively well of shared environment in which the parent’s attitudes defined, as on average 50% of our genes come from our provide a path from the parent’s environment to the mother and 50% come from our father. This leads to child’s environment), all of which are pressures toward the fact that biological children of tall parents are more similarity in parent–child attitudes. The failure of this likely to be tall than are the biological children of short parent–child correspondence to reach 1.0 presum- parents, though even for a relatively straightforward ably reflects the pressure toward dissimilarity+ coming additive physical trait like height, the relationship is from the unshared environment, but since the genetic far from determinative. similarity of a parent–offspring pair is only .5, there “Environment” is all of the nongenetic external fac- is as much genetic dissimilarity as there is similarity. tors that influence trait variation across a population. Thus, trait dissimilarity, like trait similarity, is an unde- These influences range broadly from the earliest bio- termined mixture of genetic and environmental influ- logical environment of the womb, to the physical en- ences. Our inability to tease apart genetic heritability vironment of a childhood house, to the social environ- and environment, whether shared or unshared, in these ment of the adult workplace. In the case of adult height, parent–child studies is a direct result of the fact that some of the obvious environmental factors are prenatal there is no measured variation in genetic similarity nutrition, the adequacy of childhood and adolescent across the data set of parent–child pairs (i.e., all bi- diet, and exposure to chemical agents that can inhibit ological offspring share the same average of 50% of growth. the variable genetic code with each parent). Environmental influences can be further divided into This inability of standard parent–child observations two subcategories: the shared environment and the to distinguish genetic heritability from parental social- unshared, or unique, environment. The shared envi- ization (or other features of the shared environment) is ronment is all of the shared external influences that something that has long been understood, but largely we would typically think of as leading to trait similar- ignored in modern social science. Fortunately, twins ity between individuals. Siblings, for example, might provide a powerful “natural experiment” by introduc- share similar childhood environments, including sim- ing known genetic variation into analyses of the sources ilar parental interactions, a similar physical environ- of trait variability. By shifting the focus from the sim- ment, and similar nutrition. If the siblings happen to ilarity between parents and offspring to the similarity be twins, they would also share a more similar prenatal between two siblings, we can take advantage of the environment.4 In the case of adult height, a shared fact that some siblings vary in well-known ways in the environmental factor, such as a regional diet limited in degree of their genetic correlation. protein and specific nutrients, could lead to similarity in height across the entire population of a region. The unshared environment is all of the distinctive POLITICS AND GENETICS: PREVIOUS external influences that we would typically think of as FINDINGS AND OUR EXPECTATIONS Comparisons of the correlations of MZ and DZ twins 4 However, recent research suggests that the prenatal environment on a wide variety of variables have been conducted, is so important that it can cause variation even in fetuses inhab- with intriguing results. Using appropriate modeling iting the uterus at the same time. Prescott, Johnson, and McArdle (1999) present evidence that MZ twins sharing the same chorion, the techniques including controls for parental traits and outermost extraembryonic membrane, are more similar in terms of assortative mating, it is possible to partition the ex- personality and cognitive abilities that MZ twins in separate chorions. planatory powers of heredity, shared environment, and

156 American Political Science Review Vol. 99, No. 2 nonshared environment on any given variable. These 2001 and Eaves, Eysenck, and Martin 1989). For ex- techniques have been valuable for epidemiological ample, one of psychology’s “Big 5” personality traits is traits, intelligence, personality, social attitudes such as general “openness” and it seems likely degree of open- those connected to religion, psychological interests, ness is relevant to the political arena as well. Liberals and behaviors such as risk-taking propensities (for a and conservatives, on average, differ in their openness thorough review, see Bouchard and McGue 2003). Of to atheism, homosexuality, communism, immigration, most interest to us are the findings pertaining to social and countercultural activities. These differences may attitudes and behaviors. At first, researchers were so be entirely due to enculturation, but then again, they confident that social attitudes were not heritable that may not be, and we will never know without testing for they employed such items as controls. Quickly they the effects of genetics. discovered that other controls would have to be found Based on behavioral geneticists’ study of religion, because most social attitudes consistently displayed it seems that group identification is something that a surprising measure of heritability (see, e.g., Crelia is heavily influenced by the environment, especially and Tesser 1996, Scarr and Weinberg 1981, and Tesser shared environment, and is mostly unconnected to ge- 1993). netics. Children of Methodists are likely to be Metho- Political attitudes were never a central focus in this dists not because there is a gene for Methodism or even research stream but many of the patterns found in other a personality particularly oriented toward Methodism, social attitudes should be present for political attitudes but because of parental socialization. Thus, even as atti- as well, and this assumption guided the formulation tudes connected to religiosity and religious beliefs and of our expectations. Since the social attitudes tested activities (e.g., Sabbath observance, church authority, to date have demonstrated a strong heritable compo- belief in heaven, religious fundamentalism, frequency nent, frequently stronger than attitude covariance at- of attendance) were found to be shaped more by ge- tributable to shared environment, we predict that polit- netic inheritance than by parental views on those issues ical attitudes will also be heavily heritable. Heritability (for details, see Bouchard et al. 1999, Eaves, Martin, estimates calculated by previous researchers for atti- and Heath 1990, Maes et al. 1999, and Martin et al. tudes associated with psychological conservatism are 1999), identification with a particular religious group quite high, while the relevant models typically show was shaped more by socialization and almost not at little or no effect for shared environment (the remain- all by genetics. We expect to find a similar pattern der is likely the result of nonshared environmental with political party identification. Children are eager factors). Notably, these findings come from studies of to belong to the groups their parents belong to and twins in settings as disparate as Australia, Virginia, and parents are frequently eager to encourage children in Minnesota, and the findings of the Minnesota study, this regard. Assuming these identifications have some utilizing twins reared apart, conform well to the stickiness into early adulthood, our core expectation other studies of twins raised together (for a summary, is that party identification will be influenced more by see Bouchard and McGue 2003).5 Careful studies of parental socialization (shared environment) than by adopted children confirm the finding that genetics mat- genetic inheritance but that this pattern will be re- ter more than parentally created environment in influ- versed for political attitudes with inheritance playing encing social attitudes and behaviors, personality traits, aroleatleastaslargeasthesharedenvironment.By and intelligence.6 predicting a large influence for genetic inheritance, we We further predict that attitudes on political is- depart from typical behavioralist expectations antic- sues tracking most closely to central personality traits ipating that political attitudes will be predominantly should be the most heritable since personality traits influenced by environmental factors, rendering genetic are generally heritable and since the heritability of so- inheritance largely, if not completely, inconsequential. cial attitudes is likely derivative of the heritability of various personality traits (see Bouchard and Loehlin DATA AND METHODS Since twin studies have not been conducted by po- 5 Conservatism is not unusual in this regard. Rushton, Littlefield, and litical scientists, political attitudes have been at best Lumsden (1986, 7340) find that approximately 50% of the variance asidelight,andproperlyrefinedmeasuresofpolitical in altruism is the result of “direct genetic inheritance,” with family variables have not been constructed and employed (the environment responsible for 0%. 6 Adoption studies measure the correlation of biological parents and heritability of political behavior has not been analyzed adopted children where the biological parents have had no contribu- at all). Nonetheless, some previously employed vari- tion to the rearing (environment) of the child. The most recent adop- ables in twin studies have political relevance. For ex- tion study, utilizing surveys of Korean-American adoptees randomly ample, the heritability of conservatism is frequently as- assigned to families in the United States, concludes that roughly 75% sessed (see, e.g., Bouchard et al. 1990, Eaves, Eysenck, of variance in children’s educational attainment is attributable to the educational attainment of their biological parents, and only 25% is and Martin, 1989, and Martin et al. 1986), and even attributable to the adoptive parents, thus dramatically confirming though conservatism is viewed by the scholars who the earlier findings of a substantial correlation between biological do twin studies more as a psychological trait than a parents and adopted children and a surprisingly paltry correlation political ideology, measures of it include political items. between adoptive parents and children (Sacerdote 2004). This par- allels, with an entirely distinct methodology, the basic finding of the Of most relevance here is the Wilson–Patterson twin studies (see Plomin et al. 1997, 1998 and Rhee and Waldman (W–P) Attitude Inventory. This inventory is admin- 2002). istered by presenting subjects with a short stimulus

157 Are Political Orientations Genetically Transmitted? May 2005 phrase such as death penalty or royalty and eliciting fact a continuous trait). The correlations are computed asimpleagree,disagree,oruncertainresponse.The separately for male/male and female/female twin pairs broadest version of the W–P inventory includes 50 to provide an appropriate comparison, since all MZ items, 25 of which contribute positively to the con- twins are same-sex pairs, while DZ twins are a mix servatism score and 25 of which contribute negatively of same-sex and opposite-sex pairs (in other words, to the conservatism score. While some of the items re- female/male DZ twin pairs are excluded from the anal- late to a heavily social conception of conservatism—–for ysis). Without this control, the presence of any male/ example, pajama parties, nudist camps, computer mu- female differences would spuriously deflate the cor- sic, and horoscopes—–others have a much more direct relations for DZ pairs relative to the same-sex MZ political content—–for example, disarmament, social- pairs. ism, patriotism, and death penalty. Studies typically Heritability is typically estimated by subtracting the utilize reduced sets of W–P items or modify individual correlation for DZ pairs from the correlation for MZ items to better suit the country in which the items are pairs and then doubling the resulting difference. At one being administered. For political science this presents extreme, if the correlations are the same for MZ and two frustrations. The list of politically relevant items DZ pairs, suggesting that genetic similarity plays no is tantalizing but limited and unfocused, and the re- role in similarity for that particular trait, then the result sults are often presented only for the entire combined will be an estimate of heritability of zero. At the other scale, making it difficult to assess the contribution extreme, a purely genetic additive trait should produce of the directly political items to the overall index of a correlation of .5 for DZ pairs and 1.0 for MZ pairs, re- heritability. sulting in an estimate of heritability of 1.0 (1.0 .5 .5, We were granted access to the data for the W–P items and 2 .5 1.0). In a similar way, we can estimate− = the in the United States and were able to conduct compa- influence∗ of= shared environment, as opposed to shared rable, though more limited, twin correlation analyses genetic material, by doubling the correlation for DZ from published results of an Australian study.7 The U.S. pairs and then subtracting the correlation for MZ pairs. study included information on thousands of twin pairs Again, a purely genetic additive trait should produce in Virginia, supplemented with twin pairs recruited acorrelationof.5forDZpairsand1.0forMZpairs, through the cooperation of AARP. A subset of these resulting in an estimate of the impact of shared en- twins and their relatives has been asked questions re- vironment of zero (2 .5 1.0, and 1.0 1.0 0). At garding their social attitudes, including numerous items the other extreme, if the∗ correlations= are− the same= for from the W–P inventory. MZ and DZ pairs, suggesting that genetic similarity Abriefexplicationoftwinmethodologyshouldhelp plays no role in similarity for that particular trait, then readers make independent sense of the tables. The the result will be an estimate of the impact of shared standard techniques in behavioral genetics are based environment that is equal to the MZ or DZ correlation on correlation analysis (in the case of limited response (e.g., if MZ DZ .4, then 2 .4 .8, and .8 .4 .4). items like the W–P inventory, the actual measure is the Whatever is= left over= is taken∗ to be= attributable− to= the polychoric correlation coefficient, a technique that is unshared environment. appropriate when individual subjects are using a lim- ited set of categories to express location on what is in THE HERITABILITY OF POLITICAL 7 Our thanks go to Professor Lindon Eaves at Virginia Common- ATTITUDES wealth University for making the VA30K data available to us. The data collection methods for both studies are summarized in Lake Table 1 contains the results of a standard polychoric et al. 2000 as follows: “The Australian sample was ascertained correlation analysis for the 28 W–P items available in through two cohorts of twins. The first cohort was recruited in 1980– the Virginia 30K data set and for a select set of addi- 1982 from a sampling frame which comprised 5967 twin pairs aged tional items to provide some sense of perspective for 18 years or older (born 1893 to 1964) then enrolled on the Aus- the level of these correlations. Even the quickest glance tralian NHMRC Twin Registry (ATR). Responses were obtained from 3808 complete pairs ...and these were followed up with a sec- at the results in Table 1 is enough to set aside the tradi- ond mailed questionnaire in 1988–1990 with responses from 2708 tional view that genes do not play any role in explaining complete pairs. ...The second cohort of twins, born 1964–1971, was political attitudes. All 28 of the MZ correlations are recruited from the ATR in 1989 and was mailed similar question- larger than their corresponding DZ correlations, and naires in 1989–1991, with responses from 3,769 individuals of 4269 eligible pairs. ...In total there were 21,222 respondents in the Aus- in every case the difference is statistically significant at tralian sample, of whom 20,945 had valid scores for EPQ Neuroti- the .01 level. Far from typically being at or near zero, cism. The United States twins were ascertained from a population- none of the 28 heritability estimates falls in the single based birth registry for the Commonwealth of Virginia and from digit range, and more than half of the 28 items have avolunteersamplethroughtheAmericanAssociationofRetired heritability estimates of .3 or more. Heritability ranges Persons (AARP), described in detail by Truett et al. (1994). Their first-degree relatives and spouses were recruited in a similar fashion from a high of .41 to a low of .18, all suggesting that the to the Australian sample, and in total there were 24,905 respondents influence of heredity on political attitudes is very real, (of 29,080) with valid scores for Neuroticism and for whom the zy- and given the diverse range of items included here, gosity of the proband twins could be determined. The response rates this genetic influence is also pervasive. So the view were 70% for twins and 45% for relatives” (224–25). The original U.S. twin data collection was funded in part by NIH grants GM30250 that heritability of social and political attitudes will be and AG04954, by ADAMHA grants AA06781, AA07728, AA07535, nonzero but small relative to shared environment is and MH40828, and by a gift from R. J. R. Nabisco. also called into question. We see from Table 1 that the

158 American Political Science Review Vol. 99, No. 2

TABLE 1. Genetic and Environmental Influences on Political Attitudes: The 28 Individual Wilson–Patterson Items Polychoric Correlation MZ DZ Shared Unshared Heritability, Environment, Environment, z for (MZ–DZ) Attitude Item Corr. n Corr. n 2 (MZ DZ) (2 DZ) MZ 1 MZ Differencea ∗ − ∗ − − School Prayer 0.66 2,687 0.46 1,774 0.41 0.25 0.34 9.83 Property Tax 0.47 2,643 0.27 1,748 0.41 0.06 0.53 7.66 Moral Majority 0.42 2,614 0.22 1,717 0.40 0.03 0.58 7.16 Capitalism 0.53 2,609 0.34 1,720 0.39 0.14 0.47 7.85 Astrology 0.48 2,631 0.28 1,721 0.39 0.09 0.52 7.39 The Draft 0.41 2,641 0.21 1,753 0.38 0.02 0.59 6.94 Pacifism 0.34 2,576 0.15 1,686 0.38 0.04 0.66 6.43 Unions 0.44 2,661 0.26 1,752 0.37− 0.07 0.56 6.89 Republicans 0.48 2,627 0.30 1,734 0.36 0.12 0.52 6.91 Socialism 0.43 2,616 0.25 1,726 0.36 0.07 0.57 6.53 ForeignAid 0.41 2,669 0.23 1,771 0.35 0.06 0.59 6.42 X-Rated Movies 0.63 2,685 0.46 1,783 0.35 0.28 0.37 8.15 Immigration 0.45 2,658 0.29 1,748 0.33 0.12 0.55 6.20 Women’sLiberation 0.46 2,666 0.30 1,779 0.33 0.13 0.54 6.27 Death Penalty 0.56 2,684 0.40 1,775 0.32 0.24 0.44 6.83 Censorship 0.40 2,629 0.25 1,718 0.30 0.10 0.60 5.36 Living Together 0.67 2,679 0.52 1,771 0.30 0.37 0.33 7.54 Military Drill 0.38 2,625 0.24 1,733 0.29 0.09 0.62 5.24 Gay Rights 0.60 2,658 0.46 1,767 0.28 0.32 0.40 6.26 Segregation 0.38 2,653 0.24 1,743 0.27 0.11 0.62 4.83 Busing 0.43 2,670 0.30 1,766 0.26 0.16 0.57 4.92 Nuclear Power 0.42 2,646 0.29 1,744 0.26 0.16 0.58 4.84 Democrats 0.47 2,639 0.34 1,726 0.26 0.21 0.53 4.96 Divorce 0.47 2,659 0.34 1,765 0.26 0.21 0.53 4.99 Abortion 0.64 2,668 0.52 1,768 0.25 0.39 0.36 6.23 Modern Art 0.43 2,662 0.30 1,765 0.25 0.18 0.57 4.69 FederalHousing 0.36 2,665 0.26 1,766 0.20 0.16 0.64 3.61 Liberals 0.44 2,629 0.35 1,734 0.18 0.26 0.56 3.40 28-item mean 0.47 2,648 0.31 1,748 0.32 0.16 0.53 Source: Access to the data provided by Eaves et al., principal investigators, Virginia 30K twin study (see note 7). a The MZ–DZ correlation difference is statistically significant for all of the table items at the 0.01 level or above.

impact of shared environment exceeds that of heredity ing for the remainder. Within the half that is accounted for only four of the 28 items, and the mean estimate of for by shared influences, genetic influences, in contra- heritability for the 28 W–P items is .32, compared to diction to behavioralist expectations, are roughly twice ameanestimateofsharedenvironmentalinfluenceof as influential as environmental influences. .16. While the individual items provide interesting vari- The second-to-last column in Table 1 reports the esti- ation, the purpose of the W–P inventory is to provide mates for the proportion of the variation in an attitude an overall index of conservatism. We compute a simple that is attributable to the unshared environment. As index by assigning a value of 1toany“conservative” described above this is essentially a residual variance response (i.e., a “yes” to an+ item like Death Penalty category, reflecting such factors as random choice as or a “no” to an item like Women’s Liberation) and 1 well as external influences such as the unique expe- to any “liberal” response (i.e., a “no” to an item like− rience of each individual, including those from child- Death Penalty or a “yes” to an item like Women’s hood, and later influences in life that have been termed Liberation). Items where the respondent chose a non- “adult socialization” in the political science literature. commital (?) response are coded as zero. When these Across the 28 W–P items the estimate of the impact individual scores are summed across the 28 items they of unshared environment varies from about one-third yield an index that varies from a potential low of 28 (for School Prayer) up to about two-thirds (for Paci- (indicating a set of uniformly “liberal” responses)− to fism) of the overall variation. The average impact of the ahighof 28 (indicating a set of uniformly “conser- unshared environment for these items is .53, or roughly vative” responses).+ The actual index scores for the half of the overall variation. The summary picture for twins in the study range from 26 to 26, with the this set of political attitudes, then, is that shared influ- median response falling between− 2and+ 3. Given ences (genetic and environmental) account for about the far more continuous nature of+ this overall+ index, half of the variation in these political reactions, with we can now utilize the more traditional Pearson’s cor- unique individual and environmental factors account- relation coefficient. The results for the overall index

159 Are Political Orientations Genetically Transmitted? May 2005

TABLE 2. Genetic and Environmental Influences on Political Attitudes: Summary Index and Additional Non–Wilson–Patterson Items Correlation

MZ DZ Shared Unshared Heritability, Environment, Environment, z for (MZ–DZ) Attitude Item Corr. n Corr. n 2 (MZ DZ) (2 DZ) MZ 1 MZ Differencea ∗ − ∗ − − Pearson’s correlation coefficient 28-item index score 0.65 2,107 0.43 1,384 0.43 0.22 0.35 8.93 Partialcorr.forparent, 0.64 173 0.37 131 0.53 0.11 0.36 3.08 same index Opinionation 0.39 2,107 0.20 1,384 0.36 0.02 0.61 5.77 Polychoric correlation coefficient 28-item mean 0.47 2,648 0.31 1,748 0.32 0.16 0.53 Educational Attainment 0.86 2,683 0.66 1,771 0.40 0.46 0.14 16.40 PartyAffiliation 0.55 2,417 0.48 1,554 0.14 0.41 0.45 2.99 Mean of affect toward Reps. 0.48 2,633 0.32 1,730 0.31 0.17 0.52 5.94 and Dems. Source: Access to the data provided by Eaves et al., principal investigators, Virginia 30K twin study (see note 7). a The MZ–DZ correlation difference is statistically significant for all of the table items at the 0.01 level or above. are presented in Table 2 and clearly support a powerful .36 and the estimate for shared environment is only role for heredity in influencing conservatism, at least .02. The estimate for unshared environment is high, as measured by the W–P inventory. The estimate for at .61, falling near the top of the range for individual heritability is .43, higher than for any of the individ- items. To the extent that there is a family effect on ual items. The estimate for shared environment is .22, political opinionation, it would appear to be entirely a falling within the upper range of the individual items, genetic one, with the remaining roughly two-thirds of while the estimate for unshared environment is only the variation being due to nonshared factors. .35, falling very near the bottom of the range for indi- Two items from the survey that are not a part of the vidual items. The overall picture is again a very strong W–P inventory are included in Table 2. Party affiliation role for heredity and a less powerful, but clear role for is the most clearly political of the items in the broader shared environment. What is different for the overall questionnaire, and it is useful here on its own, as well index is that the role of shared influences (genetic and as in contrast to the attitudinal items. Party identifi- environmental) account for almost two-thirds of the cation is distinct among U.S. political attitudes both variation in the index (compared to about one-half in our conception of it as an identification, and hence for the individual items), with unique individual and as something at least potentially distinct from simple environmental factors accounting for only about one- item evaluation, and in its established tendency to cor- third of the variation. This decline in the role of unique relate well between parent and child (see Jennings and individual and environmental factors seems sensible, Niemi 1968). This distinctiveness is apparent in Table 2. as we are moving from individual and highly specific As we expected, the pattern for party identification is items that could involve a host of unique experiential, nearly the exact reverse of that for the average attitude associational, and informational perturbations to an item. Heritability for party affiliation is relatively low index where those idiosyncratic features of individual (r .14), while shared environment is much stronger items have the opportunity to cancel each other out. (r = .41). Note also that not one of the 28 W–P items The W–P items can also be used to construct a rough has= an average heritability that is as low as that for party index of political opinionation by taking advantage of affiliation, and likewise, not one of the 28 items has an the frequency of ? responses. The number of times average coefficient for the impact of shared environ- that a respondent chose a yes or no response over a ment that is as high as that for party affiliation. Clearly, neutral ? response was summed to produce an index party identification is, at least for the United States, a that varies from zero to 28, with a 28 indicating that the different sort of beast than reactions to issue items. respondent was willing to express a directional opinion In this regard it is particularly interesting that the on all 28 items and a score of zero indicating that the two major parties also appear in the W–P battery, but respondent was unwilling to offer a directional opin- here they are objects of affect rather than labels of pos- ion on any of the 28 items. The median for this index sible identification, and the “pro” or “con” reactions to is 21 yes or no response choices of 28 possible. The the parties that these items pick up do not exhibit the results for the overall index clearly support a power- same patterns of genetic and environmental influence ful role for heredity in influencing political opiniona- that we see for party affiliation. In fact, if we average tion, at least as it is captured by the admitted rough the polychoric correlation for the “Democrats” item gauge of the frequency of nonneutral responses to the with the correlations for the “Republicans” item and W–P inventory items. The estimate for heritability is compute the resulting estimates we get a heritability

160 American Political Science Review Vol. 99, No. 2 estimate of .31 and a shared environment estimate of DZ twins of these parents will be as genetically alike .17, almost exactly the same as the mean results for all on this one trait as MZ twins are on this trait. Across a 28 attitude items. It would appear that affect toward study population, the higher the proportion of spouses the major parties is largely a matter of genetic predis- that share identical genetics for a trait, the closer the position but that, just as the political socialization liter- DZ correlation will be to the MZ correlation. Since ature has concluded all along, party identification itself heritability of a trait is estimated as 2 (MZ DZ), the is primarily the result of parental socialization. This increased similarity of DZ and MZ pairs∗ will− lead to an pattern is intriguing in and of itself but it also should underestimation of heritability for this genetic trait. give pause to those who would dismiss the findings This is important for our assessment of the heritabil- on attitude items as the product of some methodolog- ity of political attitudes. If there is a tendency for people ical quirk of twin studies. If estimates of heritability to choose mates with similar positions on political is- are somehow artificially inflated, why does this alleged sues, then the estimates of heritability in Tables 1 and 2 contamination not occur for party identification? are biased. Fortunately for us, the direction of the bias Table 2 also reports the results for a summary is uniform and conservative. Any measurable tendency indicator of educational attainment from the survey. toward assortative mating on political orientation will We include it here partly because it reflects an actual push up the DZ twin correlation while leaving the behavior, if only a self-reported one, and partly be- MZ correlation unaffected, and this reduction in the cause it carries the role of genetics more directly into MZ–DZ gap will have the related effect of lowering the world of actual and meaningful social variation. estimates of heritability. Note also that any increase in Educational attainment is also useful as an example similarity of DZ twins will inflate the estimate of the of a behavior that is traditionally thought to be heav- importance of shared environment, as the estimation ily influenced by shared environment, particularly by formula of (2 DZ) MZ makes clear. parental example, expectations, and resources. This tra- The immediate∗ empirical− question is how much of ditional view is supported by the shared environment a role assortative mating plays in political issue posi- estimate of .46, a figure higher than any of the estimates tions. A quick answer can be found by looking at the for the 28 attitude items and even somewhat higher interspouse polychoric correlations for the individuals than the estimate for party ID. What may surprise included in the Virginia 30K study. The average inter- readers is that as important as shared environment is spouse polychoric correlation for the 28 items is .41 and to educational attainment, heredity, at .40, is almost as the individual correlations range from a low of .26 for important. Taken together, family effects are almost Censorship to a high of .64 for School Prayer. While the entire story for variation in education attainment. some of this interspouse similarity could plausibly be The estimate for the impact of the unshared environ- attributed to persuasion effects taking place after mate ment is only .14, a value markedly lower than any other choice rather than to assortative mating, the levels of in the table. similarity are probably too high to dismiss assortative mating entirely. This is confirmed by a preliminary look ASSORTATIVE MATING at the impact of controlling for assortative mating on these 28 attitude items. The Virginia 30K study in- Assortative mating is a particular concern here. As de- cludes data for parents of twins in the study, including tailed above, the assumption that DZ twins, like any parents’ individual responses to the same W–P items other pair of biological siblings, share on average 50% that the twins responded to. The usable sample size of the variable genetic code is crucial to the estimation does drop substantially when we restrict our analysis of heritability. This contrasts with MZ twins, where the to only twin pairs with completed W–P results for both shared proportion is 100%, and the DZ level forms parents (there are a total of 304 pairs of male/male the baseline for separating genetics from the shared or female/female twins with complete twin and parent environment. What may not be immediately apparent W–P data, compared to approximately 4,400 pairs in is that the assumption that purely genetic traits in DZ the twin only analysis in Table 1). This effectively limits twins will on average correlate at .50 is itself built on the us to an assortative mating analysis that focuses on the assumption that their biological parents will on average overall index score, rather than looking at each item in correlate at .00 for the same traits. In other words, the the inventory individually. assumption is that the parents are not related to each The approach we used is to compute the partial cor- other in any close degree, and this is typically true, as relation for twin similarity in the overall index for the 28 close relatives generally do not mate, and the amount W–P items, controlling for (partialing out) the influence of average shared genetic code drops geometrically as of the degree of parental similarity on the overall index. we move away in relatedness and quickly approaches The implication for relative twin agreement is simple; zero. This assumption that mates are genetically uncor- if parental agreement results from assortative mating, related on the trait of interest is, however, violated if then the resulting increase in genetic similarity will mate choice is itself based on the trait of interest. If, increase DZ twin correlations (the more alike geneti- for example, parents have identical genetic codes for a cally the parents are on a trait, the more alike siblings trait of interest, then the shuffling of that genetic code will be on a trait). Controlling for parental similarity produced by sexual reproduction will not result in any will therefore reduce the size of the DZ twin corre- variation among DZ twins, or any other siblings, with lations. However, parental agreement resulting from regard to their genotype for that trait. In other words, assortative mating and the resulting increase in genetic

161 Are Political Orientations Genetically Transmitted? May 2005

TABLE 3. Comparison of Australian and U.S. Estimates of Genetic and Environmental Influences on Political Attitudes Virginia 30K Data Australian Data

Shared Shared Heritability, Environment, Heritability Environment 2 (MZ – DZ) (2 DZ) – MZ 2 (MZ – DZ) (2 DZ) – MZ ∗ ∗ ∗ ∗ Attitude Item Male Female Mean Male Female Mean Attitude Item Male Female Mean Male Female Mean Astrology 0.49 0.33 0.41 0.01 0.14 0.06 Horoscopes 0.30 0.32 0.31 0.07 0.18 0.13 − Pacifism 0.46 0.34 0.40 0.09 0.03 0.06 Disarmament 0.26 0.48 0.37 0.09 0.08 0.01 − − − − Censorship 0.55 0.20 0.37 0.12 0.17 0.02 Censorship 0.30 0.48 0.39 0.11 0.03 0.04 − − Socialism 0.33 0.36 0.35 0.05 0.08 0.07 Socialism 0.04 0.24 0.14 0.38 0.24 0.31 Military Drill 0.42 0.24 0.33 0.02 0.12 0.05 Military Drill 0.32 0.42 0.37 0.20 0.08 0.14 − Immigration 0.29 0.35 0.32 0.16 0.10 0.13 Nonwhite 0.06 0.20 0.07 0.50 0.23 0.37 − Immigration Death Penalty 0.27 0.35 0.31 0.27 0.19 0.23 Death Penalty 0.42 0.54 0.48 0.10 0.03 0.04 − Women’s 0.23 0.35 0.29 0.08 0.17 0.13 Working 0.22 0.32 0.27 0.15 0.16 0.16 Liberation Mothers Segregation 0.34 0.23 0.29 0.04 0.14 0.09 Apartheid 0.44 0.38 0.41 0.09 0.08 0.09 Modern Art 0.31 0.21 0.26 0.11 0.22 0.16 Modern Art 0.40 0.22 0.31 0.02 0.26 0.12 − Abortion 0.26 0.24 0.25 0.29 0.44 0.36 Legalised 0.10 0.42 0.26 0.50 0.25 0.38 Abortion Divorce 0.20 0.28 0.24 0.22 0.21 0.21 Divorce 0.16 0.44 0.30 0.31 0.10 0.21 Mean 0.35 0.29 0.32 0.08 0.16 0.12 Mean 0.24 0.37 0.31 0.21 0.12 0.16 Source: Access to the original U.S. data provided by Eaves et al., principal investigators, Virginia 30K twin study. Australian data computed from Martin et al. 1986, Table 1, p. 4365. similarity will not increase MZ twin correlations (MZ age estimate of heritability rises to .53, and the average twins are already genetically identical, regardless of estimate of the impact of shared environment drops to parental similarity or dissimilarity). Therefore, control- .11. Note that the traditional socialization account of ling for parental similarity should have no effect on the attitude formation is not at odds with this last finding. size of the MZ twin correlations. In contrast, if parental If the issue positions of parents are in conflict, then we agreement results from persuasion or from a shared en- would hardly expect this shared conflicted setting to vironment for the couple, then the impact of parental yield sibling agreement.8 agreement has no genetic implications and operates on their offspring solely through its influence on the COMPARATIVE POLITICAL GENETICS: offsprings’ shared environment. This should produce EVIDENCE FROM AUSTRALIA relatively higher correlations of equal magnitude for both MZ and DZ twins and, therefore, lead to roughly Even with a data set as large as the Virginia 30K, ques- comparable reductions in both the MZ and the DZ tions may arise over the extent to which conclusions correlations when we partial out the effect of parental are bound by time and geography. As a result, it is agreement. helpful to note results from a quite different context The results for a partial correlation analysis control- and a slightly different time period. Table 3 presents ling for parental agreement are reported in Table 2, acomparisonofthekeysummaryresultsinTable1 on the row just below the results for the overall index. from the Virginia 30K study to comparable results in For MZ twins the issue of whether their parents agree the Australian data described before (Truett et al. 1994; or disagree on a particular item makes little difference see also Lake et al. 2000). While the Australian study (.65 without control versus .64 after partialing out the effect of parental agreement). In contrast, the correla- tion between DZ twins decreases modestly when the 8 The same sort of control for parental agreement that was applied impact of parental agreement is removed (.43 with- to the W–P inventory was applied to the party affiliation analysis. Be- out control versus .37 after partialing out the effect of cause this is only a single item, the results are much less reliable than those averaged across the 28 items. However, despite the fact that parental agreement). Further, the tendency of assor- assortative mating clearly takes place with regard to party ID (only tative mating to deflate estimates of heritability while 24 of the 543 parent pairs had opposite party affiliations), the general inflating estimates of the impact of shared environment pattern of party ID being due more to shared environment than to is clear. Without controls, the estimate of heritability heredity holds up. Using a very broad definition of disagreement (i.e., anything short of exact agreement on a five-point scale), the for the overall index is .43 and the average estimate shared environment estimate weakens modestly but remains high, at of the impact of shared environment is .22. When the almost twice the heritability estimate in the subset of twin pairs with impact of parental agreement is partialed out, the aver- parents in some degree of disagreement on party affiliation.

162 American Political Science Review Vol. 99, No. 2 utilized a larger set of W–P items (50 in all, compared responsive to particular environmental conditions (see to 28 in the U.S. study), the items were a mix of po- Boyd and Richerson 1985 and Masters 1993). litical and social items, and only six items appeared in And this conditioning influence of genetics on com- exactly the same form in both studies. An additional six plex social behaviors is not the product of a single gene items were similar enough, in our judgment, to merit but rather numerous genes that, to make matters more comparison, and they are included in Table 2 with the complicated, appear to combine in configural as op- Australian wording italicized. posed to additive ways. The same set of multiple genes The broad picture from Table 3, and its comparison may influence behavior in different ways depending to Tables 1 and 2, is one of remarkable similarity. on the order in which they express themselves and The mean heritability for the 12 item subset of the the manner in which they interact with other genes. Virginia 30K data is .32 for the full 28 items in Table 1 Recent discoveries also suggest that biological mark- and .31 for the 12-item subset of the Australian data. ers of phenotypic manifestations include the manner Themeanestimatefortheeffectofsharedenvironment in which DNA is packed in the nucleus, particularly for the 12 item subset of the Virginia 30K data is .12 the physical location of genes relative to other genes compared to .16 for the full 28 items in Table 1 and .16 and to the histones that help to give DNA its structure. for the 12-item subset of the Australian data. Thus the An accurate understanding of gene expression appears general pattern of a relatively greater role for hered- to require knowledge not just of the sequence of nu- ity compared to shared environment detailed above in cleotides (e.g., ATCAGG) that constitutes the gene the discussion of the U.S. data in Tables 1 and 2 also itself but also of the context in which each gene re- applies to the Australian data in Table 3. While most sides, thus forming an interesting parallel to the way of the individual items also have broadly comparable we must try to understand the organisms (e.g., human results in the two countries, a few, specifically “social- beings) genes help to construct (for a good summary, ism” and “immigration” (“nonwhite immigration” in see Kosack and Groudine 2004; also see Lykken 1999). the Australian study), are noticeably different. In both Individual genes for behaviors do not exist and no cases the U.S. pattern of substantially higher relative one denies that humans have the capacity to act against heritability is reversed in the Australian data, where we genetic predispositions. But predictably dissimilar cor- see evidence of relatively higher shared environmental relations of social and political attitudes among peo- effects. Whether these are meaningful reflections of ple with greater and lesser shared genotypes suggest differences in how these items relate to deeper political that behaviors are often shaped by forces of which orientations is not clear, but they are in any case the the actors themselves are not consciously aware, a exceptions rather than the rule.9 point that is made with some force by Bargh and Chartrand (1999), Marcus (2002), Marcus, Neuman, and MacKuen (2000), McDermott (2004), and Wegner THE GENETICS OF POLITICAL IDEOLOGY (2002). It is not biological determinism to posit the exis- The possibility that attitudes and behaviors are influ- tence of complex collections of genes that increase the enced by genetic variables is an emotionally charged probability that certain people will display heightened topic so it is important that readers understand the or deadened response patterns to given environmental claims being made. Partitioning the origins of human cues. And it is not antibehavioralism to suggest that traits, whether they be physiological or behavioral, into true explanations of the source of political attitudes the discrete, quantifiable components of genetic inheri- and behaviors will be found when we combine our cur- tance, shared environment, and unshared environment rently detailed understanding of environmental forces should not be taken to imply that these components with a recognition that genetic variables subtly but im- work separately. Rather these numbers only provide a portantly condition human responses to environmental rough indication of the influence of three categories of stimuli. independent variables that are intimately intertwined. (Moreover, they are estimates of the ability of inde- pendent variables to account for variance in the de- IMPLICATIONS FOR POLITICS pendent variables not for the variables themselves.) As mentioned earlier, gene–culture interaction is the It is important to note that none of the data or argu- key to understanding the source of political attitudes ments presented in this paper indicates that extant em- and behaviors, just as it is the key to understanding pirical knowledge about political socialization is use- most physical and behavioral aspects of the human less. In fact, it strongly reinforces many of the most condition. Genes do not work in isolation and instead salient findings in that research stream. We know from generally influence the extent to which organisms are that research, for example, that if both parents share a political identification, there is a high degree of likeli- hood that their offspring will have that same political identification (Jennings and Niemi 1968; Tedin 1974). 9 In this case, the different results with regard to socialism could Our “twin study” results confirm this finding. One reflect different meanings of the phrase in the two countries. In of the peculiar findings in the political socialization Australia, the term socialism is closer to a party identification la- bel, whereas in the United States it has more loaded ideological literature even makes more sense when a role for ge- connotations. Likewise, the addition of the qualifier “nonwhite” to netic inheritance in conceded. Scholars have occasion- immigration raises questions of what the key stimulus is. ally puzzled over the fact that family arrangements and

163 Are Political Orientations Genetically Transmitted? May 2005 styles of operation have little if any impact on the ex- concerns? In light of the new findings, one distinct pos- tent to which there is a match between parental and sibility is that easy “gut” issues tend to be those that offspring political attitudes on a wide variety of items are more heritable. (see Jennings and Niemi 1968, 180–83). Fathers do not To the extent that political ideologies are inherited have more influence over sons, and mothers do not and not learned, they become more difficult to manip- have more influence over daughters; fathers are not ulate. Conservative parents who try to make their chil- generally more influential; the distribution of power dren conservative by carefully controlling their chil- within the family is irrelevant to parent–child correla- dren’s environments are probably overestimating the tions (i.e., neither highly autocratic, highly permissive, importance of those environments. Offspring of such nor middling arrangements affect the extent to which parents are likely to end up being conservative but attitudes are correlated); the degree to which children less because of the environment created by the parents and parents feel close to each other does not matter; the than the genes passed along by the parents. A politi- frequency with which the family discusses politics does cal match between parents and children should not be not much affect correspondence between offspring and taken to be the result of a socialization process—–that parent views (though, as we would have predicted since is, the active postnatal transmission of views—–just as it is based on active socialization, party identification is political mismatches between parent and child should more sensitive to family arrangements); and the extent not be taken as evidence against a role for genetics. to which politics is important to the parents is also ir- Parent–child mismatches are distinctly possible given relevant. Scholars grounded in traditional behavioral- the uncertainties of meiosis (the random selection of ism have difficulty accounting for these “perplexing just 50% of each parent’s DNA) and the possibility for configurations” (Jennings and Niemi 1968, 183), but occasional errors in the transcription and translation recognizing that the correlations between the views of of genes (mutations). These mismatches are likely to parents and children derive more from genetics than be the primary cause of the fact that some children familial socialization makes it much less surprising that rebel against the views of their parents but most do the strength of these correlations is not reliant on fam- not—–a pattern that environmental factors have never ily arrangements (for an example of political science explained satisfactorily. work that does posit a role for genetics, see Peterson Finally, we go into somewhat greater detail to il- 1983). lustrate the manner in which results such as ours can Still, the substantive findings we present here of- be of use in understanding the divisions characterizing fer a direct challenge to common assumptions and virtually all polities and, certainly, the United States interpretations that political attitudes and behavioral in the early twenty-first century. Remember, genes in- tendencies are shaped primarily or even exclusively fluence people’s outlooks and personalities, and it is by environmental, especially familial, factors. Setting these broad features that then predispose individuals aside the important special case of party identification, toward suites of specific attitudes. This interpretation we find that political attitudes are influenced much likely explains the otherwise puzzling consistency in more heavily by genetics than by parental socialization. ideological divisions that is present across space and For the overall index of political conservatism, genet- time. The package of attitudes held, for example, by ics accounts for approximately half of the variance in conservatives in the modern United States is remark- ideology, while shared environment including parental ably similar to that held by conservatives in other cul- influence accounts for only 11%. And in the case of tures and at earlier times in American history (on the the variance in people’s tendencies to possess political durability of the liberal–conservative spectrum in the opinions at all, regardless of their ideological direction, United States, see Poole and Rosenthal 1997). Envi- genetics explains one-third of the variance, and shared ronmental determinists have no convincing explana- environment is completely inconsequential. tion for the pervasiveness of this division but genetics What are the implications of these findings for po- does. litical science? Acknowledging a role for heritability If, as our results suggest, there is a genetic basis for in politics affects our understanding of, first, political the varying political views people hold, and if, as seems issues, second, political learning, and, third, political probable, genetic transmission frequently affects clus- cleavages. Inherited attitudes seem to be demonstrably ters of political attitudes, we are likely to observe broad different than acquired attitudes. Tesser (1993) pro- but distinct political phenotypes. The number of these vides evidence that attitudes higher in heritability are phenotypes may vary, but for purposes of illustration manifested more quickly, are more resistant to change, we discuss two probable orientations. One is charac- and increase the likelihood that people will be attracted terized by a relatively strong suspicion of out-groups to those who share those particular attitudes. It has long (e.g., immigrants), a yearning for in-group unity and been known that certain political issues seem “hard” strong leadership, especially if there is an out-group to people, and others seem “easy,” presumably because threat (“Do not question the President while we are some issues trigger “gut responses” while others do not at war with terrorists”), a desire for clear, unbending (Carmines and Stimson 1980, 79), but no explanation moral and behavioral codes (strict constructionists), a has yet been offered for why given issues do or do fondness for swift and severe punishment for violations not elicit gut responses. Why do social, more than eco- of this code (the death penalty), a fondness for system- nomic, issues tend to hit people in the gut, even though atization (procedural due process), a willingness to tol- both constitute ongoing and equally complex societal erate inequality (opposition to redistributive policies),

164 American Political Science Review Vol. 99, No. 2 and an inherently pessimistic view of human nature Such an account is speculative at this point but is (life is “nasty, brutish, and short”). fully consistent with the findings presented here, with The other phenotype is characterized by relatively previous research on the durability of political ide- tolerant attitudes toward out-groups, a desire to take ologies, and with recent events in the United States. amorecontext-dependentratherthanrule-basedap- Accounts of the 2004 election, for example, that do not proach to proper behavior (substantive due process), invoke this fundamental difference in orientation have an inherently optimistic view of human nature (people fallen flat. Issues did not determine vote choice for the should be given the benefit of the doubt), a distaste many citizens who expressed disagreement with exist- for preset punishments (mitigating circumstances), a ing economic policies and/or the war in Iraq yet still preference for group togetherness but not necessarily voted for the incumbent president, George W. Bush. unity (“We can all get along even though we are quite Indeed, if the focus remains on issues, the resultant different”), suspicion of hierarchy, certainty, and strong description of the American public is grossly at odds leadership (flip-flopping is not a character flaw), an with reality. Morris Fiorina’s (2005) creative analysis aversion to inequality (e.g., support for a graduated of survey responses indicates that Americans can be income tax), and greater general empathic tendencies placed in the middle on many important issues, but if (rehabilitate, don’t punish). this is true, then what explains the vitriol and intensity Common political usage would call the first pheno- of feeling displayed by so many ordinary Americans in type conservative and the second liberal, but we seek 2004? phrases that are less connected to political ideologies Issues do not explain Americans’ politics. Many and that indicate that these two phenotypes run to the Americans admit that they do not follow or under- very orientation of people to society,leadership, knowl- stand the issues (Hibbing and Theiss-Morse 2002), and edge, group life, and the human condition. Thus, we to the extent they do, they support whatever their label the first “absolutist” and the second “contextual- preferred politician and party seems to support (Page ist.” This fundamental dimension offers a credible pre- and Jones 1979). In the 1990s, a Democratic president cursor to basic cleavages manifested in a broad range of (Bill Clinton) transformed welfare to workfare; then in human social activity: politics (conservatives/liberals), the 2000s, his Republican successor (George W. Bush) religion (fundamentalists/secular humanists), law (pro- greatly expanded federal involvement in both educa- cedural/substantive due process), education (phonics/ tion and the provision of prescription drugs for senior whole language), art (traditional form-based real- citizens. If the enactors of these policies were reversed, ism/modern free-form impressionism), sports (foot- the groups of citizens displaying support for the policies ball/frisbee), medicine (traditional AMA/wholistic), also would have reversed. Similarly, if a Republican morality (enduring standards/situational ethics), and president had committed adultery with a young intern scientific inquiry (formal/empirical). In our view, all or if a Democratic president had dramatically wors- of these vexing perennial dichotomies are related cul- ened the deficit and taken the country to war in a far-off tural expressions of a deep-seated genetic divide in land on the basis of undeniably incorrect beliefs about human behavioral predispositions and capabilities. We the opponents’ nuclear and chemical weapons capabil- certainly are not asserting that everyone holds one of ities, the positions of most voters on the acceptability of these two orientations. Even if the individual genes in- these conditions would be completely reversed. Issue volved with absolutism or contextualism tend to move positions generally reflect divisions; they do not create together, this does not mean they always do. Some them. individuals may carry, say, an absolutist’s aversion to Instead, the most accurate account of voting behav- out-groups but a contextualist’s rejection of a univer- ior in 2004 moves beyond issues to the basic, partially salistic behavioral code. Moreover, genes not included genotypic orientations described above. This sort of in these central packages, perhaps those related to ex- broad orientation is not far removed from what most troversion, ambition, and intelligence, often muddy the commentators are trying to capture by reference to a waters. “moral” division in the electorate, but without tying it More importantly, let us not forget that a heritable to specific moral issues such as gay rights. The chasm in- component of 50% for political ideology and proba- spiring so much hostility between citizens of the United bly somewhat higher for the absolutist-contextualist States in the early twenty-first century did not divide dimension still leaves plenty of opportunity for the en- supporters and opponents of privatizing Social Secu- vironment to alter attitudes and behaviors—–and even rity; it did not even divide supporters and opponents orientation. An individual with a contextualist geno- of gun control. Rather, as has typically been the case, type who has been repeatedly victimized by out-group it divided absolutists and contextualists. members, or who has simply spent a great deal of time And the prospects for eliminating this divide are listening to persuasive absolutists, may adopt attitudes not promising. Since mate choice appears to be heav- that run against type. Thus, even if a political system ily tilted toward those with similar social and politi- started with two pure genotypes, it would soon dis- cal attitudes, no genetic melting pot exists for these play a fascinating array of expressed orientations and traits. Thus, the evidence presented here on assortative beliefs, intensity levels, and degrees of involvement mating should be quite sobering to those in search of even as the system would continue to revolve around unity and togetherness. If anything, the heritability of the central division between absolutists and context- orientation in combination with assortative mating ualists. may exacerbate the current divide.

165 Are Political Orientations Genetically Transmitted? May 2005

But admitting that genetics influences political at- Bouchard, T. J., Jr., D. T. Lykken, M. McGue, N. L. Segal, and titudes could actually help to mute societal divisions. A. Tellegen. 1990. “Sources of Human Psychological Differences: Currently, absolutists and contextualists simply do not The Minnesota Study of Twins Reared Apart.” Science 12 (Octo- ber): 223–28. connect, and the result is frustration. To contextualists, Bouchard, T. J., Jr., Matt McGue, D. T. Lykken, and A. Tellegen.1999. absolutists appear simplistic and selfish; to absolutists, “Intrinsic and Extrinsic Religiousness.” Twin Research 2(June): contextualists appear naive and indecisive. Each side 88–98. talks past, and is authentically miffed by,the other. Rec- Boyd, Robert, and Peter J. Richerson. 1985. Culture and the Evolu- tionary Process.Chicago:UniversityofChicagoPress. ognizing that our political antagonists probably have a Breland, Keller, and Marian Breland. 1961. “The Misbehavior of different genetic predisposition to people, life, human Organisms.” American Psychologist 16 (9): 681–84. nature, and politics may serve to ease frustrations Brewer, Marilyn B. 2000. “Superordinate Goals Vs. Superordinate and, eventually, to improve communications across the Identity as Bases of Cooperation.” In Social Identity Processes,ed. chasm. If absolutists spent more time trying to think Dora Capozza and Rupert Brown. London: Sage. Campbell, Angus, Philip E. Converse, Warren E. Miller, and Donald like contextualists and contextualists trying to think E. Stokes. 1960. The American Voter. New York: John Wiley. like absolutists, understanding would be increased and Carmines, Edward G., and James A. Stimson. 1980. “The Two Faces debates could become more constructive. As frustrat- of Issue Voting.” American Political Science Review 74 (March): ing as it may be to debate with someone who holds such 78–91. Caspi, Avshalom, Karen Sugden, Terrie E. Moffitt, Alan Taylor, Ian different orientations, value exists in recognizing that W. Craig, Honalee Harrington, Joseph McClay, Jonathan Mill, intransigence is not the result of willful bullheadedness Judy Martin, Antony Braithwaite, and Richie Poulton. 2003. “In- but, rather, genetically driven differences in orienta- fluence of Life Stress on Depression: Moderation by a Polymor- tion. phism in the 5-HTT Gene.” Science 18 (July): 386–89. The exciting next step is to understand the reason Converse, Philip E. 1964. “The Nature of Belief Systems in Mass Publics.” In Ideology and Discontent, ed. David Apter. New York: such distinct orientations have evolved and lasted. Free Press. Evolutionary psychologists tend to assume that all Cook, Timothy E. 1985. “The Bear Market in Political Socialization enduring traits are adaptive (for a dissenting view, and the Costs of Misunderstood Psychological Theories.” Ameri- see Gould 2000) since natural selection drives out can Political Science Review 79 (December): 1079–93. Crelia, R. A., and A. Tesser. 1996. “Attitude Heritability and At- variation and makes adaptive traits ever more com- titude Reinforcement.” Personality and Individual Differences 21 mon. In this organism-based interpretation, whichever (November): 803–8. orientation—–absolutism or contextualism—–is evolu- Easton, David, and Jack Dennis. 1969. Children in the Political Sys- tionarily superior should soon come to numerically tem.NewYork:McGraw–Hill. dominate the other. This is possible but unlikely. An Eaves, L. J., H. J. Eysenck, and N. G. Martin. 1989. Genes, Culture, and Personality: An Empirical Approach. San Diego: Academic alternative group-based interpretation sees variation Press. itself as adaptive (see Alford and Hibbing 2004 and Eaves, L. J., N. G. Martin, and A. C. Heath. 1990. “Religious Affilia- Sober and Wilson 1998). The benefits of genetic vari- tion in Twins and Their Parents.” Behavior Genetics 20 (January): ation are most easily observed in the ability of dif- 1–22. Eysenck, H. J. 1954. The Psychology of Politics.London:Routledge. ferential immune systems to prevent a group of or- Fiorina, Morris P. 1981. Retrospective Voting in American National ganisms from being completely wiped out by a single Elections. New Haven, CT: Yale University Press. pathogen, but it is easy to imagine how sociopolitical Fiorina, Morris P. 2005. Culture War? The Myth of a Polarized variation could also create more viable groups. In fact, America.NewYork:PearsonLongman. computer simulations give support to the hypothesis Gould, Stephen Jay. 2000. “More Things in Heaven and Earth.” In Alas, Poor Darwin,ed.HillaryRoseandStevenRose.NewYork: that divergent individual-level social behaviors, such as Harmony Books. cooperation and defection, are beneficial at the group Greenstein, Fred I. 1960. “The Benevolent Leader: Children’s Im- level (Hammond 2000). As loathe as contextualists and ages of Political Authority.” American Political Science Review 54 absolutists are to admit it, the presence of the other (December): 934–43. Hammond, Ross. 2000. “Endogenous Transition Dynamics in Cor- orientation may make a society stronger. ruption: An Agent-Based Computer Model.” Brookings Institu- tion. Typescript. Hibbing, John R., and John R. Alford. 2004. “Accepting Authorita- tive Decisions: Humans as Wary Cooperators.” American Journal of Political Science 48 (January): 62–76. REFERENCES Hibbing, John R., and Elizabeth Theiss-Morse. 2002. Stealth Democ- racy: Americans’ Beliefs about How Government Should Work. Adorno, Theodore W.,Else Frenkel-Brunswik, Daniel Levinson, and New York: Cambridge University Press. R. N. Sanford. 1950. The Authoritarian Personality.NewYork: Jennings, M. Kent., and Richard G. Niemi. 1968. “The Transmis- Harper. sion of Political Values from Parent to Child.” American Political Alford, John R., and John R. Hibbing. 2004. “The Origin of Politics: Science Review 62 (March): 169–83. An Evolutionary Theory of Political Behavior.” Perspectives on Jennings, M. Kent, and Richard G. Niemi. 1991. “Issues and Inheri- Politics 2(December):707–23. tance in the Formation of Party Identification.” American Journal Bargh, J. A., and T. L. Chartrand. 1999. “The Unbearable Automatic- of Political Science 35 (November): 970–88. ity of Being.” American Psychologist 54 (July): 462–79. Kosack, Steven T., and Mark Groudine. 2004. “Gene Order and Bouchard, T. J., Jr. 1998. “Genetic and Environmental Influences on Dynamic Domains.” Science 22 (October): 644–47. Intelligence and Special Mental Abilities.” Human Biology 70 (2): Lake, Robert I. E., Lindon J. Eaves, Hermine H. M. Maes, Andrew C. 257–59. Heath, and Nicholas G. Martin. 2000. “Further Evidence Against Bouchard, T. J., Jr., and J. C. Loehlin. 2001. “Genes, Personality, and the Environmental Transmission of Individual Differences in Neu- Evolution.” Behavior Genetics 31 (January): 23–73. roticism from a Collaborative Study of 45,850 Twins and Relatives Bouchard, T. J., Jr., and Matt McGue. 2003. “Genetic and Environ- on Two Continents.” Behavior Genetics 30 (May): 223–33. mental Influences on Human Psychological Differences.” Journal Laswell, Harold. 1930. Psychopathology and Politics.Chicago:Uni- of Neurobiology 54 (1): 4–45. versity of Chicago Press.

166 American Political Science Review Vol. 99, No. 2

Lodge, Milton, Kathleen McGraw, and Pat Stroh. 1989. “An Poole, Keith, and Howard Rosenthal. 1997. Congress: A Political– Impression-Driven Model of Candidate Formation.” American Economic History of Roll Call Voting. New York: Oxford Univer- Political Science Review 83 (June): 399–420. sity Press. Lykken, D. T. 1998. “The Genetics of Genius.” In Genius and the Prescott, Carol A., Ronald C. Johnson, and John J. McArdle. 1999. Mind: Studies of Creativity and Temperament in the Historical “Chorion Type as a Possible Influence on the Results and Inter- Record,ed.A.Steptoe.Oxford:OxfordUniversityPress. pretation of Twin Study Data.” Twin Research 2(December):244– Lykken, David T. 1999. Happiness: What Studies on Twins Show Us 49. about Nature, Nurture, and the Happiness Set Point. New York: Rhee, S. H., and I. D. Waldman. 2002. “Genetic and Environmental Golden Books. Influences on Antisocial Behavior: A Meta-analysis of Twin and Maes, H. H., M. C. Neale, N. G. Martin, A. C. Heath, and L. J. Eaves. Adoption Studies.” Psychological Bulletin 128 (May): 490–529. 1999. “Religious Attendance and Frequency of Alcohol Use.” Twin Ridley, Matt. 2003. Nature Via Nurture.NewYork:HarperCollins. Research 2 (June): 169–79. Rushton, J. Philipe, Christine H. Littlefield, and Charles J. Lumsden. Marcus, Gary. 2004. The Birth of the Mind.NewYork:BasicBooks. 1986. “Gene-Culture Coevolution of Complex Social Behavior: Marcus, George E. 2002. The Sentimental Citizen: Emotion in Demo- Human Altruism and Mate Choice.” Proceedings of the National cratic Politics.UniversityPark,PA:PennStateUniversityPress. Academy of Sciences 1 (October): 7340–43. Marcus, George E., John L. Sullivan, Elizabeth Theiss-Morse, and Sacerdote, Bruce. 2004. “What Happens When We Randomly Assign Sandra L. Wood. 1995. With Malice Toward Some. New York: Children to Families?” National Bureau of Economic Research Cambridge University Press. Working Paper. Typescript. Marcus, George E., W. Russell Neuman, and Michael MacKuen. Scarr S., and R. Weinberg. 1981. “The Transmission of Author- 2000. Affective Intelligence and Political Judgment. Chicago: Uni- itarianism in Families: Genetic Resemblance in Social-political versity of Chicago Press. Attitudes.” In Race, Social Class, and Individual Differences, ed. Martin, N. G., L. J. Eaves, A. C. Heath, R. Jardine, L. M. Feingold, S. Scarr. Hillsdale, NJ: Erlbaum. and H. J. Eysenck. 1986. “Transmission of Social Attitudes.” Pro- Searing, Donald, D. J. J. Schwartz, and A. E. Lind. 1973. “The ceedings of the National Academy of Sciences 15 (June): 4364–68. Structuring Principle: Political Socialization and Belief Systems.” Masters, Roger D. 1993. Beyond Relativism: Science and Human American Political Science Review 67 (June): 415–32. Values. Hanover, NH: University Press of New England. Sears, David O. 1989. “Whither Political Socialization Research? McCloskey, Herbert. 1958. “Conservatism and Personality.” TheQuestionofPersistence.”In Political Socialization, Citizenship American Political Science Review 52 (March): 27–45. Education, and Democracy,ed.O.Ichilov.NewYork:Teachers McDermott, Rose. 2004. “The Feeling of Rationality: The Meaning College Press. of Neuroscientific Advances for Political Science.” Perspectives on Sober, Elliott, and David Sloan Wilson. 1998. Unto Others: The Evo- Politics 2(December):691–707. lution and Psychology of Unselfish Behavior. Cambridge, MA: Merelman, Richard M. 1986. “Revitalizing Political Socialization.” Harvard University Press. In Political Psychology, San Francisco: Jossey–Bass. ed. Richard Stouffer, S. 1955. Communism, Conformity, and Civil Liberties.New Herrmann. York: Doubleday. Monroe, Kristen Renwick. 2004. The Hand of Compassion. Tedin, Kent L. 1974. “The Influence of Parents on the Political Princeton, NJ: Princeton University Press. Attitudes of Adolescents.” American Political Science Review 68 Orbell, John, Tomonori Morikawa, Jason Hartwig, James Hanley, (November): 1579–92. and Nicholas Allen. 2004. “Machiavellian Intelligence as a Basis Tesser, A. 1993. “The Importance of Heritability in Psychologi- for the Evolution of Cooperative Dispositions.” American Political cal Research: The Case of Attitudes.” Psychological Review 100 Science Review 98 (March): 1–17. (January): 129–42. Page, Benjamin L., and Calvin Jones. 1979. “Reciprocal Effects of Truett, K. R., L. J. Eaves, E. E. Walters, A. C. Heath, Policy Preferences, Party Loyalties, and the Vote.” American Po- J. K. Hewitt, J. M. Meyer, J. Silberg, M. C. Neale, N. G. Martin, and litical Science Review 73 (December): 1071–89. K. S. Kendler. 1994. “AModel for Analysis of Family Resemblance Peterson, Steven A. 1983. “Biology and Political Socialization: A in Extended Kinships of Twins. Behavior Genetics 24 (January): Cognitive Developmental Link?” Political Psychology 4 (2): 265– 35–49. 88. Wegner, Daniel M. 2002. The Illusion of Conscious Will. Cambridge, Plomin, R. 1990. “The Role of Inheritance in Behavior.” Science 13 MA: MIT Press. (April): 183–248. Wiggins, J., and P. Trapnell. 1997. “Personality Structure: The Re- Plomin, R., D. W. Fulker, R. Corley, and J. C. DeFries. 1997. “Nature, turn of the Big Five.” In R. Hogan, J. Johnson, and S. Biggs, ed. Nurture, and Cognitive Development from 1 to 16 years: A Parent- Handbook of Personality Psychology. San Diego: Academic Press. Offspring Adoption Study.” Psychological Science 8 (November): Zaller, John R. 1992. The Nature and Origins of Mass Opinion.New 442–47. York: Cambridge University Press. Plomin, R., R. Corley, A. Caspi, D. W. Fulker, and J. DeFries. 1998. Zhang, Xiaodong, Paul R. Gainetdinov, Jean-Martin Beaulieu, “Adoption Results for Self-Reported Personality: Evidence for Tatyana D. Sotnikova, Lauranell H. Burch, Redford B. Williams, Nonadditive Genetic Effects?” Journal of Personality and Social David A. Schwartz, K. Ranga, R. Krishnan, and Marc G. Caron. Psychology 75 (July): 211–18. 2005. “Loss of Function Mutation in Tryptophan Hydroxlylase-2 Plomin, R., J. C. DeFries, G. E. McClearn, and P. McGuffin. 2001. Identified in Unipolar Major Depression.” Neuron 6(January): Behavioral Genetics. 4th ed. New York: Worth. 11–16.

167 American Political Science Review Vol. 102, No. 2 May 2008 doi:10.1017/S0003055408080209 Genetic Variation in Political Participation JAMES H. FOWLER University of California, San Diego LAURA A. BAKER University of Southern California CHRISTOPHER T. DAWES University of California, San Diego he decision to vote has puzzled scholars for decades. Theoretical models predict little or no varia- tion in participation in large population elections and empirical models have typically accounted T for only a relatively small portion of individual-level variance in turnout behavior. However, these models have not considered the hypothesis that part of the variation in voting behavior can be attributed to genetic effects. Matching public voter turnout records in Los Angeles to a twin registry, we study the heritability of political behavior in monozygotic and dizygotic twins. The results show that a significant proportion of the variation in voting turnout can be accounted for by genes. We also replicate these results with data from the National Longitudinal Study of Adolescent Health and show that they extend to a broad class of acts of political participation. These are the first findings to suggest that humans exhibit genetic variation in their tendency to participate in political activities.

hy do people vote? The classic paradox viduals are self-interested and fully optimizing in their of turnout has puzzled theorists for years behavior show that the equilibrium amount of voter W(Aldrich 1993; Downs 1957; Feddersen and turnout approaches zero as the population becomes Sandroni 2006; Riker and Ordeshook 1968). When one large (Palfrey and Rosenthal 1985). Yet in spite of this person votes, everyone with the same preferences ben- theoretical result, millions of people do vote, suggest- efits from the increased likelihood that their preferred ing that something other than self-interest or optimiz- outcome will result. Yet those who do vote must bear ing behavior drives their decision (Bendor, Diermeier, the cost of time and effort required to learn about elec- and Ting 2003; Feddersen and Sandroni 2006; Fowler tion alternatives and go to the polls. In large popula- 2006b). In addition, the fact that millions of people tions, the probability that a single vote will change the abstain suggests that there may be inherent variation outcome of an election is miniscule (Gelman, King, in the human tendency to participate in politics. and Boscardin 1998), meaning that even very small Empirical models of turnout and political partici- costs to the individual typically outweigh the expected pation have tried to explain this variation using nu- benefits he or she would receive from voting. As a merous covariates inspired by a vast literature (Plutzer result, classic game theoretic models that assume indi- 2002; Timpone 1998; Verba, Schlozman, and Brady 1995), including demographic factors like age (Strate et al. 1989), gender (Schlozman et al. 1995), race James H. Fowler is Professor, Political Science Department, University of California, San Diego, 9500 Gilman Drive 0521, (Verba, Schlozman, and Brady 1993), marital status La Jolla, CA 92093-0521. ([email protected] or by Web at (Stoker and Jennings 1995), education (Leighley and http://jhfowler.ucsd.edu.) Nagler 1992a), income (Leighley and Nagler 1992b), Laura A. Baker is Professor, Psychology Department, University occupational prestige (Nie, Powell, and Prewitt 1969a; of Southern California, 3620 South McClintock Ave., Los Angeles, Nie, Powell, and Prewitt 1969b), and home owner- CA 90089-1061. Christopher T. Dawes is Professor, Political Science Department, ship (Highton and Wolfinger 2001); attitudinal and be- University of California, San Diego, 9500 Gilman Drive 0521, La havioral factors like interest in the campaign (Verba, Jolla, CA 92093-0521. Schlozman, and Brady 1995), access to political in- We thank T. K. Ahn, John Aldrich, Robert Bates, Ethan Bueno formation (DiMaggio, Hargittai, and Neuman 2001), de Mesquita, Nicholas Christakis, Gary Cox, Eric Dickson, Jeff general political knowledge (Galston 2001), strength Gill, Don Green, Zoli Hajnal, Ben Highton, Robert Huckfeldt, Kosuke Imai, Gary Jacobson, Cindy Kam, Sam Kernell, Gary of partisanship (Huckfeldt and Sprague 1992), feelings King, Thad Kousser, Michael Laver, Arend Lijphart, Peter Loewen, of civic duty (Blais and Young 1999), internal and exter- Gerry Mackie, Ann Pearson, Maggie Penn, Mark Pletcher, Eric nal efficacy (Finkel 1985), political trust (Hetherington Plutzer, Sam Popkin, Pete Richerson, Brian Sala, Ethan Scheiner, 1999), church attendance (Cassel 1999), personal skill Darren Schreiber, John Scott, Jas Sekhon, David Singer, Oleg Smirnov, Matt Stephenson, John Zaller, Langche Zeng, and Alan acquisition (Brady, Verba, and Schlozman 1995), hu- Zuckerman for helpful comments. We also thank participants in manitarianism (Jankowski 2007), altruism (Fowler seminars at Loyola University of Chicago, the Harris School at the 2006a), and patience (Fowler and Kam 2006); social University of Chicago, Stanford University, University of California- factors like interpersonal communication (McLeod, Irvine, Vanderbilt, Univeristy of Illinois Urbana-Champaign, Uni- Scheufele, and Moy 1999), social identification (Fowler versity of Minnesota, University of Nebraska, University of South- ern California, Washington University at St. Louis, and at panels and Kam 2007), group consciousness (Miller, Gurin, for the 2007 Behavior Genetics Association Annual Meeting, the and Gurin 1981), socialization (Cho 1999), the status 2006 Midwest Political Science Association Annual Meeting and of neighbors (Huckfeldt 1979), political disagreement the 2007 American Political Science Association Annual Meeting. (Mutz 2002), and social capital (Lake and Huckfeldt Funding for this research was provided by the Institute of Gov- ernment Affairs at the University of California, Davis. The contact 1998); and institutional factors (Jackman and Miller author can be reached by email at [email protected] or by web at 1995) like closeness of the election (Shachar and http://jhfowler.ucsd.edu. Nalebuff 1999), contact from political organizations

233 Genetic Variation in Political Participation May 2008

(Wielhouwer and Lockerbie 1994), campaigns something of an injustice. The problem is that while we can (Ansolabehere et al. 1994), civic education (Somit examine environmental variables directly, we can usually et al. 1958), polling locations (Gimpel and Schuknecht only infer genetic effects, and so our natural tendency is to 2003), and barriers to registration (Rosenstone and slight the latter perspective. In short, our procedures, fol- Wolfinger 1978). lowing the line of least methodological resistance, impinge heavily upon our theoretical perspectives.” (1044) Yet in spite of this everything-but-the-kitchen-sink approach, these models usually fit poorly to the data In spite of Merelman’s exhortation, genetic studies (Matsusaka and Palda 1999). For example, one promi- of participation were not forthcoming. Scholars con- nent model includes 32 variables but accounts for only tinued to focus on personality factors underlying par- 31% of the variance in turnout (Plutzer 2002). More- ticipation like efficacy (Finkel 1985) and self-esteem over, the theories underlying these empirical models (Sears 1987) without mentioning the fact that these fac- typically ignore genetic or biological sources of varia- tors may themselves be heritable. A few scholars have tion. Although political scientists are unlikely to op- consistently argued on general principle that genes pose the idea that biology plays a role in political must play a role in political behaviors like participa- participation, in print we hardly ever include this as a tion (Carmen 2004; Masters 1990; Somit and Peterson possibility. For example, the large literature on the role 1998) but they have not empirically tested their genetic of parents in voter turnout nearly always suggests that hypotheses. As a result, the current state of scientific the link between parent and child is the result of the knowledge on the heritability of political involvement transmission of norms rather than the transmission of is limited. genes (Plutzer 2002). As a result, our best work on the In this article we conduct three tests of the hypothesis subject frequently leaves the impression that political that part of the variation in political participation can participation is determined exclusively by environmen- be attributed to genetic factors. The results of all three tal factors. of our tests suggest that individual genetic differences Recently, social scientists have learned that vari- make up a large and significant portion of the variation ation in basic political attitudes like liberalism and in political participation, even taking socialization and conservatism can be attributed to both genes and other environmental factors into account. Our results environment (Martin, et al. 1986; Alford, Funk, and show participation is heritable and suggest that political Hibbing 2005; Eaves and Hatemi 2008; Hannagan science as a discipline should be thinking more about and Hatemi 2008), even as early as adolescence biological sources of variation in political behavior. In (Abrahamson, Baker, & Caspi, 2002). While the choice particular, we argue that these results open the door of a particular candidate or party does not appear to to an untapped realm of causal theories and empirical be heritable (Alford, Funk, and Hibbing 2005; Hatemi tests that will help us to improve our understanding of et al. 2007), it remains an open question whether or not one of the most basic acts of citizenship and democratic the act of voting (or, more broadly, any act of political government. participation) is heritable. Given the difficulty scholars have had in explaining participation solely based on environmental accounts, we hypothesize that a signifi- TWIN STUDIES cant portion of the variation in voter turnout behavior can be attributed to genetic factors. In order to estimate the heritability of voting behavior, Although we are not the first to suggest a link be- we study the turnout patterns of (identical) monozy- tween genes and political participation, this study is the gotic (MZ) twins who were conceived from a single first attempt to test the idea empirically. Some early fertilized egg and (non-identical) dizygotic (DZ) twins work studied the importance of personality in political who were conceived from two separate eggs. MZ twins participation, but this literature focused exclusively on share 100% of their genes, while DZ twins share only environmental factors, asserting that people who are 50% on average. Thus, if voting behavior is heritable, reared in similar ways will have similar personalities MZ twins should exhibit more concordance (both twins (Lane 1959; Levinson 1958) or that the role of person- vote or both twins abstain) than DZ twins. Moreover, if ality was to mediate social influences on participation we assume that MZ twins and DZ twins share compa- (Krause et al. 1970). Other early work explored the rable environments (more on this assumption below), importance of adolescent socialization in the develop- then we can use these concordances to estimate explic- ment of political behaviors, but these scholars never itly the proportion of the overall variance attributed to considered the genetic link between parent and child. genetic, shared environmental, and unshared environ- Merelman (1971) addressed this shortcoming, arguing mental factors. Very few differences have been found that both genes and environment are probably impor- between twins and non-twins (Kendler et al. 1995), tant. In fact, he explicitly recommended the use of therefore we expect the results for twins to be general- twin studies to investigate the heritability of political izable to a non-twin population. participation. In his view, the main reason heritability The twin study design has been shown to be an ex- had been ignored was due to the difficulty in statistical tremely powerful tool for identifying the relative de- design and testing: gree to which genetic and environmental factors in- fluence an observed outcome (Evans, Gillespie, and “[T]his natural tendency to examine one environmental Martin 2002; Neale and Cardon 1992). The basic twin factor after another ad infinitum does a genetic explanation model assumes that the variance in observed behavior

234 American Political Science Review Vol. 102, No. 2 can be partitioned into additive genetic factors (A), and THE COMPARABLE ENVIRONMENTS environmental factors which are shared or common to ASSUMPTION co-twins (C), and unshared environmental (E). This is the so-called ACE model. The role of genotype and Some scholars have objected to the assumption that environment are not measured directly but their influ- MZ and DZ environments are comparable, arguing ence is inferred through their effects on the covariances that the identical nature of MZ twins cause them to between twin siblings (Neale and Cardon 1992). No be more strongly affiliated and more influenced by observed covariates are needed in the model because one another than their non-identical DZ counterparts. the degree to which they contribute to variance is a If so, then greater concordance in MZ twins might part of one of three variance components (A, C, and merely reflect the fact that their shared environments E). More formally, these components are derived from cause them to become more similar than DZ twins. known relationships between three observed statistics However, studies of twins raised together have been (Evans, Gillespie, and Martin 2002): validated by studies of twins reared apart (Bouchard 1998), suggesting that the shared environment does not exert enhanced influence on MZ twins. Moreover, σ2 σ2 σ2 σ2 P = A + C + E personality and cognitive differences between MZ and COV σ2 σ2 DZ twins persist even among twins whose zygosity has MZ = A + c been miscategorized by their parents (Bouchard and 2 2 McGue 2003), indicating that being mistakenly treated COVDZ 1/2σA σc , = + as an identical twin by one’s parents is not sufficient to where σ2 is the observed phenotypic variance (the generate the difference in concordance. And, although P MZ twins are sometimes in more frequent contact with same for MZ and DZ twins), COVMZ and COVDZ are the observed covariances between MZ and DZ each other than DZ twins, it appears that twin sim- 2 2 2 ilarity (e.g., in attitudes and personality) may cause co-twins, and σA, σC, σE are the variance components for genes, common environment, and unshared envi- greater contact rather than vice versa (Posner, Baker, ronment, respectively. These relationships yield three and Martin 1996). Finally, contrary to the expectation equations and three unknowns, so it is possible to in- that the influence of the unshared environment would fer the unobserved portions of variance attributable to tend to decrease concordance over time once twins each factor. reach adulthood, MZ twins living apart tend to become Since the variance components are not directly ob- more similar with age (Bouchard and McGue 2003). servable, the ACE model’s assumption of additivity cannot be tested and more complicated relationships TURNOUT IN THE SOUTHERN CALIFORNIA are possible. For example, it is possible that genes in- TWIN REGISTRY teract with the environment (GxE) or with other genes (GxG) to yield variation in behavior, or at a higher To assess the heritability of turnout behavior, we ob- level phenotypes interact with the environment (PxE) tained electronic voter registration records for 3.8 mil- (Turkheimer and Waldron 2000). We limit our analysis lion voters from Los Angeles County with complete to the ACE model but point out that if a strong effect vote histories for eight elections (three primary, two for genes is found in the additive model, then genes statewide, and three general) from 2000 to 2005 and are also likely to play a role in more complex specifi- matched them to the Southern California Twin Reg- cations. istry (Baker et al. 2006), a list of MZ and DZ twins Finally, it is important to clarify the difference be- who live in the Los Angeles area. A principal advan- tween the common environment (C) and the unshared tage of this approach is the use of field evidence based environment (E) in the twin model. Common environ- on third-party observations of actual voter behavior ment includes the family environment in which both rather than self-reports. This type of data is rarely used twins were raised, as well as any other factor to which in twin studies and is an especially important source both twins were equally exposed. In contrast, the un- for evaluating political participation since a significant shared environment includes idiosyncratic influences number of individuals who did not vote typically report that are experienced individually. It is possible to have that they did (Karp and Brockington 2005). unshared environmental exposure as a child (twins may About 30% of the adult population in Los Angeles have different friends with different political beliefs) County is not registered to vote, so we cannot include and to have shared environments as an adult (twins them in our sample. We cannot merely assume that all may see the same election results). Thus, the distinc- unregistered twins chose not to vote—–for example, it is tion between common and unshared environment does possible that they died or moved out of the county and not correspond directly to family—–nonfamily or adult- registered elsewhere. However, focusing on registered child differences in factors that influence a given be- individuals allows us to exclude those who might gen- havior. Moreover, there may be a similarity in the ob- erate false concordance because they are ineligible to jective environment but twins may have idiosyncratic vote due to foreign citizenship status—–this is a partic- experiences that influence their effective environment ular concern in Los Angeles County where 22% of the that create an unshared rather than a common en- total population are foreign citizens (2000 U.S. Census). vironmental influence on variation in the phenotype It also allows us to avoid false concordance generated (Turkheimer and Waldron 2000). by individuals with cognitive or literacy deficits who are

235 Genetic Variation in Political Participation May 2008 not capable of voting since these individuals probably than DZ twins to live at the same address (p 0.69) do not register. or in the same postal code (p 0.84). Thus,= greater Twin registry and voter registration records were concordance in MZ twins is probably= not due to higher matched by surname, first name, birthdate, place of frequency of contact. birth, and zip code. Full matches were automatically It is important to note that we do not need to com- included in our data. Partial matches on three or more pare similarities between co-twins to test the compa- of these attributes were manually checked and included rable environments assumption. For example, if we in the data if the failure to match fully was determined show that MZ twins are more similar than DZ twins to be the result of a typographical error. We restricted for income, then it means income attainment might our search to same-sex twin pairs because MZ twins be heritable but it has no bearing on whether or not are always same sex and DZ twins are not. Including MZ and DZ twins come from essentially similar envi- opposite-sex twin pairs would complicate the analysis ronments. Conversely, we might find that MZ and DZ because we would have to assess whether differences in co-twins are equally similar on income, but this would concordance between MZ and DZ twins are the result not imply their environments were the same. The fact of closer social affiliation between same-sex pairs. that MZ and DZ twins are drawn from households that Out of 878 same-sex twins (535 MZ, 343 DZ) on the have similarly distributed income suggests that there registry who live in Los Angeles County, this procedure is nothing unique about MZ twins that could cause a yielded vote histories for 396 twins—–168 MZ twins and spurious difference in the similarity of turnout via a 102 DZ twins in matched pairs, and 79 MZ and 47 difference in income. For example, if MZ twins were DZ “singletons” where we found one twin in the pair much richer than DZ twins, they might be more similar but not the other.1 A Mann Whitney U test suggests since wealthier households vote more. Since they are that the difference in the success rate for matching not richer, we can reject this possible explanation for between MZ twins (48.6%) and DZ twins (43.4%) was why we find a difference in the similarity of turnout not significant (p 0.14). between MZ and DZ twins. Although we did= not have access to information One might worry about the house values in Table 1, about the twins’ socioeconomic status for the entire because they suggest that the subject pool is drawn sample, we were able to use their addresses to look up from the more affluent part of the population (the estimated home values and square feet on the home average single family house at this time in Los An- appraisal web site zillow.com.2 We also examined data geles county sold for about $600,000). However, this from previous studies in which subsets of the matched would only be important if it had a systematic effect on twins had participated through the Southern California turnout. Table 2 shows that DZ twins and MZ twins do Twin Registry. 3 Although not available for the entire not exhibit systematically different rates of turnout in sample, these prior data are used to evaluate possi- the different elections. Table 2 also shows that turnout ble differences between MZ and DZ pairs that might rates for all twins were somewhat higher than those for explain their voting behavior. the population, but this should not bias estimates of the To test the comparable environments assumption for magnitude of the difference in concordance between our sample, we performed a series of tests on the mean MZ and DZ twins. This is because the variances are difference between MZ and DZ twins for a number of not systematically different from the population (the variables (see Tables 1 and 2). High p-values in Mann twin variance is higher than the population variance in Whitney U tests suggest that differences between two elections and lower in three). If the twin variance types of twins are not significant for rates of turnout were much lower than the population variance, this (p 0.79), Democratic Party membership (p 0.84), would compress the difference in the MZ and DZ twin Republican= Party membership (p 0.83), third= party concordances, causing us to underestimate heritabil- membership (p 0.88), age (p =0.25), house value ity.4 Conversely, if the twin sample variance were much (p 0.49), house= square footage= (p 0.86), and lot higher than the population variance we would overes- square= footage (p 0.15). Furthermore,= t-tests of data timate the effect of heritability. The lack of a systematic obtained from previous= studies of subsets of these twins difference in the sample and population variances sug- revealed no differences in their education level (p gests that mean differences will not generate bias in the 0.72) or personality, including extraversion (p 0.38)= estimates. and neuroticism (p 0.92). Thus, the similarity= of the MZ and DZ twin samples= suggests that differences in concordance cannot be explained by mean differences INITIAL RESULTS AND THE BAYESIAN in political participation, political affiliation, persona- ACE MODEL lity, education, or other socioeconomic factors. We also note that in our sample MZ twins are not more likely Figure 1 shows two two-dimensional density plots of the number of elections in which each twin chose to vote (MZ twins on the left, DZ twins on the right). The 1 The statistical power of twin studies is maximized when DZ twins exceed the number of MZ twins by a factor of 3 or 4 to 1 so not only is our total number of twins small but the ratio is not optimal. 4 For example, in the extreme case where all twins are perfectly However, this affects efficiency and not bias. concordant and the turnout rate 100%, the variances shrink to 0, 2 Thanks to John Zaller for this suggestion. the concordances for both MZ and= DZ twins grow to 1, and the dif- 3 See Baker et al. (2006) for a description of studies conducted using ference between the concordances would also shrink to 0, suggesting the Southern California Twin data. 0heritability.

236 American Political Science Review Vol. 102, No. 2

TABLE 1. Summary Statistics, by Zygosity, Los Angeles Sample MZ Twins DZ Twins Difference of Standard Standard Means Test Mean Error Mean Error p-value Voter file data Turnout Rate, All Elections 0.57 0.03 0.58 0.03 0.79 Democrat 0.51 0.05 0.52 0.05 0.84 Republican 0.24 0.04 0.25 0.05 0.83 Third Party 0.05 0.02 0.05 0.02 0.88 Age 36.8 2.5 33.6 2.8 0.25 Same Address 0.47 0.07 0.52 0.08 0.69 Same Postal Code 0.54 0.07 0.64 0.07 0.84 Zillow.com data Was House in Zillow? 0.71 0.04 0.71 0.05 0.89 House Value 821,729 40,577 784,421 49,412 0.49 House Square Feet 2148 111 2106 137 0.86 Lot Square Feet 8062 392 9117 1014 0.15 SCTP data Education Highest Grade 15.48 0.36 15.25 0.55 0.72 Extraversion 0.66 0.04 0.71 0.04 0.38 Neuroticism 0.43 0.03 0.43 0.05 0.92 Note:ThesedatashowthatwecouldfindnosignificantdifferencesintheMZandDZtwinsamples,suggestingthattheyare drawn from comparable environments. Data are derived from three sources: 1) the Los Angeles County voter registration and vote history files for matched twins, 2) housing characteristics for 71% of the matched twins found on zillow.com on October 25, 2006; and 3) education and personality information for 15% of the matched twins (this subsample is limited to those who participated in previous studies in which education and personality questions were asked). We utilized Mann–Whitney U tests to analyze differences in means in the voter registration and zillow data and t tests for the SCTP data.

TABLE 2. Comparison of Mean Turnout and Variance in Turnout in Twin Sample and General Population in Los Angeles County, by Election Mar Nov Mar Nov Oct Mar Nov Nov 2000 2000 2002 2002 2003 2004 2004 2005 Primary General Primary General Statewide Primary General Statewide Alltwins 0.54 0.76 0.36 0.57 0.68 0.44 0.84 0.46 (N 396) (0.25) (0.18) (0.23) (0.25) (0.22) (0.25) (0.13) (0.25) = MZ twins 0.55 0.75 0.36 0.58 0.71 0.42 0.86 0.49 (N 247) (0.25) (0.19) (0.23) (0.24) (0.21) (0.24) (0.12) (0.25) = DZ twins 0.53 0.79 0.37 0.56 0.62 0.46 0.80 0.42 (N 149) (0.25) (0.17) (0.23) (0.25) (0.24) (0.25) (0.16) (0.24) = Population 0.48 0.68 0.26 0.45 0.55 0.38 0.79 0.47 (0.25) (0.22) (0.19) (0.25) (0.25) (0.24) (0.17) (0.25) Note:Variancesareshowninparentheses. color of each square indicates the number of obser- co-twins made different decisions for MZ 1.45, DZ vations at each point, so for example, there is a strong 2.00). = = mode for MZ twins where each twin voted exactly twice In the behavior genetics literature a simple com- (the point 2,2 contains about 7% of the MZ sample). parison of polychoric correlations is frequently used With this representation we lose resolution because it as a first test of the rate of twin concordance in be- is possible for both twins to vote in the same number havior (for a detailed explanation of this method, of elections without voting at the same time (e.g. twin see Alford, Funk, and Hibbing 2005). In our pooled 1 might vote in two primaries and twin 2 might vote in observations, the correlation in turnout was signifi- two general elections). Nonetheless, patterns start to cantly higher (p 0.006) between the MZ twins (0.71) emerge. There appear to be more observations on the than the DZ twins= (0.50).5 Another simple and direct main diagonal for MZ twins, and DZ twins appear to be way to see if zygosity influences co-twin similarity is more likely to have large differences in the frequency DeFries-Fulker regression (DeFries and Fulker 1985). they vote. We can analyze the pattern of voting sta- tistically by examining the number of times each twin pair differs (one votes and one abstains). A simple t 5 Because the concordance in DZ twins is greater than half the con- cordance in MZ twins, the common environment may play a role in test of the absolute difference in co-twin voting behav- voting. As a result it is appropriate to model twin-only data with an ior suggests that MZ twins are significantly more simi- ACE model instead of the alternative ADE model that assumes the lar than DZ twins (p 0.045, mean number of times common environment plays no role. =

237 Genetic Variation in Political Participation May 2008

FIGURE 1. Comparison of Co-Twin Voting Behavior in Los Angeles by Zygosity

In this method, the dependent variable is each twin’s In this model τ is a normally distributed continuous behavior and the independent variables are zygosity, variable that corresponds to the individual’s latent the co-twin’s behavior, and an interaction of the two. propensity to vote, δ is the discriminating power of each If the interaction term is significant, it means that MZ election, and α is the threshold identifying the point twins are statistically more likely to exhibit the same at which the likelihood of voting is greater than ab- characteristics than DZ twins. We use a general esti- staining in each election (also known as the “difficulty mating equation (GEE) to correct for multiple obser- parameter” in item-response models). The parameter vations on the same twin pair and find that the inter- δ is analogous to loadings in a simple factor model, action coefficient is indeed significant (Wald statistic which allow each election to have a different weight 4.38, p 0.036). = in the underlying tendency to vote (Eaves et al. 2005). However,= these measures are only a crude guide In order for this model to be identified we fix the total since they treat every choice as the same and they make variance of the latent trait (τ)toone. no provision for the unique information contained in Next, we assume that the latent tendency to vote is each election. For example, suppose everyone voted in influenced by additive genetic factors, shared environ- the first election but only half voted in the second—–the ment, and unshared environment. These three factors first election would not be very informative about the completely account for the three different kinds of vari- individual tendency to vote since there was no vari- ance that it is possible for us to diagnose in a model of ation, but the second would be very informative. To identical twins. We model this assumption using three take advantage of the differing discriminatory power random effects variables for MZ twins: of each election, we employ a generalized latent vari- τMZ A C E , able model, otherwise known as a two-parameter item ij = i + i + ij response model (Clinton, Jackman, and Rivers 2004). We assume there is a single latent propensity to vote un- where Ai is the family genetic factor, Ci is the family derlying all eight observed turnout decisions.6 We also shared environment factor, and Eij is the individually- assume that both genetic and environmental effects op- experienced unshared environment factor. For DZ erate through a common pathway (Eaves et al. 2005). twins the tendency to vote is modeled using four ran- The model can be specified as a generalized linear dom effects variables: mixed-effects model where subject j is a member of DZ τij A1i A2ij Ci Eij , family i choosing to vote (Tijk 1) or abstain (Tijk = + + + = = 0) in election k. We assume the probability that an where A is the family genetic factor shared by both individual will vote in election k (a binary choice) is 1i twins, A2ij is the individually-inherited genetic factor that is unique to each twin, and C and E are the same Pr(Tijk 1) !(δkτij αk), i ij = = − as for MZ twins. where the ! function that links the latent tendency to It is important to reiterate that there are no observed vote to a probability is a logit: covariates in any of the models. In particular, none of the measured environmental variables we exam- 1 !(x) . ined in Table 1 are included. Everything on the right = 1 exp( x) + − hand side involves latent variables whose effects are estimated solely from the observed participation deci- 6 ACronbachtest(α 0.78) reveals that these eight elections are sions. Adding covariates to the right hand side would reliable measures of a= single scalar latent value for the propensity to not affect the variance decomposition because they vote. would merely reduce the magnitude of the most-closely

238 American Political Science Review Vol. 102, No. 2 related component. For example, suppose that neigh- netic effect for DZ twins, is fixed to be half the variance borhood context influences political participation of A, the family genetic effect for MZ twins, reflecting among twins who live apart as adults. If so, we might the fact that DZ twins on average share half as many include a factor in the model like average neighbor- genes as MZ twins. Moreover, DZ twins are also influ- hood income. If we inserted this variable as an additive enced by individually-specific genes A2 that are drawn factor that directly influences the individual’s turnout from the same distribution as the shared genes since propensity, it might reduce the magnitude of the un- on average half their genes are shared and half are not. shared environmental variance since it would partially These assumptions about the genetic variance help to account for some of it. However, we would have to add distinguish shared genes from the shared environment the variance explained by mean neighborhood income variable C that is assumed to have the same variance to the unshared environmental variance to estimate its for both MZ and DZ twin families, and the residual total influence. Thus, in essence, the latent factors indi- unshared environment variable Efrom which a unique cate the total additive influence of all possible genetic draw is made for each individual. and environmental variables that could be included the If we tried to estimate all three components of vari- model. ance simultaneously the model would not be identified, Traditionally, the typical approach to estimate the so we fix the variance of the unshared environment 2 2 2 components of variance has been structural equation σE 1andthenusetheestimatesofσA and σC to derive modeling (SEM), however Bayesian methods are in- the= proportion of variance generated by each factor. creasingly being viewed as a superior modeling ap- This procedure generates estimates for the influence of 2 2 2 2 2 proach (Burton et al. 1999). For our modeling task heritability h σA/(σA σC σE), common environ- there are two main advantages to using a Bayesian 2 2 = 2 2 + 2 + ment c σC/(σA σC σE), and the unshared envi- model. First, discrete phenotypes (like the dichoto- =2 2 +2 +2 2 ronment e σE/(σA σC σE). Since the underlying mous decision to vote or abstain) present computa- components= are not constrained,+ + the estimated propor- tional challenges for SEM software packages because tions can range anywhere between 0 (the component the likelihoods contain high-dimensional integrals that has no effect on variance) and 1 (the component is cannot be evaluated in closed form and thus must solely responsible for all observed variance). be evaluated numerically (van den Berg, Beem, and In some cases, the estimate for c2 will be close to 0, Boomsma 2006). As a result, scholars have begun to so we can test the hypothesis that the common envi- 7 use Markov Chain Monte Carlo (MCMC) algorithms. ronment matters by dropping it from the ACE model, These algorithms evaluate the integrals using random creating an AE model (alternatively we could drop A draws rather than evaluating them analytically. In par- to create a CE model). If the AE model fits better than ticular, simulation studies suggest that MCMC meth- the ACE model, then it suggests a weak or insignificant ods perform better than SEM for models like ours. For role for the common environment. Procedurally, the example, Kuhnert and Do (2003) show that a Bayesian difference between the ACE and AE model is that binary response model identifies the correct model the random effect for the common environment is not more often than a comparable SEM model in cases 2 estimated and σC 0. To compare the fit of ACE and where the simulated heritability is low or medium (both AE models we used= the deviance information criterion performed equally well in cases of high heritability). (DIC), a Bayesian method for model comparison anal- Another advantage of the Bayesian approach is that ogous to the Akaike Information Criterion (AIC) in credible intervals for the variance component esti- maximum likelihood estimation. Models with smaller mates do not rely on large-sample theory that may DIC are considered to have the best out of sample pre- not be appropriate for twin studies with small sample dictive power (Gelman et al. 2004). The DIC penalizes sizes (Chen, Manatunga, and Williams 1998). In an ex- models for deviance (Dbar), which captures model fit, tensive simulation study, Burton et al. (1999) showed and the effective number of parameters (pD), which that a Bayesian binary response model based on a rel- captures model complexity. atively small sample of 250 families yielded variance In our MCMC procedure we use vague prior distri- component point estimates and credible intervals that butions to ensure they do not drive model results. For µ exhibited no significant bias. we use a mean-zero normal distribution with variance Replicating the methods used in this litera- 1,000,000 and for the precision parameters associated ture, we assume that our unobserved random ef- with σ2 and σ2 we use a pareto distribution with shape 8 2 A C fects are normally distributed : A N(0, σA), A1 parameter equal to 1 and location parameter equal 2 2 ∼ 2 ∼ 9 N(0, σA/2), A2 N(0, σA/2), C N(0, σC), and E to 0.01. In addition, we use convergence diagnostics 2 ∼ ∼ ∼ N(0, σE). Notice that the variance of A1,thefamilyge- to be sure we have reached the stationary posterior distribution.10 7 Recent studies have successfully applied Bayesian methods to ge- netic models using binary data (Kuhnert and Do 2003; van den Berg, Beem, and Boomsma 2006), survival analysis (Do et al. 2000), nonlin- 9 We experimented with priors using different distributions. We tried ear developmental change and GxE interaction (Eaves and Erkanli a gamma with shape parameters 0.001, 0.01, and 0.1 and scale para- 2003), item response theory (Eaves et al. 2005), longitudinal models meters 1,000, 100, and 10, respectively. We also tried uniform (Burton et al. 2005), and multivariate models for ordinal data (van priors (0,10), (0,20), and (0,100) on σA and σC but found they had den Berg et al. 2006). essentially no effect on the final estimates. 8 The choice of normal distributions is for convenience—–they im- 10 To ensure that the models converged to what we believe to be ply τ is normally distributed as well since the sum of two normally their target posterior distribution, we began sampling from the joint distributed random variables is also distributed normal. posterior distribution after convergence was established using the

239 Genetic Variation in Political Participation May 2008

How do the assumptions we make in this model compare to assumptions political scientists typically FIGURE 2. Heritability of Voter Turnout in make in their models? Consider a simple logit model Los Angeles of turnout. Like the ACE model presented here, a logit model (1) implicitly assumes that there are in- dependent normal data generating processes that (2) influence a latent variable that is (3) transformed via a logistic function into a probability, which (4) itself is also a latent, unobserved variable. The ACE model is somewhat more complex, but not more so than state- of-the art Bayesian item-response models (Clinton, Jackman, Rivers 2004) which also include assumptions about prior distributions and multilevel latent factors.

RESULTS The results of the ACE model suggest that 53% of the variance in turnout behavior can be accounted for by additive genetic effects (h2). The 95% credible interval (C.I.) for the estimate is (10%, 89%), indicating that we can reject the hypothesis that genes do not con- tribute to variation in turnout. The ACE model also suggests that the environment is important, with the Note:TernaryplotshowstheposteriorBayesiandistributionof 2 shared environment (c )accountingforabout35%of estimated components of total variance in an ACE model of the variance (C.I. 2%, 73%) and the unshared envi- voter turnout among subjects in the Southern California Twin 2 ronment (e2)accountingfor12%(C.I.3%,26%).Fig- Registry (SCTR). Mean heritability (h )isestimatedtobe53%. Colors indicate credible areas calculated by using 10,000 pos- ure 2 shows the 95% credible area of the joint estimates. terior draws to estimate a three-dimensional kernel density. The Notice that the contribution of the common environ- dark areas indicate the highest density regions with the most ment is close to zero. This suggests that an AE model, credible estimates, while the light areas contain 95% of the where the common environment variable is assumed to draws (i.e., the probability that the true coefficients lie outside the colored regions is p 0.05). be zero, may be more appropriate. Indeed, measures of = model fit indicate that an AE model is superior to the ACE model (see Appendix A-1). Although one may be concerned that our analysis lacks power because our the actual distribution. None of the discrepancies are sample of 396 subjects is small, multiple observations statistically significant, indicating reasonable fit for the per individual improve the precision of the estimates, overall model (Gelman et al. 2004). and the credible intervals in the posterior indicate that To test the sensitivity of our method we also em- there is an extremely low probability (p < 0.0001) that ployed a traditional structural equation model (SEM) voting behavior is not heritable. to fit the data, using Mplus to estimate the A, C, To ensure that our model is consistent with the data, and E components of variance in a common factor we use it to generate replicated values of the depen- underlying the individual election turnout variables. dent variable from the predictive distribution for each The Mplus software provides maximum likelihood es- simulated parameter in the model and compare these timates in genetic models for observed categorical vari- replicated values with the observed dependent vari- ables (Prescott 2004). Variance in the common factor was explained primarily by genetic factors (A 67%; able (Gelman et al. 2004). In order to summarize the = discrepancy between the model and data, a relevant S.E. 38%), with non-significant effects of shared en- vironment= (C 27%; S.E. 34%) and non-shared measure must first be chosen. Using this measure, a = = posterior predictive p-value may be calculated to eval- environment (E 7%; S. E. 8%). All alternatives (Bayesian and non-Bayesian)= = suggested that turnout uate the fit of the model to the observed data (Gelman 2 et al. 2004). Specifically, the predictive p-value is the behavior is heritable, with mean h consistently es- proportion of the replicated datasets for which the dis- timated to be greater than 50%. We also explored crepancy measure equals or exceeds its realized value. whether heritability estimates differed by types of elec- Large and systematic differences between the repli- tions (primary vs. general, close vs. not close), however the power for our sample was too low to detect any cated and observed data, resulting in p-values close to 11 zero or one, suggest a poor fitting model. In Table A-2 significant difference. in the Appendix we compare the predicted percent- age of individuals voting in zero to eight elections to 11 We estimated separate election-type-specific heritability parame- ters within the same model. This is equivalent to including a dichoto- Brooks and Gelman (1998) statistic (values of less than 1.1 on each mous interaction term on the A parameter. When we compared the parameter indicate convergence). For the Los Angeles voting models four closest elections to the other four elections, the difference in the “burn-in” period was 500,000 iterations and for the Adolescent heritability was insignificant ( 13%, 95% C.I. 71%, 50%). Simi- Health voting and political participation models it was 1 million iter- larly, the difference in heritability− between the− three primaries and ations. The Los Angeles and Adolescent Health models respectively the three general elections was also insignificant ( 16%, 95% C.I. were thinned by 100 and 200 for the posterior sample. 46%, 70%). + −

240 American Political Science Review Vol. 102, No. 2

One potential objection to our model is that by in- INDEPENDENT REPLICATION IN THE ADD cluding an election-specific fixed effect (the “difficulty” HEALTH STUDY parameter in the model), we automatically remove “in- stitutional variation” from the model (e.g. the procedu- Given the narrow geographic region of our study, we ral differences between primary and general elections decided to conduct an independent replication of the which may influence turnout). To determine the ex- results using data from a nationally representative sam- 12 tent to which including fixed effects for each election ple. The National Longitudinal Study of Adolescent in the model may be artificially deflating the amount Health (Add Health) is a study that, among other top- of variance to be explained, we also generated results ics, explores the causes of health-related behavior of from a model in which the difficulty parameters were adolescents in grades 7 through 12 and their outcomes removed and turnout was purely a function of the latent in young adulthood.13 Three waves of the Add Health propensity to vote and election factor loadings. This study have been completed: Wave I was conducted in robustness check ensures that institutional variation is 1994-1995, Wave II in 1996, and Wave III in 2001–2002. included in total variance. The results of this model In Wave I of the Add Health study, researchers cre- indicate heritability of 51% (C.I. 9%, 89%). ated a genetically informative sample of sibling pairs Another potential objection to our model is in fixing based on a screening of a sample of 90,118 adolescents. the genetic variance in DZ twins to be half the value These pairs include all adolescents that were identified of MZ twins, which is tantamount to assuming that DZ as twin pairs, half-siblings, or unrelated siblings raised twins share exactly 50% of their genes—–in reality, there together. Twins and half biological siblings were sam- is some variance from pair to pair in the amount shared pled with certainty. The Wave I sibling-pairs sample resulting from the small number of recombinations has been found to be similar in demographic compo- on each chromosome that are possible. The empirical sition to the full Add Health sample (Jacobson and distribution has been estimated to be approximately Rowe 1998). Nearly 80% of the sibling-pairs sample normal with a mean of 50% and a variance of 0.13% participants in Wave I also participated in Wave III (Visscher et al. 2006). When we incorporate this distri- (Haberstick et al. 2005) and the demographic char- bution in the Bayesian model instead of assuming an acteristics of the sibling-pairs sample did not change exact figure of 50%, the heritability estimate and con- significantly over the course of the three waves (Hopfer fidence intervals are nearly identical (53%, C.I. 10%, et al. 2005). The total number of twins who participated 89%). in Wave III was 1,082 (442 MZ and 640 DZ), with 806 We reiterate that an important assumption of classi- twins (442 MZ and 364 DZ) in same sex pairs. cal twin studies is that MZ and DZ twins share compa- The Add Health data has been used in a wide va- rable social environments. Therefore, greater similarity riety of twin studies (Harris et al. 2006). As a result, of the phenotype in MZ twins compared to DZ twins there have been several analyses of the comparable indicates the degree to which genes influence the phe- environments assumption for MZ and DZ twins. One notype. If this assumption is violated, it is possible that of these studies claimed to find the environments were the estimated genetic effect is inflated. In our study, vi- not comparable (Horwitz, Videon, and Schmitz 2003), olation of the “equal environments” assumption likely but other scholars have pointed to serious deficiencies would have produced significant differences between in this work (Freese and Powell 2003).14 In consonance MZ and DZ twins in the distribution of turnout, party with most studies of the Add Health twin data, we affiliation, education, and socioeconomic status. Be- conduct our own assessment of equal environments cause the distributions of these variables do not ap- in Table 3 and find no significant differences in MZ pear to differ for the two types of twins, any possi- and DZ environments for several socioeconomic and ble overestimation of the genetic effect is likely to be politically relevant variables. small. In Wave III of the Add Health study, respondents Another factor to consider is assortative mating. One provided information about their recent political ac- assumption of the ACE model is that the distribution tivity that will permit analysis of both voting and other of parent genotypes is independent. If political partici- kinds of participation. This includes one question about pation is heritable and if people who participate in pol- voting: “Did you vote in the most recent presidential itics tend to have children with other politically-active individuals, then this will increase the concordance in participatory behavior in their children. However, the 12 Women make up 49% of the study’s participants, Hispanics 12.2%, effect of this assortment is to increase the degree of Blacks 16.0%, Asians 3.3%, and Native Americans 2.2%. Partici- concordance in offspring, making it harder to detect pants in Add Health also represented all regions of the country: the differences in MZ and DZ twins. For example, per- Northeast made up 17% of the sample, the South 27%, the Midwest fect assortment and perfect genetic transmission would 19%, and the West 17%. 13 yield a concordance of 1.0 for both MZ and DZ twins, Detailed information about the Add Health study can be found and this lack of difference in the concordance would at www.cpc.unc.edu/projects/addhealth. 14 For example, Horwitz et al. (2003) showed that including observed suggest that heritability plays no role. As a result, the social variables in a twin model causes the p-value on the genetic com- more assortative mating there is, the more it biases ponent for males trying alcohol to change from being just below 0.05 downward the estimate of heritability. Thus, if the pos- to just above it. Freese and Powell (2003) note that this is unsurprising sibility exists that people choose mates based in part on since adding variables to a regression can have a substantial effect on efficiency.Even worse, they point out that Horwitz et al. (2003) do not their disposition to participate in politics, then the ACE acknowledge that their own fit statistics indicate the models with and model estimates will be conservative—–heritability will without social variables are statistically indistinguishable, suggesting actually be underestimated. that the model with additional variables should be rejected!

241 Genetic Variation in Political Participation May 2008

TABLE 3. Summary Statistics, by Zygosity, Add Health Sample MZ Twins DZ Twins Difference of Standard Standard Means Test Mean Error Mean Error p-value Add Health data TurnoutRate 0.45 0.56 0.48 0.50 0.40 Democrat 0.55 0.50 0.58 0.49 0.50 Republican 0.41 0.49 0.40 0.49 0.79 Age 22.0 1.61 21.8 1.71 0.20 Same Address 0.37 0.48 0.32 0.47 0.15 Income 28,860 23,734 25,385 17,098 0.25 Education Highest Grade 13.25 1.89 13.36 2.16 0.44 Note:ThesedatashowthatwecouldfindnosignificantdifferencesintheMZandDZtwinsamples,suggestingthatthey are drawn from comparable environments.

election?” It also includes five questions about other (Tij 1) or abstain (Tij 0) in the election. As in the kinds of political participation: “Which of the following Los= Angeles voting model,= the observed phenotypes types of organizations have you been involved with in are dichotomous variables and we assume τ is a con- your volunteer or community service work in the last tinuous variable that maps to the individual’s latent 12 months?” (“political clubs or organizations”) propensity to vote via a logit function. In fact, the “Which of the following things have you done during only difference is that we restrict δ1 1toidentifythe the last 12 months?” (“contributed money to a political model since there is only a single election= for subjects party or candidate”; “contacted a government official in the Add Health data. regarding political or community issues”; “run for a The only difference between the Add Health model public office”; “run for a non-public office”; “attended of political participation and the Los Angeles voting apoliticalrallyormarch”).Duetolowincidence,we model is that the dichotomous outcome variables in pooled the two “run for office” questions to create the former indicate whether subjects participated in avariableindicatingwhetherthesubjectranforany various acts of participation rather than whether or office, public or nonpublic. We performed a factor anal- not they voted in various elections. The latent tendency ysis of these five variables that suggested they all relate to participate in political activities in the Add Health to an underlying tendency to participate in politics. A sample is modeled in the same manner as the latent Cronbach test of internal consistency (α 0.61) re- tendency to vote in the Los Angeles sample. veals that it is reasonable to include these variables= in TheresultsofbothreplicationsusingtheAddHealth amodelinwhichasinglescalarlatentvalueforpar- study show that participatory behavior is heritable. Fig- ticipation is being estimated (see Verba, Schlozman, ure 3 shows that about 72% of the variance in turnout and Brady 1995, who report a similar α for a scale of behavior can be attributed to genes (95% C.I. 32%, participation that includes these items). 93%). The shared environment accounts for 20% of It is important to note that there are several differ- the variance (95% C.I. 1%, 57%), but an AE model ences between the Los Angeles sample and the Add without common environment actually fits the data Health sample. First, Add Health is nationally repre- better than the ACE model (see Appendix). Figure 4 sentative, suggesting that the results are more likely shows that genetic effects account for 60% (C.I. 11%, to generalize to the population outside Los Angeles. 91%) of the variance in political participation with the Second, Add Health includes subjects who were eligi- shared environment having little effect (18%, C.I. 1%, ble but not registered to vote. This is important because 54%). Once again, an AE model without shared en- the act of registration itself may be an important part of vironment fits better, suggesting that most variance the decision to vote. Third, Add Health relies on self- can be attributed to genetic and unshared environmen- reported turnout instead of official records meaning tal factors. In summary, both Add Health replications it is more susceptible to overreporting than the Los yield estimates of heritability that are similar in mag- Angeles sample. Fourth, Add Health is restricted to nitude to the 53% estimate for heritability in the Los young adults in their late teens and twenties (all eligible Angeles sample, suggesting the heritability of political to vote)—–thus, while it increases generalizability with participation is robust.15 respect to geography, socioeconomic composition, and local political conditions, it decreases generalizability with respect to age. Finally, Add Health includes data 15 We also re-ran the Los Angeles voting model, AddHealth voting on turnout for just a single election compared to eight model, and AddHealth political participation model with separate heritability, common, and shared environment components for males in the Los Angeles data. As a result, the greater ef- and females. This was done to ensure pooling males and females ficiency of a larger sample may be partially offset by is appropriate. The DIC for the Los Angeles gender-specific voter fewer observations per individual. model was higher than for the pooled model indicating the pooled There are also some small differences in the mod- model fits the data better. The DIC for the AddHealth gender- specific model is lower than the pooled model, however the male eling of the Add Health data. The Add Health voting and female heritability estimates are nearly identical (0.69 for males model is based on a single election, k 1 ,therefore and 0.66 for females). Finally, the DIC for the AddHealth political subject j is a member of family i choosing={ } to vote participation index is higher for the gender-specific model.

242 American Political Science Review Vol. 102, No. 2

It is important not to confuse these estimates with FIGURE 3. Heritability of Voter Turnout in those from other models in the turnout literature. Add Health They are not comparable. For example, we referred to another study (Plutzer 2002) earlier in which envi- ronmental factors account for only 31% of the vari- ance in turnout, but many of the variables in that model might well include genetic effects (for example, parental turnout might in part be a proxy for genetic as- sociation). It is also possible that there are as-yet undis- covered or unmeasured environmental factors that will improve the fit of that model. Nor can we state with certainty that genetic effects are somehow more important than environmental ef- fects. Although we estimate that genetic variation ac- counts for more than 50% of the variance in partici- pation in all three tests, these estimates are based on a simple additive genetic model that undoubtedly masks richer and more complex gene-environment interac- tions. We therefore strongly discourage readers from perceiving these results as a horse race between genes and environment. In fact, our results suggest that both genes and environment matter, and our job now is to Note:TernaryplotshowstheposteriorBayesiandistribution look closer at both to understand better how nature of estimated components of total variance in an ACE model and nurture work together to create the political phe- of voter turnout among subjects in the National Longitudinal nomena we observe in the world. Study of Adolescent Health (Add Health). Mean heritability (h 2) is estimated to be 72%. Colors indicate credible areas cal- culated by using 10,000 posterior draws to estimate a three- dimensional kernel density. The dark areas indicate the highest DISCUSSION density regions with the most credible estimates, while the light areas contain 95% of the draws (i.e., the probability that the true The fact that we have found that genetic variation in coefficients lie outside the colored regions is p 0.05). = voting, and political participation in general, should not be surprising given the large number of behaviors that have already been found to be heritable (Bouchard and FIGURE 4. Heritability of Political McGue 2003; Turkheimer 1998). However, our goal is Participation in Add Health not simply to show that political behavior can be added to this long list of behaviors. Instead, we suggest that our findings are the first step in a research agenda with the goal of uncovering biological sources of partici- patory behavior, a finding that would have important implications for political science in general and studies of voting behavior in particular. Political scientists have typically not focused on the role of genetic and biological factors in political be- havior (Alford, Funk, and Hibbing 2005), which has potentially biased our interpretations of several impor- tant phenomena. For example, if political participation is heritable, it would help to explain why models based primarily on environmental variables fit poorly to ob- served behavior (Matsusaka and Palda 1999). It would also conform to two well-known features of voting. First, parental turnout behavior has been shown to be one of the strongest predictors of turnout behavior in young adults (Plutzer 2002). Although this has previ- ously been interpreted as the result of social influence, Note:TernaryplotshowstheposteriorBayesiandistributionof the findings here suggest it may be mostly due to her- estimated components of total variance in an ACE model of itability since the shared environment appears to play political participation among subjects in the National Longitudi- only a small (if any) role. Second, turnout behavior nal Study of Adolescent Health (Add Health). Mean heritability (h 2)isestimatedtobe60%.Colorsindicatecredibleareas has been shown to be habitual—–the majority of people calculated by using 10,000 posterior draws to estimate a three- either always vote or always abstain (Fowler 2006b; dimensional kernel density. The dark areas indicate the highest Gerber, Green, and Shachar 2003; Green and Shachar density regions with the most credible estimates, while the light 2000; Miller and Shanks 1996; Plutzer 2002; Verba and areas contain 95% of the draws (i.e., the probability that the true Nie 1972). Scholars previously interpreted this as the coefficients lie outside the colored regions is p 0.05). = result of reinforcement learning, but given the small

243 Genetic Variation in Political Participation May 2008 effect of environmental variation it might also be The frontier before us is vast. Future work should largely due to inherent genetic variability. explore the interaction effects of genes and environ- While the results here suggest a significant role for ment on participation. These studies will help us to genes, they are completely silent on the specific mech- learn what the causal mechanisms are that link genes anism that links genes to participation. Therefore, the which have taken millions of years to evolve to large- next step in this line of research must move beyond esti- scale political behavior which is an extremely recent mates of heritability and attempt to identify why genes phenomenon on the scale of human evolution. Evi- matter so much. There are many possible mechanisms dence of political behavior in chimpanzees (de Waal one could imagine, but here we speculate on a few. 1998 [1982]), capuchins (Brosnan, Freeman, and de The theoretical literature on voting has centered on Waal 2006), and early human societies (Boehm 1999) rational, self-interested models (Aldrich 1993; Downs suggests that it may have, in part, adapted genetically 1957; Riker and Ordeshook 1968) that have great diffi- to small-scale interactions, but it is an open question culty explaining high turnout in large populations. One whether or not these small-scale adaptations influence popular extension to these models is to assume that large-scale political participation. The obvious place to some individuals experience an extra benefit from vot- start is with factors for participation that have already ing (the “D” term as Riker and Ordeshook called it) been identified like cognition and efficacy, which also that has nothing to do with the outcome. Instead, this have a genetic basis (McGue and Bouchard 1998). It is benefit comes from the satisfaction of fulfilling a civic also possible that genes influence political participation duty or of contributing to the democratic process. In via their effect on personality traits that have not yet other words, these models posit that there is inherent been linked to it, like their effect on assertiveness or heterogeneity in the desire to vote. While many schol- competitiveness. Thus an important area of research ars believe this argument is plausible (notably Aldrich will study the extent to which the link between genes (1993, p. 266) argues “most of the action is probably in and participation can be explained by genetic variation the intrinsic values of voting per se”), not a single one in inherent personality attributes. has suggested that this heterogeneity may have genetic Future research should also begin the work of iden- origins. Thus, our results suggest that a fruitful avenue tifying genes that are implicated in political behavior. for future research is to study whether or not variation It is extremely unlikely that such efforts will uncover in feelings of civic duty intermediate the relationship a“votinggene”,however,theresultspresentedhere between genes and political participation. suggest that there is some (possibly large) set of genes Amorerecentextensiontotherationalmodelposits whose expression—–in combination with environmental that voters get utility for behaving “ethically” as a way factors—–regulates political participation. Finding out of coordinating high participation equilibria between which genes they are and what physical function they competing groups (Sandroni and Feddersen 2006). This have will improve our understanding of the biological argument is also plausible, but since it is based on processes that underlie these complex social behaviors equilibrium analysis, it is agnostic about the origin and may also shed light on their evolutionary origin of the preference for ethical behavior. The evidence (Fitzpatrick et al. 2005). here suggests that genetics may play a role. The eth- Finally, we offer a note of caution. Heritability stud- ical voting model works equally well in small groups ies have shown that genes account for some of the and large populations, so it is possible that the ethi- variance in a very large set of human behavior, in- cal mechanisms underlying equilibrium evolved genet- cluding activities like television watching that are ex- ically in small-scale settings in early human societies tremely recent in human history and not (yet) relevant and then continued to have an influence as humans to genetic evolution. In particular, Turkheimer (1998) became involved in the larger-scale behavior of recent argues that these results have been well known in other history. disciplines for a very long time, but expectations that Another possibility is that variation in voting and they would lead to the discovery of specific “deeper” participation are related to variation in prosocial be- biological explanations of human behavior have largely havior. A wide range of studies have already shown been disappointed. There are simply too many genes astronggeneticbasisforprosocialpersonalityand and too many causal steps between genes and behav- behavior (McGue, Bacon, and Lykken 1993; Rushton ior to expect that genetic analysis will ever lead to et al. 1986; Scourfield et al. 2004; Cesarini et al. 2008). improved understanding. Moreover, high heritability This literature suggests that innate dispositions play a for a phenotype does not guarantee that it will be significant role in an individual’s willingness to partic- possible to identify specific genes that contribute to ipate in social activities or to engage in acts that pri- it. For example, in cancer genetics the least heritable marily benefit others. Meanwhile, observational stud- cancers have been the most amenable to molecular ies (Edlin, Gelman, and Kaplan 2007; Jankowski 2002; genetic analysis, because they are rare and caused by Jankowski 2007) and laboratory experiments (Fowler single genes of large effect. Highly heritable cancers are 2006a; Fowler and Kam 2007) suggest that prosocial more common and highly polygenic and it is therefore attitudes and behavior are important factors for ex- harder to identify genes for them (Risch 2001).16 plaining voter turnout and political participation. Thus, However, the recent revolution in genotyping genes may influence voting and political participation presents possibilities that were not available to because they influence a generalized tendency to en- gage in social behavior. 16 Thanks to Eric Turkheimer for bringing this idea to our attention.

244 American Political Science Review Vol. 102, No. 2 behavior geneticists when they first uncovered evi- be mediated by partisanship (Dawes and Fowler 2008). dence of the heritability of complex social behaviors. Thus, the realization that participation is heritable has Scholars have already begun discovering specific genes already helped to generate additional evidence that associated with political behavior, which may be the may be applied to existing theories of turnout, parti- first few pieces in the puzzle to understanding the biol- sanship, and prosocial behavior, and it also yielded new ogy that underlies it. For example, two studies (Fowler theories about the effect of the serotonin and dopamine and Dawes 2008; Dawes and Fowler 2008) recently system on participation. Therefore, although it may identified variants of three genes that are positively cor- not surprise behavior geneticists that participation is related with voter turnout. The genes they studied are heritable, it seems premature to argue that heritability known to influence social behavior via the dopamin- studies will not bear fruit in political science. These ergic and serotonergic systems, suggesting that voting studies provide the first step needed to excite the imag- may, in fact, be a prosocial act. Moreover, the associa- inations of a discipline not used to thinking about the tion between one of these genes and turnout appears to role of biology in human behavior.

APPENDIX

TABLE A1. Summary of Model Results Deviance Common Unshared Information Heritability Environment Environment Criterion h 2 c2 e2 (DIC) Dbar pD Los Angeles Turnout ACE 0.53 0.35 0.12 2643.4 2351.6 291.9 (0.10, 0.89) (0.02, 0.73) (0.03, 0.26) AE 0.86 0.14 2639.2 2347.9 291.3 (0.71, 0.95) (0.05, 0.29) Add Health Turnout ACE 0.72 0.20 0.09 852.8 532.6 320.2 (0.32, 0.93) (0.01, 0.57) (0.05, 0.15) AE 0.91 0.09 850.5 528.3 322.3 (0.85, 0.95) (0.05, 0.15) Add Health Political Participation ACE 0.60 0.18 0.23 615.3 490.8 124.5 (0.11, 0.91) (0.01, 0.54) (0.04, 0.59) AE 0.70 0.30 605.5 489.8 115.7 (0.31, 0.93) (0.07, 0.69) Note:Theseresultsshowthatweconsistentlyfoundthatalargeproportionofvarianceinturnoutandpoliticalparticipation behavior is due to heritability and that the best fitting models are those that assume a role for heritability and the unshared environment (but not the common environment). The first column describes each model. ACE models estimate a parameter for genetic (A), common environment (C), and unshared environment (E ); AE models assume the common environment has no effect. Columns 2, 3, and 4 show the mean estimated proportion of total variance attributable to heritability (h 2), common environment (c2), and unshared environment (e2), with 95% credible intervals indicated in parentheses below each estimate. Model fit is assessed using the deviance information criterion (DIC), which penalizes models for deviance (Dbar), capturing model fit, and the effective number of parameters (pD), capturing model complexity. The results show that the AE model generates the best fit for all three samples. The empirical means, 95% credible intervals, and DICs reported for the Los Angeles voting models are based on 10,000 draws from the posterior distribution.

TABLE A2. Posterior Predictive Checks Discrepancy Measure: Realized Predicted 95% CI %Votingin Value Value (Predicted) p-value No elections 9.1 7.7 [5.3, 10.4] 0.17 One election 12.4 13.5 [10.6, 16.7] 0.74 Twoelections 12.4 13.5 [10.4,16.7] 0.72 Threeelections 11.1 11.1 [8.3,14.1] 0.46 Fourelections 13.4 11.5 [8.6,14.4] 0.09 Five elections 8.8 10.9 [8.1, 13.9] 0.90 Sixelections 11.9 10.7 [7.8,13.6] 0.23 Sevenelections 11.4 11.3 [8.3,14.4] 0.43 Eight elections 9.6 9.8 [7.1, 12.6] 0.60

245 Genetic Variation in Political Participation May 2008

REFERENCES Eaves, L., A. Erkanli, J. Silberg, A. Angold, H. H. Maes, and D. Foley. 2005. “Application of Bayesian Inference Using Gibbs Sampling Abrahamson, Amy C., Laura A. Baker, and Avshalom Caspi. 2002. to Item-Response Theory Modeling of Multi-Symptom Genetic “Rebellious teens? Genetic and environmental influences on the Data.” Behavior Genetics 35 (6): 765–80. social attitudes of adolescents.” Journal of Personality and Social Eaves, Lindon J., and Peter K. Hatemi, Peter. 2008. “Transmission Psychology 83: 1392–1408. of attitudes toward abortion and gay rights: Parental socialization Aldrich, John H. 1993. “Rational Choice and Turnout.” American or parental mate selection?” Behavior Genetics,fothcoming. Journal of Political Science 37 (1): 246–78. Edlin, Aaron, Andrew Gelman, and Noah Kaplan. 2007. “Voting as Alford, J. R., C. L. Funk, and J. R. Hibbing. 2005. “Are Political Ori- aRationalChoice:WhyandHowPeopleVotetoImprovethe entations Genetically Transmitted?” American Political Science Well-Being of Others.” Rationality and Society forthcoming. Review 99 (2): 153–67. Evans, D. M., N. A. Gillespie, and N. G. Martin. 2002. “Biometric Ansolabehere, S., S. Iyengar, A. Simon, and N. Valentino. 1994. Genetics.” Biological Psychology 61: 33–51. “Does Attack Advertising Demobilize the Electorate.” American Feddersen, T., and A. Sandroni. 2006. “A Theory of Participation in Political Science Review 88 (4): 829–38. Elections.” American Economic Review 96 (4): 1271–82. Baker, Laura A., Mafalda Barton, Adrian Raine, and James H. Finkel, S. E. 1985. “Reciprocal Effects of Participation and Political Fowler. 2006. “The Southern California Twin Register at the Uni- Efficacy: A Panel Analysis.” American Political Science Review versity of Southern California: Ii.” Twin Research and Human 29 (4): 891–913. Genetics 9(6):1–8. Fitzpatrick, M. J., Y. Ben-Shahar, H. M. Smid, L. E. M. Vet, G. E. Bendor, J., D. Diermeier, and M. Ting. 2003. “A Behavioral Model Robinson, and M. B. Sokolowski. 2005. “Candidate Genes for of Turnout.” American Political Science Review 97 (2): 261–80. Behavioural Ecology.” Trends in Ecology and Evolution 20 (2): Blais, A., and R. Young. 1999. “Why Do People Vote? An Experi- 96–104. ment in Rationality.” Public Choice 99 (1–2): 39–55. Fowler, James H. 2006a. “Altruism and Turnout.” Journal of Politics Boehm, Christopher. 1999. Hierarchy in the Forest: The Evolution of 68 (3): 674–83. Egalitarian Behavior.Cambridge:HarvardUniversityPress. Fowler, James H. 2006b. “Habitual Voting and Behavioral Turnout.” Bouchard, T. J. 1998. “Genetic and Environmental Influences on Journal of Politics 68 (2): 335–44. Adult Intelligence and Special Mental Abilities.” Human Biology Fowler, James H., and Christopher T. Dawes. 2008. “Two Genes 70 (2): 257–79. Predict Voter Turnout.” Journal of Politics 70 (3), TBA. Bouchard, T. J., and M. McGue. 2003. “Genetic and Environmen- Fowler, James H., and Cindy D. Kam. 2006. “Patience as a Political tal Influences on Human Psychological Differences.” Journal of Virtue: Delayed Gratification and Turnout.” Political Behavior 28 Neurobiology 54 (1): 4–45. (2): 113–28. Brady, D., S. Verba, and K. L. Schlozman. 1995. “Beyond Ses: A Fowler, James H., and Cindy D. Kam. 2007. “Beyond the Self: Altru- Resource Model of Political Participation.” American Political ism, Social Identity, and Political Participation.” Journal of Politics Science Review 89 (2): 271–94. 69 (3): 811–25. Brosnan, Sarah F., Cassiopeia Freeman, and Frans B. M. de Waal. Freese, Jeremy, and Brian Powell. 2003. “Tilting at Twindmills: Re- 2006. “Partner’s Behavior, Not Reward Distribution, Determines thinking Sociological Responses to Behavioral Genetics.” Journal Success in an Unequal Cooperative Task in Capuchin Monkeys.” of Health and Social Behavior 44 (2): 130–35. American Journal of Primatology 68: 713–24. Galston,W.A.2001.“PoliticalKnowledge,PoliticalEngagement,and Burton, P. R., K. J. Scurrah, M. D. Tobin, and L. J. Palmer. 2005. Civic Education.” Annual Review of Political Science 4: 214–34. “Covariance Components Models for Longitudinal Family Data.” Gelman, Andrew, John B. Carlin, Hal S. Stern, and Donald B. Rubin. International Journal of Epidemiology 34 (5): 1063–77. 2004. Bayesian Data Analysis. New York: Chapman & Hall/CRC. Carmen, Ira H. 2004. Politics in the Laboratory: The Constitution of Gelman, Andrew, Gary King, and W. John Boscardin. 1998. “Es- Human Genomics.Madison:UniversityofWisconsinPress. timating the Probability of Events That Have Never Occurred: Cassel, C. A. 1999. “Voluntary Associations, Churches, and So- When Is Your Vote Decisive?” Journal of the American Statistical cial Participation Theories of Turnout.” Social Science Quarterly Association 93 (441): 1–9. 80 (3): 504–17. Gerber, A. S., D. P. Green, and R. Shachar. 2003. “Voting May Be Cesarini, David, Christopher T. Dawes, James H. Fowler, Magnus Habit-Forming: Evidence from a Randomized Field Experiment.” Johannesson, Paul Lichtenstein, Bjorn¨ Wallace. 2008. “ Heritabil- American Journal of Political Science 47 (3): 540–50. ity of Cooperative Behavior in the Trust Game.” Proceedings of Gimpel, J. G., and J. E. Schuknecht. 2003. “Political Participation the National Academy of Sciences 105 (10): 3721–26. and the Accessibility of the Ballot Box.” Political Geography Chen, Ming-Hui, Amita K. Manatunga, and Christopher J. Williams. 22 (5): 471–88. 1998. “Heritability Estimates from Human Twin Data by Incorpo- Green, D. P., and R. Shachar. 2000. “Habit Formation and Political rating Historical Prior Information.” Biometrics 54 (4): 1348–62. Behaviour: Evidence of Consuetude in Voter Turnout.” British Cho, W. K. T. 1999. “Naturalization, Socialization, Participation: Im- Journal of Political Science 30: 561–73. migrants and (Non-) Voting.” Journal of Politics 61 (4): 1140–55. Haberstick, B. C., J. K. Lessem, C. J. Hopfer, A. Smolen, M. A. Clinton, Joshua, Simon Jackman, and Doug Rivers. 2004. “The Sta- Ehringer, D. Timberlake, and J. K. Hewitt. 2005. “Monoamine tistical Analysis of Roll Call Data.” American Political Science Oxidase a (Maoa) and Antisocial Behaviors in the Presence of Review 98 (2): 355–70. Childhood and Adolescent Maltreatment.” American Journal of Dawes, Christopher T., and James H. Fowler. 2008. “Partisanship, Medical Genetics Part B (Neuropsychiatric Genetics) 135b: 59–64. Voting,andtheDopamineD2ReceptorGene.”(January31,2008). Hannagan, Rebecca, and Peter K. Hatemi. 2008. “AComment on the Available at SSRN: http: //ssrn.com/abstract=1089365. Genetics of Political Attitudes Debate.” Perspectives on Politics, de Waal, Frans. 1998 [1982]. Chimpanzee Politics: Power and Sex forthcoming. among Apes.Baltimore,MD:JohnsHopkinsUniv.Press. Harris, Kathleen Mullan, Carolyn Tucker Halpern, Andrew Smolen, DeFries, J. C., and D. W. Fulker. 1985. “Multiple Regression Analysis and Brett C. Haberstick. 2006. “The National Longitudinal Study of Twin Data.” Behavior Genetics 15 (5): 467–73. of Adolescent Health (Add Health) Twin Data.” Twin Research DiMaggio, P., E. Hargittai, and W. R. Neuman. 2001. “Social Impli- and Human Genetics 9(6):988–97. cations of the Internet.” Annual Review of Sociology 27: 307–36. Hatemi, P. K., S. E. Medland, K. I. Morley, A. C. Heath, and N. G. Do, K. A., B. M. Broom, P. Kuhnert, D. L. Duffy, A. A. Todorov, Martin. 2007. “The Genetics of Voting: An Australian TwinStudy.” S. A. Treloar, and N. G. Martin. 2000. “Genetic Analysis of the Behavior Genetics 37 (3): 435–48. Age at Menopause by Using Estimating Equations and Bayesian Hetherington, M. J. 1999. “The Effect of Political Trust on the Random Effects Models.” Statistics in Medicine 19 (9): 1217–35. Presidential Vote, 1968–1996.” American Political Science Review Downs, Anthony. 1957. An Economic Theory of Democracy.New 93 (3): 311–26. York: Harper. Highton, Benjamin, and Raymond E. Wolfinger. 2001. “The First Eaves, L., and A. Erkanli. 2003. “Markov Chain Monte Carlo Ap- Seven Years of the Political Life Cycle.” American Journal of Po- proaches to Analysis of Genetic and Environmental Components litical Science 45 (1): 202–09. of Human Developmental Change and G X E Interaction.” Be- Hopfer, C. J., D. Timberlake, B. Haberstick, J. M. Lessem, M. A. havior Genetics 33 (3): 279–99. Ehringer, A. Smolen, and J. K. Hewitt. 2005. “Genetic Influences

246 American Political Science Review Vol. 102, No. 2

on Quantity of Alcohol Consumed by Adolescents and Young Miller, Warren E., and J. Merrill Shanks. 1996. The New American Adults.” Drug and Alcohol Dependence 78: 187–93. Voter.Cambridge,Mass.;London:HarvardUniversityPress. Horwitz, A. V., T. M. Videon, and M. F. Schmitz. 2003. “Rethinking Mutz, Diana C. 2002. “The Consequences of Cross-Cutting Networks Twins and Environments: Possible Social Sources for Assumed for Political Participation.” American Journal of Political Science Genetic Influences in TwinResearch.” JournalofHealthandSocial 46 (4): 838–55. Behavior 44 (2): 111–29. Neale, M. C., and L. R. Cardon. 1992. Methodology for Genetic Huckfeldt, R., and J. Sprague. 1992. “Political Parties and Electoral Studies of Twins and Families.Dordrecht:KluwerAcademicPub- Mobilization: Political Structure, Social Structure, and the Party lishers. Canvass.” American Political Science Review 86 (1): 70–86. Nie, Norman H., G. B. Powell, and K. Prewitt. 1969a. “Social Struc- Huckfeldt, Robert. 1979. “Political Participation and the Neigh- ture and Political Participation—–Developmental Relationships.” borhood Social Context.” American Journal of Political Science American Political Science Review 63 (2): 361–78. 23 (3): 579–92. Nie, Norman H., G.B. Powell, and K. Prewitt. 1969b. “Social Struc- Jackman, R. W., and R. A. Miller. 1995. “Voter Turnout in the Indus- ture and Political Participation—–Developmental Relationships.” trial Democracies During the 1980s.” Comparative Political Studies American Political Science Review 63 (3): 808–32. 27 (4): 467–92. Palfrey, Thomas R., and Howard Rosenthal. 1985. “Voter Participa- Jacobson, K. C., and D. C. Rowe. 1998. “Genetic and Shared En- tion and Strategic Uncertainty.” American Political Science Review vironment Influences on Adolescent BMI: Interaction with Race 79 (1): 62–78. and Sex.” Behavior Genetics 28: 265–75. Plutzer, Eric. 2002. “Becoming a Habitual Voter: Inertia, Resources, Jankowski, Richard. 2002. “Buying a Lottery Ticket to Help the and Growth in Young Adulthood.” American Political Science Re- Poor-Altruism, Civic Duty, and Self-Interest in the Decision to view 96 (1): 41–56. Vote.” Rationality and Society 14 (1): 55–77. Posner, S., L. A. Baker, and N. G. Martin. 1996. “Social Contact and Jankowski, Richard. 2007. “Altruism and the Decision to Vote: Ex- Attitude Similarity in Australian Twins.” Behavior Genetics 26: plaining and Testing High Voter Turnout.” Rationality and Society 123–34. 19 (1): 4–34. Prescott, C. A. 2004. Behavior Genetics 34: 17–39. Karp, J. A., and D. Brockington. 2005. “Social Desirability and Re- Riker, W. H., and P. C. Ordeshook. 1968. “A Theory of the Calculus sponse Validity: A Comparative Analysis of Overreporting Voter of Voting.” American Political Science Review 62 (1): 25–42. Turnout in Five Countries.” Journal of Politics 67 (3): 825–40. Risch, N. 2001. “The Genetic Epidemiology of Cancer: Interpreting Kendler, K. S., Nick G. Martin, A. C. Heath, and L. J. Eaves. 1995. Family and Twin Studies and Their Implications for Molecular “Self-report psychiatric symptoms in twins and their nontwin rel- Genetic Approaches.” Cancer Epidemiology: Biomarkers and Pre- atives: are twins different? American Journal of Medical Genetics vention 10 (7): 733–41. (Neuropsychiatric Genetics) 60: 588–91. Rosenstone, Steven J., and R. E. Wolfinger. 1978. “Effect of Registra- Krause, Merton S., Kevin Houlihan, Mark I. Oberlander, and tion Laws on Voter Turnout.” American Political Science Review Lawrence Carson. 1970. “Some Motivational Correlates of Atti- 72 (1): 22–45. tudes toward Political Participation.” Midwest Journal of Political Rushton, J. P., D. W. Fulker, M. C. Neale, D. K. B. Nias, and Science 14 (3): 383–91. H. J. Eysenck. 1986. “Altruism and Aggression—–the Heritabil- Kuhnert, Petra M., and Kim-Anh Do. 2003. “Fitting Genetic Models ity of Individual-Differences.” Journal of Personality and Social to Twin Data with Binary and Ordered Categorical Responses: Psychology 50 (6): 1192–98. AComparisonofStructuralEquationModellingandBayesian Schlozman, Kay L., Nancy Burns, Sidney Verba, and Jesse Donahue. Hierarchical Models.” Behavior Genetics 33 (4): 441–54. 1995. “Gender and Citizen Participation: Is There a Different Lake, R. L., and Robert Huckfeldt. 1998. “Social Capital, Social Voice?” American Journal of Political Science 39 (2): 267–93. Networks, and Political Participation.” Political Psychology 19 (3): Scourfield, J., B. John, N. Martin, and P. McGuffin. 2004. “The 567–84. Development of Prosocial Behaviour in Children and Adoles- Lane, R. E. 1959. Political Life.NewYork:FreePress. cents: A Twin Study.” Journal of Child Psychology and Psychiatry Leighley, J. E., and J. Nagler. 1992a. “Individual and Systemic In- 45 (5): 927–35. fluences on Turnout—–Who Votes 1984.” Journal of Politics 54 (3): Sears, David O. 1987. “Political Psychology.” Annual Review of Psy- 718–40. chology 38: 229–55. Leighley, J. E., and J. Nagler. 1992b. “Socioeconomic-Class Bias in Shachar, R., and B. Nalebuff. 1999. “Follow the Leader: Theory and Turnout, 1964–1988—–the Voters Remain the Same.” American Po- Evidence on Political Participation.” American Economic Review litical Science Review 86 (3): 725–36. 89 (3): 525–47. Levinson, D. J. 1958. “The Relevance of Personality for Political- Somit, A., J. Tanenhaus, W. H. Wilke, and R. W. Cooley. 1958. “The Participation.” Public Opinion Quarterly 22 (1): 3–10. Effect of the Introductory Political Science Course on Student At- Martin, Nick G., Lyndon J. Eaves, A. C. Heath, R. Jardine, L. M. titudes toward Personal Political Participation.” American Political Feingold., and H. J. Eysenck. 1986. “Transmission of Social Ati- Science Review 52 (4): 1129–32. tudes.” Proceedings of the National Academy of Sciences 83 (12): Somit, Albert, and Steven A. Peterson. 1998. “Biopolitics after Three 4364–68. Decades—–a Balance Sheet.” British Journal of Political Science 28 Masters, Roger D. 1990. “Evolutionary Biology and Political The- (3): 559–71. ory.” American Political Science Review 84 (1): 195–210. Stoker, Laura, and M. K. Jennings. 1995. “Life-Cycle Transitions and Matsusaka, J. G., and F. Palda. 1999. “Voter Turnout: How Much Can Political Participation: The Case of Marriage.” American Political We Explain?” Public Choice 98 (3–4): 431–46. Science Review 89 (2): 421–33. McGue, M., S. Bacon, and D. T. Lykken. 1993. “Personality Stability Strate, John M., Charles J. Parrish, Charles D. Elder, and Coit Ford. and Change in Early Adulthood—–a Behavioral Genetic-Analysis.” 1989. “Life Span Civic Development and Voting Participation.” Developmental Psychology 29 (1): 96–109. American Political Science Review 83 (2): 443–64. McGue, M., and T. J. Bouchard. 1998. “Genetic and Environmental Timpone, Richard J. 1998. “Structure, Behavior, and Voter Turnout Influences on Human Behavioral Differences.” Annual Review of in the United States.” American Political Science Review 92 (1): Neuroscience 21 (1): 1–24. 145–58. McLeod, J. M., D. A. Scheufele, and P. Moy. 1999. “Community, Turkheimer, Eric. 1998. “Heritability and Biological Explanation.” Communication, and Participation: The Role of Mass Media and Psychological Review 105 (4): 782–91. Interpersonal Discussion in Local Political Participation.” Political Turkheimer, Eric, and Mary C. Waldron. 2000. “Nonshared Environ- Communication 16 (3): 315–36. ment: A Theoretical, Methodological, and Quantitative Review.” Merelman, Richard M. 1971. “The Development of Policy Thinking Psychological Bulletin 126 (1): 78–108. in Adolescence.” American Political Science Review 65 (4): 1033– van den Berg, P. M., A. Setiawan, M. Bartels, T. J. C. Polderman, A. 47. W. van der Vaart, and D. I. Boomsma. 2006. “Individual Differ- Miller, Arthur H., P. Gurin, and G. Gurin. 1981. “Group Conscious- ences in Puberty Onset in Girls: Bayesian Estimation of Heritabil- ness and Political Participation.” American Journal of Political ities and Genetic Correlations.” Behavior Genetics 36 (2): 261– Science 25 (3): 494–511. 70.

247 Genetic Variation in Political Participation May 2008 van den Berg, S. M., L. Beem, and D. I. Boomsma. 2006. “Fit- Voice and Equality : Civic Voluntarism in American Politics. ting Genetic Models Using Markov Chain Monte Carlo Algo- Cambridge, Mass.: Harvard University Press. rithms with Bugs.” Twin Research and Human Genetics 9(3):334– Visscher, Peter M., Sarah E. Medland, Manuel A. R. Ferreira, 42. Katherine I. Morley, Gu Zhu, Belinda K. Cornes, Grant W. Mont- Verba, Sidney, and Norman H. Nie. 1972. Participation in America : gomery, and Nicholas G. Martin. 2006. “Assumption-Free Esti- Political Democracy and Social. New York Harper & Row, 1972. mation of Heritability from Genome-Wide Identity-by-Descent Verba, Sidney, Kay L. Schlozman, and Henry E. Brady. 1993. “Race, Sharing between Full Siblings.” PLOS GENETICS 2(3):316–25. Ethnicity, and Political Resources—–Participation in the Unites Wielhouwer, P. W., and B. Lockerbie. 1994. “Party Contacting and States.” British Journal of Political Science 23 (4): 453–97. Political-Participation, 1952–90.” American Journal of Political Sci- Verba, Sidney, Kay Lehman Schlozman, and Henry E. Brady. 1995. ence 38 (1): 211–29.

248 Political Research Quarterly OnlineFirst, published on January 6, 2009 as doi:10.1177/1065912908327607

Political Research Quarterly Volume XX Number X Month XXXX xx-xx The Heritability of Partisan Attachment © 2008 University of Utah 10.1177/1065912908327607 http://prq.sagepub.com Jaime E. Settle hosted at Christopher T. Dawes http://online.sagepub.com James H. Fowler University of California, San Diego

One of the strongest regularities in the empirical political science literature is the well-known correlation in parent and child partisan behavior. Until recently, this phenomenon was thought to result solely from parental socialization, but new evidence on genetic sources of behavior suggests it might also be due to heritability. In this article, the authors hypothesize that genes contribute to variation in a general tendency toward strength of partisanship. Using data col- lected at the Twins Days Festival, the authors compare the similarity of partisan strength in identical twins who share all of their genes to the similarity of partisan strength in nonidentical twins who share only half of their genes. The results show that heritability accounts for almost half of the variance in strength of partisan attachment, suggesting we should pay closer attention to the role of biology in the expression of important political behaviors.

Keywords: political psychology; political methodology; public opinion and political participation

he study of partisanship occupies a vast part of the socialization (Campbell et al. 1960), stemming from Tpolitical behavior literature because of the com- childhood and reflecting the influences of the imme- plexity of what it means, how it forms, and what it diate social milieu and the family (Hyman 1959; predicts (e.g., Campbell et al. 1960; Fiorina 1981; Greenstein 1965). Subsequent work built on this Niemi and Jennings 1990; Popkin 1991; Gerber and social psychological view argued that identification Green 1998). Partisanship is typically evaluated along with a particular party is based on images of that two dimensions—the strength of reported partisan party as a social group (Gerber and Green 1998; attachment and the direction of that attachment. While Green, Palmquist, and Schickler 2002; Fowler and there is much divergence of opinion on the nature and Kam 2007). The authors of The American Voter measurement of partisanship, scholars have almost (Campbell et al. 1960) essentially viewed strength of exclusively focused their attention on the socialization partisanship as a fixed factor that could be used to process and environmental determinants of the origin, predict political behavior, but they could only specu- direction, and intensity of partisanship. However, late as to why or how it was fixed. recent work has demonstrated that heredity plays a Since the 1970s, a debate in the literature has con- role in closely related political behaviors, such as tested whether partisanship is affective, nearly political attitudes (Alford, Funk, and Hibbing 2005; immutable, and emotionally based or whether it is Hatemi et al. 2007; Tesser 1993), political orientations better conceived as instrumental, changeable, and (Alford, Funk, and Hibbing 2005; Settle et al. 2008), responsive to current conditions and attitudes toward voting behavior (Fowler, Baker, and Dawes 2008; contemporary political events. As opposed to the Fowler and Dawes 2008; Dawes and Fowler 2008), and social psychological interpretations of partisanship, trust (Cesarini et al. 2008). The developing consensus instrumental theories view partisan attachment as an that genes play an important role in political behavior information shortcut that is continually updated and leads us to believe that heritability could also help to explain one of the remaining questions in the parti- Authors’ Note: We would like to thank Peter Hatemi and Rose sanship literature: what contributes to the underlying McDermott for comments and encouragement. We would also strength of a person’s partisan identity? like to thank Ira Carmen and Gene Robinson for putting together the Biology and Politics conference at the University of Illinois What Is Partisanship? at Urbana-Champaign, where we first presented this work; the organizers of the Twins Days Festival in Twinsburg, Ohio; and the National Science Foundation (grant SES-0719404) and Party identification was originally conceived as an Institute of Government Affairs at the University of California, affective attachment resulting from the process of Davis, for generous research support.

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 1 2Political Research Quarterly adjusted based on rational evaluation (Fiorina 1981; Given the importance of the question of stability of Popkin 1991). For example, Achen (1992) argued that partisanship and the well-known correlation between voters utilize a Bayesian process, prospectively judging parent and child partisan behavior, it is somewhat parties based on their observations of the party’s past surprising that this literature has ignored heritability performance and information received from a cam- as a factor in partisan attachment. As we have indi- paign. Some research indicates that voters receive cated, there are many examples of careful empirical “noisy” signals about party performance, originating studies of theories of partisan attachment, but almost at either the individual level or system level of the all have focused exclusively on environmental expla- information environment, or both. If, due to high nations. If a substantial portion of our tendency to levels of individual-level noise, voters cannot deter- attach to parties is passed from parent to child via mine party differences, they may be less likely to form genetic predispositions, it could help explain stability party attachments (Huber, Kernell, and Leoni 2005). of partisanship over time since, barring recombina- Instrumentalists have challenged the use of parti- tion or mutation, genetic factors are fixed over the sanship as an independent variable, instead asserting course of our lifetimes. Moreover, since children that it is not as stable as it was originally conceived share half of each parent’s genes, it might help to (Fiorina 1981; Franklin and Jackson 1983). For explain within-family correlations in partisanship. example, presidential approval, consumer sentiment, Recent work has shown that genetic factors specific political events, and attitudes toward particu- account for a significant proportion of variation in lar administrations affect levels of partisanship in the social attitudes (Martin et al. 1986; Tesser 1993) and population at large (Mackuen, Erikson, and Stimson political attitudes related to the direction of partisan 1989; Brody and Rothenberg 1988; Converse and identification (Alford, Funk, and Hibbing 2005; Markus 1979; Meier 1975; Page and Jones 1979). Hatemi et al. 2007; Settle et al. 2008). The strength of However, recent experimental manipulations show political opinion, defined as the percentage of non- that simulating the effects of short-term political neutral responses on a survey of political opinions, forces does not have an effect on party choice, rein- has also been demonstrated to have a genetic compo- forcing traditional notions of partisanship (Cowden nent (Alford, Funk, and Hibbing 2005). Likewise, and McDermott 2000). genetic factors are also important for political behav- Beyond the debate over what partisanship is, schol- iors like voting (Fowler, Baker, and Dawes 2008; ars have offered a series of interpretations about what Fowler and Dawes 2008; Dawes and Fowler 2008) causes it to vary, both in strength and in direction. that are known to be influenced by the tendency to Most scholarship suggests that the seeds of partisan- attach to a given party. ship are planted early in life and that strength of par- However, while heritability plays a role in many tisanship develops and changes over the life course. political behaviors, we do not expect that it will play One of the few relatively consistent findings in the lit- a role in all political behaviors (Alford and Hibbing erature is that partisanship strengthens over the course 2008), and we have reason to expect a difference in of a lifetime, although interpretations for this vary. the heritability of strength versus direction of parti- The authors of The American Voter argued that the sanship. As measured in both our sample and in strength of partisan attachment increases with age as Hatemi (2007) and Hatemi et al. (2007), identifying the result of an individual becoming more active oneself as a “strong” partisan is a reflection of the within the community and associating with social propensity toward group attachment. Research on the groups, some of which have partisan ties (Campbell et al. heritability of religion has found that while religios- 1960). While Converse (1969, 1972, 1976, 1979) sug- ity is strongly heritable, denominational affiliation is gested that partisanship is formed early in life and not (Eaves 1977; Martin et al. 1986; Bouchard and reinforced by experiences interpreted through a parti- McGue 2003; Koenig et al. 2005; Bouchard et al. san lens, Abramson (1976, 1979a, 1979b) argued that 1999; Beer, Arnold, and Loehlin 1998; Bouchard et al. generational effects and period effects play a more 2004). We conjecture that the relationship between significant role in shaping partisanship strength over strength of partisan attachment and party identifica- time. Further studies have shown that parental parti- tion is analogous to the relationship between religios- sanship has an influence on children that is strongest ity and denominational affiliation (Hatemi et al. at age eighteen but continues into adulthood and that 2007; Jennings, Stoker, and Bower 2001). That is, the issues play an increasingly important role in partisan- intensity of one’s attachment to a group may be ship formation over time (Niemi and Jennings 1991). shaped by genetic predispositions, but the selection

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 3 of the group to which one attaches is largely shaped strength of partisanship of (identical) monozygotic by parental and environmental exposures. We expect (MZ) twins, conceived from a single egg; and (non- a different pattern for ideology, which is best viewed identical) dizygotic (DZ) twins, conceived from two as an orientation toward the social world and its orga- separate eggs. MZ twins share 100 percent of their nization (Jost 2006; Alford and Hibbing 2008). genes, while DZ twins share only 50 percent on aver- Ideological orientations may be heritable because of age. Thus, if the decision to vote is based in part on their social psychological role (Jost et al. 2003; Jost genetic characteristics, MZ twins should exhibit 2006; Alford and Hibbing 2008), but the intensity of more behavioral concordance than DZ twins. Based these beliefs may not have a strong heritable compo- on the assumption that MZ twins and DZ twins share nent since the underlying psychological factors associ- comparable environments, we can use these concor- ated with liberalism (like openness) and conservatism dances to estimate explicitly the relative influence of (like conscientiousness) are directional. genetic, shared environmental, and unshared environ- In this article, we hypothesize that genes con- mental factors. tribute to variation in a general tendency toward The twin study design has been shown to be an strength of partisanship. Using data collected at the extremely powerful tool for identifying the relative Twins Days Festival in 2006 and 2007, we compare degree to which genetic and environmental factors the similarity of partisan strength in identical twins influence an observed outcome (Evans, Gillespie, and who share all of their genes to the similarity of parti- Martin 2002; Neale and Cardon 1992). The reasoning san strength in nonidentical twins who share only behind this model relies on the principle that all inde- half of the genes that vary between human beings, on pendent variables explaining political behavior can average. The results indicate that strength of partisan be put into three mutually exclusive categories: addi- attachment is heritable, and they suggest that we tive genetic factors (A), shared or common environ- should pay closer attention to the role of biology in mental factors (C), and unshared environmental the expression of important political behaviors. factors (E). We describe this model in the appendix. Although these results do not identify specific causal It is important to clarify the difference between the mechanisms underlying these genetic predisposi- common environment and the unshared environment tions, they do indicate that we should be searching for in the twin model. Common environment includes biological and genetic sources of partisan behavior. the family environment in which both twins were raised as well as any other factor to which both twins were equally exposed. In contrast, the unshared envi- Twin Studies ronment includes idiosyncratic influences that are experienced individually. It is possible to have The technique currently best suited to study the unshared environmental exposure as a child (twins contribution of heredity to political behavior is the may have different friends with different political twin study, a method used for decades by psycholo- beliefs) and to have shared environments as an adult gists but used only recently in political science. The (twins may see the same election results). Thus, the twin study model cannot tell us which genes con- distinction between common and unshared environ- tribute to a particular behavior or the mechanism by ment does not correspond directly to family–nonfamily which genes and the environment interact to produce or adult–child differences in factors that influence a certain phenotypes, but it is useful for testing the given behavior. Moreover, there may be a similarity existence of a genetic component and establishing the in the objective environment, but twins may have relative importance of the environment for a specific idiosyncratic experiences that influence their effec- behavioral outcome. tive environment that create an unshared rather than a By partitioning phenotypic variance into its con- common environmental influence on variation in the stituent components, we can estimate the role of phenotype (Turkheimer 2000). For example, twins heredity versus environment, or nature and nurture. may watch the same campaign speech but process it The “relative importance” of heredity, or the propor- differently, with different effects on their feelings tion of the total phenotypic variance due to genes towards the competing parties. transmitted by parents to their offspring, is called the Some scholars argue that the identical nature of heritability of the character (Falconer and Mackay MZ twins causes them to be more strongly affiliated 1996). We can estimate the degree to which partisan- and more influenced by one another than their non- ship strength is heritable by studying the reported identical DZ counterparts, which would indicate that

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 4Political Research Quarterly greater concordance in MZ twins might merely reflect attracts about 2,000 pairs of twins each year, who, in the fact that their shared environments cause them to addition to participating in social events, have the become more similar than DZ twins. This situation option to volunteer for a number of research studies. would violate the assumption that MZ and DZ envi- The sample consists of 353 pairs of same-sex adult ronments are comparable. However, studies of twins twins, aged eighteen or older (mean age of thirty-six), raised together have been validated by studies of twins in attendance at the annual festival in August 2006 and reared apart (Bouchard 1998), suggesting that the 2007. A condition for participation was that both twins shared environment does not exert enhanced influence in a pair were able to complete the survey. In total, 706 on MZ twins. Furthermore, even among twins whose individuals participated (75 DZ and 278 MZ same-sex zygosity has been miscategorized by their parents, pairs of twins). Zygosity was determined by self- personality and cognitive differences between MZ report, which has been used previously for studies of and DZ twins persist (Bouchard and McGue 2003), Twins Days participants (Ashenfelter and Krueger indicating that being mistakenly treated as an identi- 1994). In particular, one study of 86 Twins Days cal twin by one’s parents is not sufficient to generate subjects showed self-report to be 100 percent reliable the difference in concordance. Although MZ twins compared to a genotypic assessment of zygosity (Wise may sometimes be in more frequent contact with each et al. forthcoming). Subjects who had participated in other than DZ twins, it appears that twin similarity 2006 were excluded from taking part in 2007. (e.g., in attitudes and personality) may be the cause of Participants were asked a series of demographic greater contact rather than an effect (Posner et al. questions and a question about their partisan affilia- 1996). Finally, contrary to the prediction that the tion, using the traditional 7-point partisanship scale. influence of the unshared environment would tend to This scale was then “folded over” to produce a 4-point, reduce concordance over time once twins reach adult- directionless scale of partisan attachment. This is a hood, MZ twins living apart tend to become more variation of the standard National Election Studies similar with age (Bouchard and McGue 2003). (NES)/Michigan party identification scale,1 the most The ACE model measures the total variance in a frequently used scale of partisanship in the literature given phenotype, in this case partisanship strength, (Weisberg 1999). This scale has been found to tap and then estimates the relative contributions of into respondents’ general partisan tendencies and genetic and environmental influences separately to may be less sensitive to transitive or election-specific the total observed variance of that phenotype. The partisan preferences than other measures (Whiteley roles of genotype and environment are not measured 1988; Green and Palmquist 1990; W. Miller 1991; directly, but their influence is inferred through their Abramson and Ostrom 1991, 1992, 1994a, 1994b; effects on the covariances of twin siblings (Neale W. Miller and Shanks 1996). Moreover, this measure and Cardon 1992). While the ACE model does not has been used frequently as an explanatory variable indicate the specific causal mechanisms that interact in models of political participation (Timpone 1998; with genes and/or mediate the relationship between Verba, Schlozman, and Brady 1995). Because we are genes and strength of partisanship, it is a useful tool most interested in the long-term aspects of partisan to establish whether genes play a role and, thus, strength, the wording of this question makes it most whether they merit further study to explain aspects appropriate for our purposes. of political behavior. The ACE specification is the Table 1 shows summary statistics for study partic- simplest model of genes, shared environment, and ipants. One method of evaluating whether MZ and unshared environment; more complicated interac- DZ twins are drawn from comparable environments tions certainly occur in nature, but a strong effect for is to examine the distributions of relevant covariates genes in the additive ACE model indicates that genes between the two groups. If there are any significant are also likely to play a role in more complex speci- differences between the MZ and DZ twins, we can fications as well. interact the covariate in question with A (the additive genetic component) in the ACE model to see if it impacts the heritability estimate. For example, there Twins Days Festival Data Collection is a statistically significant difference in age between the MZ and DZ twins. However, when we add a vari- The data used in this study were gathered from a able for age to the ACE model, we do not find any sample of twins in attendance at the 2006 and 2007 evidence that the heritability of partisanship strength Twins Days festival in Twinsburg, Ohio. This festival changes with age. Similarly, we find that MZ twins

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 5

Table 1 Summary Statistics MZ Mean MZ SD DZ Mean DZ SD Difference (p-Value)

Male 0.19 0.40 0.20 0.40 .89 Age 36.96 16.69 32.43 14.84 .00 Turnout 0.69 0.46 0.66 0.47 .30 Education 0.64 0.48 0.57 0.50 .15 Income 0.41 0.49 0.36 0.48 .45 Democrat 0.30 0.46 0.35 0.48 .34 Republican 0.34 0.47 0.19 0.40 .00 Liberal 0.19 0.40 0.20 0.40 .66 Conservative 0.71 0.46 0.69 0.46 .47 Partisan intensity 1.63 0.89 1.51 0.90 .12 Ideological intensity 1.26 1.03 1.07 1.03 .02

Note: MZ = monozygotic; DZ = dizygotic. are somewhat more likely to be Republicans and With some further manipulation, it is also possible to ideologically intense.2 However, neither of these vari- show that a similar relationship holds for correlation ables significantly moderates the heritability esti- coefficients—heritability can be estimated as twice the mates. Thus, while this sample is small and not difference between the correlations, which yields a randomly selected, it does not appear to be systemat- value of .58. In other words, a simple interpretation of ically biased in a way that affects our ability to use the data suggests that genes account for about 58 per- the data to estimate the influence of genes on partisan cent of the variance in strength of partisan attachment. attachment. Finally, because the sample was self- However, this simple method does not account for selected (both into attendance at the Twinsburg the fact that our dependent variable is ordered and Festival and into the study section of the festival), we categorical, and it assumes that the latent distribution may have a disproportionate share of people who of partisan strength is normal even though we only “like to participate,” indicative of some underlying observe four distinct values. We therefore estimate a tendency to associate or form group attachments. full ACE model that includes parameters for heri- This could serve to increase the proportion identify- tability (a2), the common environment (c2), and the ing as strong partisans in our sample as compared to unshared environment (e2), as well as three thresholds the general population, but it should not bias the rel- between the four ordered categories that define the ative contributions of genes and environment to that underlying distribution of partisanship strength (see behavior. Any self-selection bias likely applies to DZ the appendix). We use the software package MX to and MZ twins equally. estimate this model (Neale et al. 2006). The results suggest that heritability (h2) generates about 46 percent of the variance in strength of parti- Results san attachment. The 95 percent CI for the estimate is (5 percent, 57 percent), indicating that we can reject In the behavior genetics literature, a simple com- the hypothesis that genes play no role in partisanship parison of polychoric correlations is frequently used strength. The ACE model also suggests that the envi- as a first test of the rate of twin concordance in ronment plays a role, although this is primarily due to behavior (for a detailed explanation of this method, the unshared environmental factors (e2), which see Alford, Funk, and Hibbing 2005). In our observa- account for 54 percent of the variance (CI = 43 per- tions, the correlation in partisan strength is signifi- cent, 67 percent). The shared environment (c2) does cantly different (p = .0002) for MZ twins (.46, 95 not seem to play a significant role (0 percent, CI = 0 percent confidence interval [CI] = .34, .57) and DZ percent, 37 percent). Measures of model fit indicate twins (.16, 95 percent CI = –.15, .48) (see Table 2). that an AE model is better than the ACE model. All of Using the three variance equations noted in the the models presented include controls for age and appendix, it is easy to show that3 gender. These control variables only influence the estimation of the three thresholds and do not enter 2 2 2 2 σ A = 2(COVMZ – COVDZ). into the estimation of a ,c,or e.

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 6Political Research Quarterly

Table 2 Partisanship Strength Model A C E –2 Log-Likelihood df ∆χ2 p

ACE .46 (.05, .57) .00 (.00, .37) .54 (.43, .67) 1,602.02 665 AE .46 (.33, .57) .54 (.43, .67) 1,602.02 666 0.00 1.00 CE .40 (.28, .50) .60 (.50, .72) 1,606.74 666 4.72 .03 E 1,645.04 657 43.01 .00

Note: The ACE model consists of additive genetic factors (A), shared or common environmental factors (C), and unshared environmen- tal factors (E). MZ = monozygotic; DZ = dizygotic. There were 278 MZ and 75 DZ twin pairs.

Table 3 Partisanship Direction Model A C E –2 Log-Likelihood df ∆χ2 p

ACE .30 (.00, .71) .37 (.00, .68) .33 (.27, .41) 2,287.26 662 AE .67 (.60, .74) .33 (.26, .40) 2,289.95 663 2.69 .10 CE .64 (.56, .71) .36 (.29, .44) 2,290.05 663 2.79 .10 E 2,434.45 664 147.20 .00

Note: The ACE model consists of additive genetic factors (A), shared or common environmental factors (C), and unshared environmen- tal factors (E). MZ = monozygotic; DZ = dizygotic. There were 278 MZ and 75 DZ twin pairs.

Table 4 Liberal/Conservative Intensity Model A C E –2 Log-Likelihood df ∆χ2 p

ACE .13 (.00, .51) .32 (.27, .48) .61 (.49, .73) 1,754.73 677 AE .41 (.28, .52) .59 (.48, .72) 1,755.78 676 1.06 .30 CE .38 (.26, .49) .62 (.51, .74) 1,754.95 676 0.22 .64 E 1,791.01 678 36.28 .00

Note: The ACE model consists of additive genetic factors (A), shared or common environmental factors (C), and unshared environmen- tal factors (E). MZ = monozygotic; DZ = dizygotic. There were 278 MZ and 75 DZ twin pairs.

Previous research indicates a modest genetic influ- although genes appear to play a role in how strongly ence on vote choice (e.g., choice between Labor ver- we attach to a given party, we do not find much evi- sus Conservative parties) but suggests that it could be dence that they influence which party will be chosen. attributed to an underlying genetic component in per- We also wondered if these results could be ceived attitudes and perceptions that intermediate the explained by variation in the tendency for individuals relationship with vote choice (Hatemi et al. 2007). to have extreme ideologies. After all, Alford, Funk, Other research indicates that individual differences in and Hibbing (2005) and Hatemi et al. (2007) found political partisanship can be attributed mostly to the that the direction of political attitudes (liberal vs. environment (Eaves, Eysenck, and Martin 1989; conservative) is heritable. If so, then ideological Olson, Vernon, and Jang 2001; Bouchard et al. 2003). intensity might also be heritable, helping to explain To test these theories in our sample, we repeated the the link from genes to ideology to partisanship. In analysis using a directional measure of partisanship other words, genes might help explain who becomes in Table 3. Although the ACE model was the best fit an extreme liberal or conservative and, therefore, of any of the models tried, we do not find a signifi- who becomes a strong Democrat or Republican. To cant role for heritability in describing the direction of test for this possibility, we ran an ACE model on partisanship. This result is consistent with prior work strength of ideology, a scale that folds over the tradi- suggesting the heritability of party choice is low tional liberal-conservative scale (see note 2). Table 4 (Alford, Funk, and Hibbing 2005). In other words, shows that we cannot reject the null hypothesis that

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 7 strength of ideology is not heritable. A is estimated to 2008; Dawes and Fowler 2008), political attitudes be 13 percent but not significantly different from 0. (Alford, Funk, and Hibbing 2005; Hatemi et al. 2007), Therefore, our strength of partisanship result cannot conservatism (Alford, Funk, and Hibbing 2005; Settle be explained as merely the by-product of a genetic et al. 2008), and trust (Cesarini et al. 2008). It also contribution to extreme ideological orientations. reinforces the importance of examining the role of Finally, it is important to note the possible effects genetic factors in explaining political behavior instead of assortative mating on our results. One assumption of focusing solely on a multitude of environmental of the ACE model is that the distribution of parent variables. genotypes is independent. If partisan strength is par- One implication of these results is that previous tially heritable and if strong partisans tend to have conceptions of the transmission and acquisition of children with other strong partisans, for example, partisanship should be reformulated. Strength of par- then there will be an increased concordance in parti- tisan attachment—and identification as an indepen- sanship strength in their children. However, this pos- dent (Mattei and Niemi 1991)—has in the past been sibility actually serves to make it more difficult to attributed to political socialization from parent to detect differences between MZ and DZ twins. For child (Campbell et al. 1960; Converse 1969; Niemi instance, if a trait follows a pattern of perfect assorta- and Jennings 1991; Hyman 1959; Greenstein 1965). tivity and is 100 percent genetically transmitted, we However, our results suggest that the correlation would observe a concordance of 1 for both MZ and between parent and child partisan behavior is more DZ twins. The finding that the concordance between likely to result from shared genes than the family the two types of twins is the same would suggest that environment. In fact, in our model we cannot reject heritability does not contribute to the expression of the hypothesis that the common environment has no the trait. Consequently, high assortativity tends to impact on strength of partisan attachment whatsoever. bias downward the estimate of heritability. If people Of course, given the broad literature on partisanship choose mates based in part on their disposition to par- and parental socialization, we doubt the effect of tisanship, then the ACE model estimates will be con- parental socialization is really zero, but these results servative and the contribution of heritability will do suggest that we can rule out common familial actually be underestimated. experience as a major contributing factor to partisan- ship strength. Our results also speak to the literature on the sta- Discussion bility and nature of partisanship over time. Partisanship was originally thought to be stable and We find that heritability plays a significant role in long-enduring (Campbell et al. 1960; Converse 1969), partisanship, accounting for almost half of the variance but this finding has been challenged (Abramson 1976, in strength of partisan identification. This heritability is 1979a, 1979b; Fiorina 1981; Popkin 1991; Achen probably not an artifact of ideological orientation since 1992; Mackuen, Erikson, and Stimson 1989; Brody strength of ideology is not significantly heritable in the and Rothenberg 1988; Converse and Markus 1979; same sample. Nor is it an artifact of heritability in the Meier 1975; Page and Jones 1979). Exposing a role direction of partisanship, which also fails to be signif- for heritability in determining partisan strength intro- icant for this sample. Instead, variation in the decision duces an important, previously uncharacterized, to identify with any political party appears to be explanation into the durability debate. Genetic strongly influenced by genetic factors. expression is stable, and we show here that genes Our findings replicate the work conducted by Peter explain some variance in the strength of partisan Hatemi in his 2007 dissertation and in Hatemi et al. attachment. Therefore, although we only observe (forthcoming). Using a different sample, with data individuals at a single point in time, we would expect collected almost twenty years after the data analyzed individuals to exhibit some degree of stability in their in Hatemi’s work, we find nearly identical results. partisan behavior. In this sense, our findings can help This replication across samples and time should reconcile the debate in the literature as to whether reduce the concern that these findings are isolated or partisanship is affective and immutable or changeable accidental. Furthermore, our finding builds on previ- and responsive to current conditions. If the stable ous research demonstrating a genetic basis for other component of partisanship is conceived of as a forms of political behavior, such as voter turnout genetic predisposition toward group affiliation, then (Fowler, Baker, and Dawes 2008; Fowler and Dawes the short-term effects that change partisanship

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 8Political Research Quarterly strength could be viewed as the unshared environ- However, the selection of a particular political mental factors that combine with genetic predisposi- party or a specific religious denomination probably tions to change partisanship strength in an individual. has little impact on fitness. Instead, it is strongly cor- This would be consistent with Scarr and related with environmental influences, such as demo- McCartney’s (1983) theory of how genetic and envi- graphic and socioeconomic factors (Alford, Funk, and ronmental differences combine to produce variation Hibbing 2005; Eaves, Eysenck, and Martin 1989; in development. They argued that the role of geno- Olson, Vernon, and Jang 2001; Bouchard et al. 2003). type determines which environments are experienced Thus, it is possible that genes more generally impact by individuals and which environments individuals the inclination and strength of a predisposition toward seek for themselves; essentially, people seek out political behavior, not its direction. This is consistent experiences to reinforce their genetic predispositions. with the finding that strong identifiers who switched Those inclined to be partisan seek out opportunities parties between elections are more likely to become to do so, which has the effect of strengthening their strong identifiers of the new party than independents partisan attachment even further. This finding helps or weak identifiers (Katz 1979). There is little reason clarify some of the debate over the endogenous ver- to think that there should be a genetic basis for spe- sus exogenous nature of partisan strength by demon- cific group attachment, as the organization, principles, strating roles for both. People may have a genetic and practices of groups change over time. In other predisposition toward developing strong attachment words, genes may contribute to the tendency toward to a political party, but there is still room for this pre- group attachment but not necessarily the groups with disposition to be shaped by both shared and unique which an individual will choose to associate. environments. Also of interest is conceptually linking the Another implication of our results is that we might genetic tendency toward partisan attachment with better think of the acquisition of partisanship in two other behaviors like political participation. Knowing distinct parts, one strongly influenced by genes and that there is a heritable component to partisan the other strongly influenced by the environment. The attachment suggests that we must reexamine the literature has already conceptualized party identifica- theoretical mechanism by which partisan strength tion as consisting of two components: a direction affects these other behaviors. For example, strong component, which indicates the specific party with partisans are more likely to vote for a candidate of which an individual identifies; and a strength compo- their party whom they do not like because of party nent, which reflects the intensity of that identification loyalty and attitudes about split-ticket voting (Converse 1976). Our results suggest that partisan (Campbell, Gurin, and Miller 1954), and strength of intensity is heritable but partisan direction is not. partisanship is one of the best predictors of straight- Our finding is consistent with the pattern of findings ticket voting (Campbell et al. 1960; Brody, Brady, from studies of other social behaviors, such as reli- and Heitshusen 1994). Strength of partisanship is gious beliefs and practices. Strength of attachment to a also related to political knowledge (Delli Carpini group, such as strength of partisanship or religiosity, and Keeter 1996), the motivation to vote (Fowler has a strong heritable component, perhaps because of 2006), and the formation of attitudes toward new its relationship to fundamental processes in early candidates and public policy issues (Campbell et al. human history. For example, we can imagine that the 1960; Converse and Markus 1979). Does a genetic strength of one’s affiliation and association with predisposition toward partisan attachment also pre- groups was of more consequence when survival dispose a person to these other political behaviors, depended more directly on group cooperation. or does affiliation with a party itself mediate these Evolutionary models of cooperation show that some other outcomes? environments favor group participation in the produc- There are several limitations in our study. First, tion of public goods, while other environments favor our sample is relatively small and self-selected. self-reliance because of the competition between con- While we do not think that this has biased our analy- tributors and free-riders (Fowler 2005; Hauert et al. sis in any way, we must keep the limitations of the 2007). These models predict heterogeneity in strate- sample in mind when generalizing to the population gies, with some individuals joining groups and others at large. Excluding those younger than age eighteen trying to survive on their own. It is possible that this from our sample means that we are not examining the heterogeneity extends to several kinds of groups, role of heritability in the initial stages of partisan including religious organizations and political parties. attachment and instead are focusing on the strength

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 9 of attachment once it has been formed. Second, our to move toward a more explanatory basis for the measure of partisan attachment, while frequently research program (Fowler and Schreiber 2008), and employed in the literature, has been subject to the cri- scholars have made significant strides in this direction tique that it does not adequately address the theoreti- (Alford and Hibbing 2008; Medland and Hatemi forth- cal differences between partisans and nonpartisans; coming). The first step, like the one we take here, is to partisanship appears to be multidimensional and non- establish which political behaviors are heritable and monotonic (Kamieniecki 1988; Petrocik 1974), and which are not. Scientists in other fields continue to consequently, standard scales of partisanship strength explore the mechanisms by which genetic variation may not adequately address how partisanship is cor- affects general psychological and behavioral tenden- related with political behavior. The 7-point scale used cies. The next step for political scientists is to apply in our study may be best for measuring attitudes this knowledge in the search for specific genes, neural toward political parties in general because the main and physiological mechanisms that may underlie polit- differences between subgroups of independent voters ical behaviors, as well as their potential evolutionary is in their orientation toward the symbols of political basis (McDermott, Fowler, and Smirnov 2008). In par- independence (Craig 1985). However, a scale such as ticular, it is extremely important to combine our new that discussed in Greene (2002) or Weisberg and understanding of biology with our prior investigations Hasecke (1999) may better measure the social- of environmental causes of political behavior. psychological identity aspect of partisan strength. For example, Fowler, Baker, and Dawes (2008) Third, the ACE model used in this study estab- originally showed that political participation is heri- lishes that genes do play a role in partisan attachment, table, and this study was followed by a molecular but it cannot expose the exact mechanism by which study showing an association between MAOA, 5HTT, genes and the environment interact to produce the and voter turnout (Fowler and Dawes 2008). In par- phenotype. To best understand partisan attachment, ticular, the association with 5HTT was moderated by in the future, we must also examine these interaction church attendance, which ceased to be have a signif- effects. The significant contribution of the unshared icant main effect on turnout. Since 5HTT has previ- environment in our study opens the door for an exam- ously been associated with social behavior, this ination of factors that could serve as mediators for the suggests that the development of a sense of commu- gene-attachment mechanism. It seems likely that we nity is more likely to be at the heart of past associa- are tapping into a general predisposition toward tions between attendance and turnout rather than the attachment, interest, or engagement and that environ- development of civic skills as some have suggested. mental factors can play an important role in channel- Alford, Funk, and Hibbing (2005) originally ing that predisposition into behavior. showed that ideological orientations were heritable, Although our use of the ACE model does not allow and then their study was followed by a molecular us to specify the contribution of any one gene in par- genetic study (Settle et al. 2008). The molecular ticular, the most likely candidate of genes identified study investigated an association between self- to date is the DRD2 gene (Dawes and Fowler 2008). reported political ideology and the 7R variant of the The A1 allele of this gene has been related to dopamine receptor D4 gene (DRD4), which has pre- decreased dopamine signaling in the brain (Jonsson viously been associated with novelty seeking. Settle et al. 1999), and the consequences of altered et al. (2008) found that the number of friendships a dopamine receptors include social detachment (Breier person has in adolescence is significantly associated et al. 1998; Farde, Gustavsson, and Jonsson 1997; with liberal political ideology among those with Jonsson et al. 1999), social alienation (Hill et al. DRD4-7R. Among those without the gene variant, 1999), antisocial personality disorder (Ponce et al. 2003), there was no association. In other words, it was the and avoidant personality types (Blum et al. 1997). All interaction of a particular gene and a particular envi- four of these behaviors could reasonably be linked ronment that mattered. Oxley et al. (2008) also fol- to a decreased tendency toward group attachment and lowed up the original heritability study with research affiliation, and studies testing for various alleles of in the political physiology of ideological orientations, this gene have shown an association between DRD2, showing that people who support conservative poli- partisan attachment, and voting (Dawes and Fowler cies are more likely to exhibit startle reflexes when 2008). presented with visual and auditory fear stimuli. The resurgence of interest in biopolitics calls for a Thus, the finding that strength of party identifica- systematic approach to the study of political heritability tion is heritable suggests that more work should be

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 10 Political Research Quarterly done. Biology appears to play a role in partisanship, Heritability, or the proportion of the variance in partisan- so the next step is to identify which genes and which ship strength explained by genetic factors, can be estimated as mechanisms are interacting with which kinds of envi- a2/(a2 + c2 + e2). We use the software package MX to estimate ronments to affect partisan identification. Novel this structural equations model (Neale et al. 2006). Since our approaches such as the use of the ACE model and dependent variable is (ordered) categorical, the model genetic association studies have the potential to revo- assumes that latent distribution of partisan strength is normal even though we only observe four distinct values. Therefore, lutionize the way we interpret the political world. In in addition to estimating a2,c2,and e2,the model estimates other parts of political science, the study of institu- three thresholds associated with the underlying distribution. tions has improved our understanding of political outcomes because it helps us understand how legisla- Note: The ACE model consists of additive genetic factors (A), tures, courts, and other bodies are constrained in their shared or common environmental factors (C), and unshared envi- ronmental factors (E). behavior. Similarly, the study of genes potentially promises better understanding of the constraints Notes imposed on basic political psychology. Genes are the institutions of the human body, and we ignore them at 1. We employ a version of the question that reads, “Generally our peril if we want to develop a full understanding speaking, do you usually think of yourself as a Republican, a of human psychology and political behavior. Democrat, or what?” Respondents are forced to select one of seven categories, consisting of strong Democrat; Democrat; inde- Appendix pendent, but closer to Democrats; independent; independent, but closer to Republicans; Republican; or strong Republican. The The ACE Model standard Michigan/National Election Studies (NES) question leads to the same 7-point scale but derives the scale from a two- More formally, the components of the ACE model are question series. derived from known relationships between three observed 2. We measured liberalism and conservatism with the answer statistics (Evans, Gillespie, and Martin 2002): to the following question: “We hear a lot of talk these days about liberals and conservatives. Here is a scale on which the political 2 2 2 2 σ P = σ A + σ C + σ E views that people might hold are arranged from extremely liberal 2 2 to extremely conservative. Where would you place yourself on COVMZ = σ A + σ C 2 2 this scale?” Response options are very liberal, liberal, slightly lib- COVDZ = 1/2σ A + σ C. eral, moderate, slightly conservative, conservative,and very con- servative. Similar to partisan intensity, ideological intensity was a 2 In this equation, σ P is the observed phenotypic variance version of the ideological scale, folding it over to use moderate as (the same for MZ and DZ twins), COVMZ and COVDZ are the minimum value and very conservative and very liberal as the the observed covariances between MZ and DZ twins, and maximum values. 2 2 2 3. The ACE model (additive genetic factors [A], shared or σ A, σ C, and σ E are the variance components for genes, common environment, and unshared environment, respec- common environmental factors [C], and unshared environmental tively. These relationships yield three equations and three factors [E]) assumes that genetic effects are only additive; there- fore there are no dominance effects. However, a dizygotic (DZ) unknowns, so it is possible to infer the unobserved portions correlation that is less than half of the monozygotic (MZ) corre- of variance attributable to each factor. lation is generally considered evidence of dominance. While this The known relationship between the phenotypic variance is the case for our point estimates, the confidence interval for the and the variances of A, C, and E, as well as the relationship DZ correlations is fairly wide due to our small sample size. To between MZ and DZ twin covariance and the variances of A formally test the null hypothesis that the DZ correlation is at least and C, allows for var(A), var(C), and var(E) to be estimated. as large as half the MZ correlation, we bootstrapped one thou- The structural equation specification of our model is as sand MZ and DZ correlation coefficients. We failed to reject the follows: null hypothesis that DZ ≥ 0.5 × MZ (p = .71), suggesting that the ACE model is the correct one to use for estimation. VAR(Partisan Strength) = a2 + c2 + e2 COV(MZ) = a2 + c2 References COV(DZ) = 0.5a2 + c2. Abramson, Paul. 1976. Generational change and the decline of This is a system of three equations and three unknowns, party identification in America: 1952-1974. American so it is identifiable. The parameter estimates are solved for by Political Science Review 70 (June): 469-78. Abramson, Paul. 1979a. Comment: On the relationship between age and party identification. Political Methodology 2 1 a 110− COVMZ 6 (Fall): 447-55. 2 c 0:510 COVDZ : Abramson, Paul. 1979b. Developing party identification: A 0 2 1 = 0 1 0 1 e 111 σPartisan Strength further examination of life-cycle, generational and period @ A @ A @ A Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 11

effects. American Journal of Political Science 23 Campbell, Angus, Philip E. Converse, Warren E. Miller, and (February): 78-96. Donald E. Stokes. 1960. The American voter. Chicago: Abramson, P. R., and C. W. Ostrom. 1991. Macropartisanship: University of Chicago Press. An empirical reassessment. American Political Science Campbell, A. G. Gurin, and W. E. Miller. 1954. The voter Review 85:181-92. decides. Evanston, IL: Row, Peterson. Abramson, P. R., and C. W. Ostrom. 1992. Response. American Cesarini, David, Christopher T. Dawes, James H. Fowler, Magnus Political Science Review 86:481-86. Johannesson, Paul Lichtenstein, and Björn Wallace. 2008. Abramson, P. R., and C. W. Ostrom. 1994a. Question wording Heritability of cooperative behavior in the trust game. and partisanship. Public Opinion Quarterly 58:21-48. Proceedings of the National Academy of Sciences 105: 3721-26. Abramson, P. R., and C. W. Ostrom. 1994b. Response. American Converse, Philip E. 1969. Of time and partisan stability. Political Science Review 88:955-58. Comparative Political Studies 2 (July): 139-71. Achen, Christopher. 1992. Social psychology, demographic vari- Converse, Philip E. 1972. Change in the American electorate. In ables, and linear regression: Breaking the iron triangle in vot- The human meaning of social change,ed. Angus Campbell ing research. Political Behavior 14 (3): 195–211. and Philip Converse. New York: Russell Sage Foundation. Alford, John, Carolyn Funk, and John Hibbing. 2005. Are politi- Converse, Philip E. 1976. The dynamics of party support: Cohort cal orientations genetically transmitted? American Political analyzing party identification. Beverly Hills, CA: Sage. Science Review 99:153-67. Converse, Philip E. 1979. Rejoinder to Abramson. American Alford, John, and John Hibbing. 2008. The new empirical biopol- Journal of Political Science 23 (February): 97-100. itics. Annual Review of Political Science 11:183-203. Converse, Philip, and Gregory Markus. 1979. Plus ca change . . . : Ashenfelter, Orley, and Alan Krueger. 1994. Estimates of the eco- The new CPS election study panel. American Political Science nomic return to schooling from a new sample of twins. Review 73:32-49. American Economic Review 84 (5): 1157-73. Cowden, Jonathan, and Rose McDermott. 2000. Short-term forces Beer, Jeremy M., Richard Arnold, and John C. Loehlin. 1998. and partisanship. Political Behavior 22 (3): 197-222. Genetic and environmental influences on MMPI factor scales: Craig, Stephen. 1985. Partisanship, independence, and no pref- Joint model fitting to twin and adoption data. Journal of erence: Another look at the measurement of party identifi- Personality and Social Psychology 74 (3): 818-27. cation. American Journal of Political Science 29 (2): Blum, K., P. Sheridan, T. Chen, R. Wood, E. Braverman, and 274-90. J. Cull. 1997. The dopamine d2 receptor gene locus in reward Dawes, Christopher T., and James H. Fowler. 2008. Partisanship, deficiency syndrome: Meta analyses. In Handbook of voting, and the dopamine D2 receptor gene. SSRN working Psychiatric Genetics, ed. Blum, K. and Noble, E.P., 407-434. paper. http://ssrn.com/abstract=1089365. Boca Raton, Florida: CRC Press. Delli Carpini, Michael X., and Scott Keeter. 1996. What Bouchard, T. J. 1998. Genetic and environmental influences on Americans know about politics and why it matters. New adult intelligence and special mental abilities. Human Biology Haven, CT: Yale University Press. 70 (2): 257-79. Eaves, Lyndon J. 1977. Inferring the causes of human variation. Bouchard, T. J., and M. McGue. 2003. Genetic and environmen- Journal of the Royal Statistical Society 149 (3): 324-55. tal influences on human psychological differences. Journal of Eaves, L. J., H. J. Eysenck, and N. G. Martin. 1989. Genes, cul- Neurobiology 54 (1): 4-45. ture, and personality: An empirical approach. London: Bouchard, T. J., M. McGue, D. Lykken, and A. Tellegen. 1999. Academic Press. Intrinsic and extrinsic religiousness: Genetics and environmen- Evans, D. M., N. A. Gillespie, and N. G. Martin. 2002. Biometric tal influences and personality correlates. Twin Research 2:88-89. genetics. Biological Psychology 61:33-51. Bouchard, T. J., N. Segal, A. Tellegen, M. McGue, M. Keys, and Falconer, D. S., and Trudy Mackay. 1996. Introduction to quan- R. Krueger. 2003. Evidence for the construct validity and her- titative genetics. 4th ed. Essex, UK: Longman. itabilty of the Wilson Patterson Conservatism Scale: A reared Farde, L., J. Gustavsson, and E. Jonsson. 1997. D2 dopamine apart twins study of social attitudes. Journal of Personality receptors and personality traits. Nature 385:590. and Individual Differences 34:959-69. Fiorina, Morris. 1981. Retrospective voting in American national Bouchard, T. J., N. Segal, A. Tellegen, M. McGue, M. Keys, and elections. New Haven, CT: Yale University Press. R. Krueger. 2004. Genetic influence on social attitudes: Fowler, James H. 2005. Altruistic punishment and the origin of Another challenge to psychology from behavior genetics. In cooperation. Proceedings of the National Academy of Behavior genetic principles: Development, personality and Sciences 102 (19): 7047-49. psychopathology,ed. L. DiLalla,pp. 89-104. Washington, Fowler, James H. 2006. Altruism and turnout. Journal of Politics DC: American Psychological Association Press. 68 (3): 674-83. Breier, A., L. Kestler, C. Adler, I. Elman, N. Wiesenfeld, A. Fowler, James H., Laura A. Baker, and Christopher T. Dawes. Malhotra, and D. Pickar. 1998. Dopamine D2 receptor density 2008. Genetic variation in political behavior. American and personal detachment in healthy subjects. American Political Science Review 102 (2): 233-48. Journal of Psychiatry 155:1440–42. Fowler, James H., and Christopher T. Dawes. 2008. Two genes Brody, R. A., D. W. Brady, and V. Heitshusen. 1994. Accounting predict voter turnout. Journal of Politics 70 (3): 579-94. for divided government. In Elections at home and abroad,ed. Fowler, James H., and Cindy D. Kam. 2007. Beyond the self: M. K. Jennings and T. E. Mann. Ann Arbor: University of Social identity, altruism, and political participation. Journal of Michigan Press. Politics 69 (3): 813-27. Brody, Richard, and Lawrence Rothenberg. 1988. The instability Fowler, James H., and Darren Schreiber. 2008. Biology, politics, of partisanship: An analysis of the 1980 presidential election. and the emerging science of human nature. Science 322 British Journal of Political Science 18:445-66. (5903): 912-914 .

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 12 Political Research Quarterly

Franklin, Charles, and John Jackson. 1983. The dynamics of party Martin, N. G., L. J. Eaves, A. C. Heath, Rosemary Jardine, Lynn identification. American Political Science Review 77:957-73. Feingold, and H. J. Eysenck. 1986. Transmission of social Gerber, A., and D. Green. 1998. Rational learning and partisan attitudes. Proceedings of the National Academy of Science 83 attitudes. American Political Science Review 42:794-818. (12): 4364-68. Green, D. P., and B. Palmquist. 1990. Of artifacts and partisan Mattei, Franco, and Richard G. Niemi. 1991. Unrealized parti- instability. American Journal of Political Science 34:872-902. sans, realized independents, and the intergenerational trans- Green, D., B. Palmquist, and E. Schickler. 2002. Partisan hearts mission of partisan identification. Journal of Politics 53 (1): and minds. New Haven, CT: Yale University Press. 161-74. Greene, Steven. 2002. The social-psychological measurement of McDermott, Rose, James H. Fowler, and Oleg Smirnov. 2008. On partisanship. Political Behavior 24 (3): 171-97. the evolutionary origin of prospect theory preferences. Greenstein, Fred. 1965. Children and politics. New Haven, CT: Journal of Politics 70 (2): 335-50. Yale University Press. Medland, Sarah E. and Peter K. Hatemi. forthcoming. Political Hatemi, Peter K. 2007. The Genetics of Political Attitudes. PhD Science, Behavior Genetics and Twin Studies: a Methodological Dissertation, University of Nebraska-Lincoln. Primer. Political Analysis. Hatemi, Peter K., John Hibbing, John Alford, Nicholas Martin, Meier, Kenneth. 1975. Party identification and vote choice: The and Lindon Eaves. Forthcoming. How much of a party is in causal relationship. Western Political Quarterly 28:496-505. your genes? Political Research Quarterly. Miller, Warren E. 1991. Party Identification, Realignment, and Hatemi, Peter K., Sarah E. Medland, K. I. Morley, Andrew C. Party Voting: Back to Basics. American Political Science Heath, and Nicholas G. Martin. 2007. The genetics of vot- Review 85:557-568. ing: An Australian twin study. Behavior Genetics 37 (3): Miller, Warren E., and J. Merrill Shanks. 1996. The new 435-48. American voter. Cambridge, MA: Harvard University Press. Hauert, Christoph, Arne Traulsen, Hannelore Brandt, Martin A. Neale, M. C., S. M. Boker, G. Xie, and H. H. Maes. 2006. MX: Nowak, and Karl Sigmund. 2007. Via freedom to coercion: Statistical modeling. 7th ed. Richmond: Virginia Commonwealth The emergence of costly punishment. Science 316 (5833): University Department of Psychiatry. 1905-7. Neale, M. C., and L. R. Cardon. 1992. Methodology for genetic stud- Hill, S., N. Zezza, G. Wipprecht, J. Locke, and K. Neiswanger. ies of twins and families. Dordrecht, the Netherlands: Kluwer. 1999. Personality traits and dopamine receptor (D2 and D4): Niemi, Richard G., and M. Kent Jennings. 1991. Issues and Linkage studies in families of alcoholics. American Journal of inheritance in the formation of party identification. American Genetics 88:634–41. Journal of Political Science 35:970-88. Huber, J., G. Kernell, and E. Leoni. 2005. Institutional context, Olson, J., P. Vernon, and K. Jang. 2001. The heritability of atti- cognitive resources and party attachments across democra- tudes: A study of twins. Journal of Personality and Social cies. Political Analysis 13:365-86. Psychology 80 (6): 845-60. Hyman, Herbert H. 1959. Political socialization. Glencoe, IL: Oxley, Douglas R., Kevin B. Smith, Matthew V. Hibbing, Free Press. Jennifer L. Miller, John R. Alford, Peter K. Hatemi, and John Jennings, Kent M., Laura Stoker, and Jake Bowers. 2001. R. Hibbing. 2008. Political attitudes are predicted by physio- Politics across generations: Family transmissions reexam- logical traits. Science 321 (5896): 1667-70. ined. Paper WP2001-15, Institute of Governmental Studies. Page, Benjamin, and Calvin Jones. 1979. Reciprocal effects of http://repositories.cdlib.org/igs/WP2001-15. policy preferences, party loyalties, and the vote. American Jonsson, E., M. Nothen, F. Grunhage, L. Farde, Y. Nakashima, Political Science Review 73:1071-89. P. Propping, and G. Sedvall. 1999. Polymorphism in the Petrocik, John. 1974. An analysis of intransitivities in the index dopamine D2 receptor gene and their relationships to striatal of party identification. Political Methodology 1:31-47. dopamine receptor density of healthy volunteers. Molecular Ponce, G., M. Jimenez-Arriero, G. Rubio, J. Hoenicka, Psychiatry 4:290-96. I. Ampuero, J. Ramos, and T. Palomo. 2003. The A1 allele of Jost, J. T. 2006. The end of the end of ideology. American the DRD2 gene (TaqI A Polymophisms) is associated with Psychologist 61 (7): 651-70. antisocial personality in a sample of alcohol-dependent Jost, J. T., J. Glaser, A. W. Kruglanski, and F. J. Sulloway. 2003. patients. European Psychiatry 18:356-60. Political conservatism as motivated social cognition. Popkin, Samuel. 1991. The reasoning voter: Communication and Psychological Bulletin 129 (3): 339-75. persuasion in presidential campaigns. Chicago: University of Kamieniecki, Sheldon. 1988. The dimensionality of partisan Chicago Press. strength and political independence. Political Behavior 10 (4): Posner, S. F., L. Baker, A. C. Heath, and N. G. Martin. 1996. 364-76. Social contact, social attitudes and twin similarity. Behavioral Katz, Richard. 1979. The dimensionality of party identification: Genetics 26 (2): 123-34. Cross-national perspectives. Comparative Politics 11:147-63. Scarr, Sandra, and Kathleen McCartney. 1983. How people make Koenig, Laura B., Matt McGue, Robert F. Krueger, and Thomas their own environments: A theory of genotype→environment J. Bouchard Jr. 2005. Genetic and environmental influences effects. Child Development 54 (2): 424-35. on religiousness: Findings for retrospective and current reli- Settle, Jaime, Christopher T. Dawes, Peter K. Hatemi, Nicholas A. giousness ratings. Journal of Personality 73 (2): 472-88. Christakis, and James H. Fowler. 2008. Friendships moderate Mackuen, Michael B., Robert S. Erikson, and James Stimson. an association between a dopamine gene variant and polit- 1989. Macropartisanship. American Political Science Review ical ideology. SSRN working paper. http://ssrn.com/abstract= 42:661-89. 1186260.

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Settle et al. / The Heritability of Partisan Attachment 13

Tesser, Abraham. 1993. The importance of heritability in psycho- Lawrence Wrightsman, 681-736. San Diego, CA: Academic logical research: The case of attitudes. Psychological Review Press. 100 (1): 129-42. Weisberg, Herbert F., and Edward B. Hasecke. 1999. The psy- Timpone, R. J. 1998. Ties that bind—Measurement, demograph- chological underpinnings of party identification. Paper pre- ics, and social connectedness. Political Behavior 20 (1): 53-77. sented at the annual meeting of the Midwest Political Science Turkheimer, E. 2000. Three laws of behavior genetics and what they Association, Chicago. mean. Current Directions in Psychological Science 9:160-64. Whiteley, P. 1988. The causal relationships between issues, can- Verba, Sidney, Kay Lehman Schlozman, and Henry E. Brady. didate evaluations, party identification, and vote choice—The 1995. Voice and equality: Civic voluntarism in American view from “Rolling Thunder.” Journal of Politics 50:961-84. politics. Cambridge, MA: Harvard University Press. Wise, P. M., J. L. Hansen, D. R. Reed, and P. A. S. Breslin. Weisberg, Herbert F. 1999. Political partisanship. In Measures of Forthcoming. Twin study of the heritability of recognition political attitudes, ed. John Robinson, Phillip Shaver, and thresholds for sour and salty taste. Chemical Senses.

Downloaded from prq.sagepub.com at UNIVERSITE DE MONTREAL on December 30, 2010 Do Genes Contribute to the ‘‘Gender Gap’’?

Peter K. Hatemi Virginia Commonwealth University Sarah E. Medland Queensland Institute of Medical Research Lindon J. Eaves Virginia Commonwealth University

The nature and mechanisms underlying the differences in political preferences between men and women continues to be debated with little consideration for the biology of sex. Genetic influences on social and political attitudes have been reported for each sex independently, yet neither the magnitude nor sources of genetic influences have been explored for significant differences between males and females. In a large sample of adult twins, respondents indicated their attitudes on contemporary social and political items. Finding significant differences in the magnitude of genetic, social, and environmental variance for political preferences, and the potential for different genes in males and females to influence these phenotypes, we provide evidence that sex modulates the effects of genetic and environmental differences on political preferences.

t is undeniable that human males and females tend While the concept of sex is explicit, the sources of to behave differently. It is widely accepted across inherent differences between males and females are I disciplines that both physiology and the environ- not. In addition to hormonal and chromosomal dif- ment make profound contributions to behavioral ferences (females have two X chromosomes while differences, and there is considerable evidence that males inherit one X and one Y chromosome), behavior these differences are in part due to genetic variation, differences may arise from variation in the timing or which may differ between the sexes (Eaves, Eysenck, magnitude of gene action, at times triggered by envi- and Martin 1989). While it is widely held that humans ronmental stimuli. Differences observed between males have evolved to possess the ability to transcend their and females are likely due in part to a combination of biology (Lumsden and Wilson 1981) and enjoy the these effects. For example, females are two to three capacity to do what they choose, their preferences are times more likely to suffer from migraines and dif- biologically influenced. However, relatively few stud- ferences in hormones such as estrogen explain some ies have explored the role of endogenous differences of this variation (Ligthart et al. 2006). In some in- on political preferences (for exceptions see Alford, stances only with certain environmental factors, such Funk, and Hibbing 2005; Fowler, Baker, and Dawes as stress, are sex differences in hormonal and genetic 2008; Hatemi et al. 2007; Hatemi et al. N.d.). Those influences on migraines present (Brandes 2006). Thus, which exist provide little clarity on whether or how even for biomedical traits, by no means is the envi- these mechanisms differ between the sexes. ronment unimportant. It very well may be that for In the biological sciences, sex is a term related to social traits the environment is the most salient in- fundamental reproductive ability; simply whether one fluence, as social behaviors take place in a social is male or female. In political research most studies context. combine gender and sex and primarily view gender as While the topic of sex differences is highly charged a social construct. Thus, most examinations of sex dif- in politics and the media, a more complete under- ferences focus on social conditions, such as gender standing of the interplay between biology and culture roles. Political scientists typically assume that sex dif- in the convoluted developmental pathway from DNA ferences reflect differences in social identity, which are to complex social behavior is a necessary step towards primarily due to differences in socialization (Chodorow resolving the biological and social origins of sex dif- 1995). As such, the vast majority of political studies ferences. Merely demonstrating that there are differ- convolute the independent effects of social forces and ent genetic pathways to behavior between the sexes anatomical sex. and that this may explain some of the manifest

The Journal of Politics, Vol. 71, No. 1, January 2009, Pp. 262–276 doi:10.1017/S0022381608090178 Ó 2009 Southern Political Science Association ISSN 0022-3816

262 genes and gender gap 263 differences in behavior in no way implies that the role ethic of rights or justice (e.g., women are caretakers of gender defined socially cannot in turn modulate and nurturers whereas men are not). While these the expression of otherwise similar biological influences differences are believed to be socially constructed, even on behavior. Ultimately ‘‘gender roles’’ may modu- Gilligan acknowledges that ‘‘Clearly these differences late the expression of ‘‘genes’’ as a function of social arise in a social context where factors of social status context. However, whether stimulated by different and power combine with reproductive biology to environmental pressures or different hormonal and shape the experience of males and females’’ (1982, 2). neurochemical pathways, the same gene or group of Neuropsychological factors, physiological differ- genes may exert different influences on political pre- ences, genetic influences, or differences in the biological ferences for each sex. Genetic influence is no more environment (e.g., hormones introduced in utero) deterministic than social influence, and in order to largely remain unaddressed as either directly influ- better explain the source of differences in political encing political preferences, or providing a source for preferences between men and women, both genetic the differences in ‘‘psychology’’ (Bussey and Bandura and social influences that affect political behavior are 1999; Correll 2004). This is inconsistent with scholar- considered. ship outside of political science. For example, Money and Ehrhardt (1972) reported that females who were Gender and Politics: the Traditional treated with androgens when still in the womb (nor- Discourse mally only present in male fetuses) displayed similar behavior patterns as males throughout their lives. The Most literature on political behavior assumes sex dif- ‘treated’ females were more vigorously active, self- ferences are socialized and remains silent about ge- assertive and competitive, preferred outdoor activities, netic or biological influences (Carroll 1988; Chodorow played with ‘‘male’’ toys, and took greater interest 1978, 1995). The main focus in the literature points in hierarchical positioning. In contrast to their peer toward the greater attraction of the Democratic Party group, once reaching adulthood, romance and mar- to women than men and the greater support women riage took second place to career advancement and give to liberal issue positions (Cook and Wilcox 1991). personal achievement. Explanations of this ‘‘gender gap’’ focused on policy With few exceptions, the literature examining sex preferences such as abortion, equal rights, violence, differences on political behaviors have taken little note foreign policy, minority rights, equity, and criminal of these and similar findings and seldom addressed justice (Manza and Brooks 1998; Norrander 1999; the importance of biological conditions related to Smith 1984; Togeby 1994). Men were more likely to sex. Rather, the focus remained on social, cultural, support the use of force or violence whereas women personal, and economic conditions (Howell and Day were less willing to use force in foreign policy and less 2000). However, an important leap in the political apt to support war (Carroll 1988). It was argued that science literature opened the door to incorporate phy- women were more concerned with crime as they see siological, biological, and evolutionary theses. Build- themselves as more vulnerable, were more in favor ing upon the work of Eaves (1977), and Martin et al, of stronger punitive measures for rape and domestic (1986), Alford, Funk, and Hibbing (2005) found sub- violence, and were more likely to convict those accused stantial genetic sources of variation for a wide array of violent crimes. However, women were also more of political attitudes. However, while implying differ- supportive of prevention policies, gun control meas- ences in genetic variation may exist between the sexes, ures, and treatment for criminal behavior (Applegate, the study did not test for significant differences in the Cullen, and Fisher 2002; Hurwitz and Smithey 1998; magnitude of genetic influence or explore the poten- Lambert 2003; Lerner and Keltner 2000). On public tial for different genes to account for the genetic in- protection or ‘‘compassion’’ issues such as education, fluence on political attitudes between men and women. welfare, minorities, the poor, sick, elderly, or un- employed, women were substantially more likely to support liberal positions (Shapiro and Mahajan 1986; Trevor 1999). An Evolutionary Explanation for Sex A considerable portion of the research on sex Differences in Political Behavior differences for political preferences has been based on Gilligan’s (1982) theoretical premise: women and men Interdisciplinary scholarship offers alternative theories approach values from different perspectives. Women’s to explain sex differences on modern day social roles ‘‘psychology’’ is based on an ethic of care versus men’s by including inherent sources of behavior (Campbell 264 peter k. hatemi, sarah e. medland, and lindon j. eaves

2002; Corning 2004; Cosmides, Tooby, and Barkow males’ greater interest in political activities is a re- 1992; Johnson 1993). The field of evolutionary psy- flection of these evolved preferences with specific con- chology stresses that modern roles, including political cern for the establishment and maintenance of social behaviors, are evolutionarily novel. Consequently, dominance and status. evolutionary selection pressures have modified brain Contemporary observations in early childhood structures, hence behavior, and too little time has reinforce these theories. Cross cultural studies of passed since prehistoric conditions for most of these children found that the largest behavior differences structures and foundations of behaviors to have been between the sexes occurs in children between three and substantially modified. Specifically, human abilities six years old, providing evidence that complex social to mitigate the problems of today did not evolve as an behaviors are inherited as well as learned (Maccoby adaptation to today’s environment, but are modifi- and Jacklin 1974; Money 2002). Males develop stron- cations of previously evolved skills from a prevailing ger aggressiveness, outwardly directed predispositions, environment that occurred over the last 2 million greater self assertiveness, contentious tendencies, ad- years. This evolutionary model has been used to ex- venturousness, and the need for individual achieve- plain the pathways of a wide variety of contemporary ment early on. Whereas females develop a more behaviors including: the nuclear family, antipathy actively sympathetic disposition, inward directed for strangers, self-motivated learning, exploratory be- demeanor, maternal impulses, heightened domestic, havior, territoriality, fear, ethical behavior, authority social, and personal concerns, as well as more interest acceptance, cooperation, play, group affiliation, aggres- in art, linguistics, and literature at a very early age sion, competitiveness, and hierarchical groups (e.g., (Cosmides and Tooby 2000; Geary 1998). Political Fehr and Fischbacher 2003; Fehr and Gachter 2000). scientists typically attribute these differences to social- Accordingly, evolutionary forces in certain do- ization. However, if socialization were the determi- mains differed between males and females in prehis- nant, the gap should widen with age or have a more toric societies, and these differing forces have led to acute effect when children are fully aware of and innate processes which contribute to behavioral differ- immersed in their social environment, and not in the ences between the sexes. Because males and females three-to-six-years-old range. shared most adaptive problems in primitive societies, the majority of human behavioral and physical adap- The Heuristic Value of Genetics tations are sexually anamorphic (e.g., all healthy humans have a prefrontal cortex, sleep, etc.). However, Evolutionary psychology strives to explain inherent adaptive problems which were continually disparate differences in behavior by reference to the long history between the sexes, such as child bearing, are expected of evolutionary adaptation. However, empirical tests to have led to sexually differentiated specializations and of this theory require approaches different from trad- preferences (Buss 1999; Tooby and Cosmides 1990). itional social science designs (e.g., Alford, Funk, and A core evolutionary pressure for women has been Hibbing 2005). The greater complexity and hetero- childbirth and childcare. Thus, it is argued that geneity of modern society compared to the Pleisto- women have evolved greater sensitivity than men to cene era (e.g., dating websites, abortion clinics, defense those in need, a stronger focus on mate selection, and of marriage acts, condoms, suicide bombers, etc.), a more acute empathetic disposition (Gangestad and presents a problem for the use of evolutionary expla- Simpson 2000; Gangestad and Thornhill 1997). In nations on modern political preferences. Specifically, similar fashion, men have evolved psychological mec- the evolutionary psychology focus on universals offers hanisms that place more value on social status, in- limited scope for empirical exploration and it is not creased competitiveness, and risk taking because the readily apparent how to test if ‘‘yesterday’s’’ biology is limiting resource of males’ reproductive success had influencing ‘‘today’s’’ political behaviors differently to do with the availability of fertile women, and be- for each sex (Kanazawa 2001; Oota et al. 2005). cause women had a propensity to choose males who However, there are clues that the genetic study are of higher status (Low 2005; McBurney, Zapp, and of individual differences may provide some insight Streeter 2005; McDermott, Fowler, and Smirnov 2008). into evolutionary history and modern behavior be- Moreover, the need for cooperative alliances and re- cause natural selection modifies the expression of gard for status and hierarchy in hunting and warfare genetic and environmental differences differently in has heightened the desire for males to establish group response to different adaptive challenges (Fisher 1930; alliances (Huberman, Loch, and o¨nculer 2004; Lerner 1954; Mather 1966). While the relationship Wrangham et al. 1999). According to Pratto (1996), between ‘‘heritability’’ and ‘‘selection’’ is unresolved genes and gender gap 265 in population genetics, human or otherwise, if sex Variance components analyses are highly infor- really has been responsible for major differences in mative in identifying genetic influence, but they evolution we expect the genetic system of today to cannot identify exactly which genes are responsible bear some footprints of natural selection. for the genetic influence. Genome wide and allelic Although there are no studies we know that empir- analyses identify regions of the genome seemingly ically examine the potential for different genetic path- correlated with a given trait, and how a specific gene ways on political attitudes between men and women, influences a trait, respectively. Numerous candidate or differentiate the variance for political traits between genes have been identified in the study of personal biological sex and social forces, behavior geneticists temperament and social behaviors and offer a starting have been addressing similar issues for some time point for exploring which genes could be related to (Martin et al. 1986). Behavioral genetic techniques political phenotypes. For example, the androgen genes developed in an attempt to understand why individ- ADRA2 and PNMT have been found to influence uals in a population differ from one another and in- harm avoidance and cooperativeness (Deupree et al. clude both genetic and environmental influences 2006); dopamine receptors DRD3, DRD4, COMT, (Medland and Hatemi forthcoming). These analyses and MAOa are correlated with risk-taking behavior, are concerned with accounting for variation around a inattention, impulsivity, spirituality, self-transcendence, population mean, but estimate effects of covariates high neuroticism, low extraversion, anxiety, reward on means. Instead of testing to see if opinions on the dependence, aggression, antisocial behavior, and voter death penalty are genetic, one would test if individual turnout (Chen et al. 1999; Fowler and Dawes 2008; differences in opinions on the death penalty are in- Harpending and Cochran 2002). In the glutamate fluenced by genes. Variance is partitioned into esti- and nicotinic systems, GRIN1 and CHRNA4 have mates of genetic, social, and unique environmental been shown to be related to cooperativeness; and in influences, thereby providing a means to explore the serotonin system, 5-HT2a, SLC6A4, 5-HTTLPR, whether genetic and social influences on certain be- and TPH are related to impulsivity, harm avoidance, haviors are more or less pronounced in either sex and assertive behavior, anxiety, aggression, creative ex- if the source of genetic influence differs between pression, and depression (Caspi et al. 2003; Hariri males and females. et al. 2002). The potential for differences in genetic influence The most profound genetic sex differences exist between the sexes have been explored for many social in the vasopressin and oxytocin systems. The genes traits (e.g., Eaves, Eysenck, and Martin 1989), however OXTR (oxytocin), and AVPR1a (vasopressin) are heritable differences between the sexes on political related to affiliative behaviors, mating, pair-bonding, preferences have not. The trait most commonly asso- parenting, and trust. These neuropeptides play an ciated with political preferences in the psychology important role in the regulation of affiliation, social and genetics literature is personality. There is over- attachment, and social recognition. While there is very whelming evidence, in numerous large-scale studies limited exploration of vasopressin and oxytocin in across cultures and continents, that personality is sub- human social behavior (for an exception see Prichard stantially influenced by genes (e.g., Bouchard 1997; et al. 2007 and Gordon et al. 2008), the investigation Jang, Livesley, and Vernon 1996). However, the evi- of the neurobiological pathways underlying complex dence is mixed regarding different genetic influences social behaviors are widely explored in animal studies by sex. Finkel and McGue (1997) found no evidence (Williams et al. 1994). Repeated experiments find oxy- that different sets of genes influences personality in tocin promotes pair bonding and mating in female each sex, nor did they find that the same genes prairie voles, but has no significant effect on males influence each sex differently for the higher order (Young et al. 2001). However, in males, but not dimensions of Emotionality. However for dimensions females, higher vasopressin uptake increases pair bond- of Alienation, Control, and Absorption the same genes ing and has been shown to be associated with increased contributed differently to the total variance for males investment in childcare (Hammock and Young 2005; and females. In one of the largest studies on person- Pitkow et al. 2001; Winslow et al. 1993). Humans and ality, Eaves et al. (1999) found that the magnitude of voles share certain neurochemical pathways, and the genetic influence differed for Extraversion, but not aforementioned studies offer a basis to explore the Neuroticism, and provided confirmatory evidence that potential for different genes in human males and while the magnitude of genetic influence differed, the females to influence similar behaviors. same genes were influencing personality in males and Rather than ignore the potential for endogenous females. influences that differ between the sexes, we challenge 266 peter k. hatemi, sarah e. medland, and lindon j. eaves the assumption that biological pathways to political Variance components estimates provide a means behaviors are the same for both sexes. Males and to partition the relative importance of individual varia- females are biologically different and should interact tion for each trait. However, any trait with a significant with and respond to the environment differently. The genetic influence does not mean that there is ‘‘a’’ gene relationship between the genetics of ‘‘now’’ and what for that trait or that the trait is an item under intense may or may not have happened historically remains evolutionary selection. One school of thought suggests ambiguous. However, if human’s current genetic that traits strongly related to fitness should be less dispositions are the result of their evolutionary past, heritable because genetic differences are removed by we expect significant differences to be present in the selection. Extensive published literature reviews give magnitude of genetic influence between men and only partial support for this view. It is also suggested women on many political issues, similar to studies that nonadditive genetic effects will predominate for which explored personality. More importantly, for so called ‘‘fitness traits’’ (Keller 2007). This view has political preferences expected to be modern repre- some empirical support in other species but is diffi- sentations of constructs that were under intense cult to demonstrate in humans due to the absence selection pressures which differed between the sexes, of controlled mating. Among the many mechanisms it is likely that different genes are responsible for that have been invoked to account for the persistence differing genetic variation between the sexes. This of genetic polymorphisms are: adaptive interdepend- expectation is strengthened by the recent findings on ence of traits; adaptive significance of allele changes the vastly different role vasopressin has in males and with shifting environment; frequency dependent se- oxytocin in females for pair bonding, mating, and lection; disruptive selection; and gene by environ- childcare. Therefore, we expect political preferences ment interaction. Any or all of the above may prevail which affect reproductive success and mating to be in a given historical context. While any attempt to influenced by different genes in males and females. relate the presence or absence of genetic variation to adaptive significance of particular traits should be met with caution, it is clear that strong directional selection will, if anything, eradicate genetic differ- Methods ences and not enhance them.

Behavior genetic population samples are often cen- Sample tered on twins, and the family members and peers of twins. While intentionally sampling nonindependent The Virginia 30,000 Health and Life Style Question- individuals violates statistical assumptions of inde- naire (VA30K) consist of the kinships of twins aged pendence, explicitly modeling the degree of relatedness 18–88 years. Here we limit our analyses to the 14,763 for the individuals in the sample corrects for these twins, including opposite sex twin pairs. The VA30K violations. The power of twin samples is based on the includes assessments of political and social attitudes, knowledge that monozygotic twins are genetically sociodemographics, personality traits, and life events. identical, whereas dizygotic twins arise from two dif- Completed questionnaires were received from 70% of ferent eggs fertilized by different sperm. Using twin pairs twins invited to participate (for details on sampling raised by the same parents in the same environment technique and demographics see Lake et al. 2000). provides a natural experiment controlling for the familial The political attitudes assessment consisted of a environment (see Medland and Hatemi forthcoming). 28-item contemporary attitudes inventory based on Using structural equation modeling (SEM), var- the Wilson-Patterson (1968) format. Three items iance can be decomposed into additive genetic (A), (Democrats, Republicans, Liberals) were excluded as common environment (C), and unique environment a more appropriate item, Party Identification, was (E) and tested for sex differences. ‘‘Additive genetic’’ is available. Church attendance (Religiosity) was also the combined genetic effect of all genes (inherited included (see Table 1 for the list of 27 items used in traits). ‘‘Common environment’’ includes that which the analyses). Previous analyses of the differences in is shared by family members and includes cultural attitudes between sexes for the VA30K are consistent norms and familial socialization. ‘‘Unique environ- to findings in the general political behavior scholar- ment’’ refers to all environmental stimuli that are ship. Attitudes on the Death Penalty, Living Together, unique to the individual (i.e., personal experience). Abortion, Nuclear Power, Capitalism, the Draft, Re- Without retest measures unique environment includes publicans, Property Tax, and Military Drill were measurement error. supported more by males, whereas attitudes on Gay genes and gender gap 267

1 TABLE 1 Quantitative Sex Limitation Variance Components (95% confidence intervals)

Parameter Estimates Females Males Modela a2 c2 e2 a2 c2 e2 Da2 Dc2 Live Together ACEbc .51 (.412.68) .16 (.102.24) .33 (.302.37) 0 (.002.34) .48 (.212.54) .52 (.522.58) .51 .32 Busing ACEb .31 (.162.31) .09 (.082.20) .60 (.552.65) .12 (002.40) .30 (.062.45) .58 (.502.66) .19 .21 Divorce ACEbc .25 (.162.29) .23 (.082.38) .52 (.472.57) .42 (.312.42) 0 (.002.07) .57 (.532.65) .17 .23 School Prayer ACEb .32 (.162.48) .37 (.222.51) .31 (.272.36) .47 (.222.62) .21 (.092.41) .32 (.262.40) .15 .16 Capitalism AEb .47 (.432.52) — .53 (.482.57 .61 (.542.67) — .39 (.332.46) .14 .00 Abortion ACEbc .26 (.122.41) .41(.272.53) .33 (.292.37) .38 (.162.51) .19 (.102.37) .43 (.362.50) .12 .22 Foreign Aid ACEb .40 (.292.45) .01 (.00.-10) .59 (.552.64) .31 (.082.49) .11 (.002.31) .58 (.512.66) .09 .10 Women’s Lib ACEbc .34 (.182.49) .18 (.052.18) .48 (.442.53) .31 (.232.39) 0 (.002.03) .69 (.612.76) .03 .18 Religiosity ACEbc .48 (.322.67) .26 (.092.41) .25 (.222.29) .47 (.002.65) .18 (.042.56) .35 (.352.45) .01 .08 Censorship AEbcd .38 (.332.42) — .62 (.582.67) .38 (.332.42) — .62 (.582.67) .00 .00 Death Penalty ACEbd .35 (.222.48) .21 (.102.31) .44 (.40.-48) .35 (.222.48) .21 (.102.31) .44 (.40.-48) .00 .00 Pacifism AEbd .31 (.272.35) — .69 (.652.73) .31 (.272.35) — .69 (.652.73) .00 .00 Segregation AEbcd .37 (.322.37) — .63 (.592.68) .37 (.322.37) — .63 (.592.68) .00 .00 Draft AEbd .37 (.322.41) — .63 (.602.68) .37 (.322.41) — .63 (.602.68) .00 .00 X-Rated AEbcd .51 (.472.56) — .49 (.462.54) .51 (.472.56) — .49 (.462.54) .00 .00 Modern Art AEbcd .40 (.362.43) — .60 (.572.64) .40 (.362.43) — .60 (.572.64) .00 .00 Moral Majority AEbd .42 (.382.47) — .58 (.532.62) .42 (.382.47) — .58 (.532.62) .00 .00 Property Tax AEbd .42 (.412.46) — .58 (.582.63) .42 (.412.46) — .58 (.582.63) .00 .00 Socialism AEbd .38 (.342.38) — .62 (.582.66) .38 (.342.38) — .62 (.582.66) .00 .00 Immigration AEbd .46 (.462.49) — .54 (.512.54) .46 (.462.49) — .54 (.512.54) .00 .00 Party ID CEbcd — .81 (.782.84) .19 (.162.22) — .81 (.782.84) .19 (.162.22) .00 .00 Astrology AEb .47(.432.47) — .53 (.482.57) .47 (.392.54) — .53 (.462.61) .00 .00 Gay Rights ACEbd .34 (.242.45) .25 (.222.34) .41 (.392.45) .34 (.242.45) .25 (.222.34) .41 (.392.45) .00 .00 Military Drill AEbd .36 (.312.40) — .64 (.632.69) .36 (.312.40) — .64 (.632.69) .00 .00 Unions AEbd .41 (.362.46) — .59 (.542.64) .41 (.362.46) — .59 (.542.64) .00 .00 Fed Housing AEb .41 (.362.46) — .59 (.542.64) .41 (.362.46) — .59 (.542.64) .00 .00 Nuclear Power AEbd .34 (.302.39) — .65 (.612.65) .34 (.302.39) — .65 (.612.65) .00 .00

Notes: (a) Only best fitting models shown (thresholds corrected for age). ACE represents a model in which all three variance components are significant; CE represents a model in which common and unique environmental influences are significant; AE represents a model in which genes and unique environment are significant; a2,c2, and e2 represent additive genetic, common environmental, and unique environmental influence respectively; D represent the difference between males and females (b) equated thresholds for MZ and DZ pairs (c) equated thresholds for males and females (d) equated variance components for males and females.3 1Portions of the quantitative sex limitation analyses were initially included as part of a larger dissertation project (see Hatemi 2007).

Rights, Federal Housing, Liberals, Democrats, Busing, place between males and females. However, while not Censorship, Modern Art, Unions, Divorce, School Prayer, all items are ideal, the index is constructed of a wide Socialism, Moral Majority,andPacifism were supported variety of items based on five underlying factors: sex more by females (Eaves et al. 1999). and mating, force and militarism, economics and The Wilson-Patterson (WP) inventory is admin- property, politics and group affiliation, and religion istered by presenting subjects with a stimulus phrase (see Eaves et al. 1999). Items related to mating and such as ‘‘Abortion’’ and eliciting an ‘‘agree,’’ ‘‘dis- sex (Living Together, X-rated, Divorce, Abortion, Gay agree,’’ or ‘‘uncertain’’ response. As with any measure, Rights) are appropriate for fitness hypotheses, while it can be argued that individual WP items may not be the items that are not provide the opportunity to ideal measures to test biological, evolutionary, or even falsify the hypotheses. political hypotheses. From an evolutionary perspec- tive, items which affect reproductive success (fitness) Analyses are the most likely to be influenced by different genes, and many of the attitudes do not appear to be items on Employing a maximum likelihood approach minimizes which divergent selection pressures would have taken the discrepancies between observed and predicted 268 peter k. hatemi, sarah e. medland, and lindon j. eaves covariance/variance matrices (i.e., the goodness-of- (C) were 1.0 in both MZ and DZ twin pairs, as both fit), estimates the model fit of the data for a range of twins were raised by the same parents, in the same parameter values, and converges at the solution when environment. The latent factor for the unique envi- it locates the parameters which produce the lowest ronment (E) is uncorrelated. The difference between log-likelihood. The parameter values that produce the genetic correlation of MZ and DZ twins provides the best solution are estimates of the magnitude of the leverage for estimating the genetic contribution to the latent sources of variance (A, C, and E). The individual differences. reliability of these estimates expressed as 95% con- Common in behavior genetic analyses, observed fidence intervals are the deviations from the esti- frequencies for the ordinal political attitude measures mates that result in a significant change in the fit were calculated and incorporated into a threshold of the model. This approach to the estimation of model that assumes that each variable has an under- genetic and environmental variance is commonly lying normal distribution of liability. In order to cor- used for the analysis of genetic and environmental rect for age effects, we used a full-information approach components in twin data (Medland and Hatemi fitting the structural models to the raw data. Thresh- forthcoming). olds are expressed as z values which discriminate be- Genetic models were fit to the political items; Mx tween categories that correspond to the frequency of 1.60, a matrix algebra application widely used for the political attitudes. Thresholds were tested for sim- genetic analysis, was used for model fitting. The path ilarity across sex and twin zygosity groups and cor- model for twin resemblance is represented in Figure 1. rected for age effects. Correlations between the latent additive genetic fac- In order to investigate the presence and nature of tors (A) are 1.0 for monozygotic twins (MZ) who genetic differences in the variance between the sexes, share 100% of their genes and 0.5 for dizygotic twins a sex limitation parameter is included (noted as AMale (DZ), including opposite sex pairs, who share on in Figure 1). The general approach to fitting a sex- average 50% of their discriminating genes. Correla- limitation model to twin data is described by Heath, tions between the latent common environment factors Jardine, and Martin (1989).The full sex-limitation model permits quantitative sex differences, which assumes the same sources of variation for males and females (same genes), but allows for differences FIGURE 1 Sex Limitation ACE Model in the magnitude of genetic and environmental effects for each trait, and qualitative sex differences, where 1 the model allows for different genes to influence the behavior for each sex. Opposite-sex twins are neces- MZ=1/DZ=0.5 sary in this process as they share the same familial environment, but experience it as members of the E C A A C E opposite sex, thereby providing a natural experiment to examine sex differences. The correlation between genetic effects in unlike sex pairs will be a function of eacea c Amale the extent to which the same genes affect both males am and females. In the extreme case, where completely different genes are expressed in males and females, Pmale Pfemale the genetic correlation is expected to be zero. The genetic correlation between opposite-sex twin pairs (OS) may be less than 0.5 if there are qualitative Notes: This path diagram is an explicit representation of the model where the expected covariance between two variables is sex specific genetic effects. If dropping the sex specific computed by multiplying together all the coefficients in a chain, genetic effect does not significantly alter the fit of the and then summing over all possible chains. For example, the model (if the difference in fit between the models is not variance of a phenotypic trait for a MZ twin is calculated as: significant), this indicates that although the genetic (a * 1 * a) + (c * 1 * c) + (e * 1 * e) = a2 + c2 + e2. “A” is additive effects may differ in their magnitude between males genetic 2*(rMZ-rDZ), “C” is common environment (2*rDZ-rMZ) and females, the influence is from the same genetic and “E” is unique environment (1 - r ). As indicated by the sex MZ source regardless of sex. Alternatively, if the model specific subscript for the path coefficients (Amale) the model allows a, c, and e, to be estimated separately for each sex. The sex which includes the separate sex specific ‘‘A’’ parameter specific parameter can be placed on either one of the sexes (but results in a better fitting model, the results indicate that only one, males in this example) and provides the same results. some, but not necessarily all, the genetic influences on genes and gender gap 269 the trait are specific to one sex (i.e., a qualitative sex 1980s. Defined sex roles are present in many aspects difference). of U.S. culture, and populations from other regions of the United States in different time periods are Model Fitting necessary for replication. Sex-limitation models assume that opposite-sex Model fitting is used as a method of hypothesis twin pairs are influenced by the common environment testing to determine the relative importance of gene- to the same extent as same-sex twin pairs for the trait tic and environmental influence. The full model, which under analysis. While it is implausible that parents’ includes three sources of variance (ACE), (a fourth would raise their opposite-sex offspring in a dissim- sex-specific source in the qualitative model), was ilar manner for political behaviors, it is also likely that tested against progressively reduced models by com- boys grow up as ‘‘future men’’ and girls as ‘‘future paring the fit of the model in which the component women.’’ Differences in peer groups, social activities, (A, C or E) is freely estimated to the fit of the model and other social processes may influence environ- in which either the A or C parameter has been set to mental similarity. However, any dissimilarity would zero. The difference in model fits, assuming that the have to affect the trait of interest. While there is no models are nested, is asymptotically distributed as a indication that potential sex differences in rearing exist chi-square distribution with the degrees of freedom for political behaviors (e.g., parents raising daughters equal to the difference in the number of estimated to be pro-choice and sons pro-life, etc.), a more ex- parameters between the two models. For example, to tended family model including parents would pro- compare the difference in fits between an ACE model vide more insight. (in which the covariation is due to the combination of It is also assumed that any differences in the additive genetic, common environmental and unique similarity of environments between DZ and MZ co- environmental effects) and a CE model (in which the twin pairs do not affect the trait under examination. covariation is due to common environmental and Empirical support for this assumption and its im- 2 environmental effects only) against a x1 distribution, plications have been widely discussed, and numerous the A parameter is dropped from the full model and methods have been employed to validate the assump- model fit is compared. Quantitative sex differences tion (for a review see Kendler and Gardner 1998). (different magnitude of genetic influence) were tested These include testing twin environmental similarity by comparing the fit of nested models that equated by controlling for perceived zygosity versus true zygo- the separate path coefficients for males and females to sity (rated by self- reports, parents, friends, teachers, the full model. Qualitative sex differences (different etc.), modeling self-report and parental report of genes) were assessed by comparing the fit of Model 1, parental treatment, including ratings of physical in which correlations between genetic or environ- similarity, as well as specific environmental measures. mental latent factors were estimated to the fit of There are extremely few traits in which unequal envir- Model 2 in which these correlations were fixed to 0.5 onments have an influence and none so far reported and 1.0, respectively. for political preferences. While unequal environ- ments may be found for dressing alike as toddlers, Model Limitations it has no influence on co-twin similarity of political behaviors later in life. Rather, what scholarship is Consistent with all empirical analyses, there are available provides evidence that MZ and DZ co-twin assumptions and limitations built into sex-limitation correlations for political preferences are statistically twin analyses. Univariate estimates do not account the same in adolescence, but greatly differ once for the possibility of gene by environment interaction children leave home (Hatemi et al. 2008). Critics of (GxE) or gene-environment covariation (rGE). More the CTD claim childhood is precisely the time period advanced twin models that include the effects of where differences in the shared environment influ- genes and environments can provide information on ence MZ and DZ twin pairs differently for political how genetic influences are expressed through and by preferences (see Charney 2008). This is apparently environmental conditions (for more on GxE and rGE, not the case. Rather, it appears that the parental see Medland and Hatemi forthcoming). Importantly, (shared) environment is responsible for keeping DZ as with all population-based samples, results are pop- co-twins more similar. However, despite these findings, ulation specific, and generalizations should be made and the numerous explorations of the twin method, only after the results are replicated on different pop- and its acceptance in the genetic, psychiatric, and ulations. Our sample is from a U.S. population in the general science scholarship, the ‘‘twin debate’’ has 270 peter k. hatemi, sarah e. medland, and lindon j. eaves found a new home in political science (e.g., Charney one of the most important changes in our conception 2008; Suhay, Kalmoe, and McDermott 2007). This is of the condition since the first pioneering descrip- not unsurprising, as political scientists are only now tions’’ (Hill and Frith 2003, 282). becoming more familiar with the genetics method- ology and literature. However, the focus of these challenges are to a large degree limited to those com- Results mitted to a theory that only social forces influence social behaviors. These include philosophical objec- The magnitude of genetic variance differed between tions to applying scientific methods to the analysis of males and females for most traits, providing strong political behavior (e.g., ‘‘They’re trying to make evidence confirming the hypothesis that social forces political science into a science, when really it’s not’’; cannot account for all the differences in individual Evan Charney quoted in Cohen 2008). Often, critics variation within and between males and females (see choose not to address the most recent and relevant Table 1). Rather, there is substantial evidence that for literature on familial modeling, including models many political preferences genes are having markedly which provide estimates of twin-specific environ- different influences on behavior in males and females. mental variance (e.g., Eaves et al. 1999; Eaves and In 9 of the 27 items significant sex differences in the Hatemi 2008; Hatemi et al. 2007b). Nor have they magnitude of genetic variance were present: Living addressed DNA based techniques that validate the use Together (.51), Busing (.19), Divorce (.17), School Prayer of twin designs. Visscher et al. (2006) used nontwin (.15), Capitalism (.14), Abortion (.12), Foreign Aid sibling’s exact genetic similarities (DNA) in a var- (.09), Women’s Lib (.03), and Church Attendance (.01). iance components model, thereby removing the as- While the difference in the magnitude of genetic sumption of equal environments. Using this method influence found between the sexes on political attitudes Visscher et al. (2006) reported genetic estimates for confirms the need to address sex when exploring genetic height similar to those reported using twin models. sources of political preferences, interpreting the quan- While the method has not yet been used for political titative results to address specific hypotheses based attitudes, the method provides strong evidence that upon evolutionary foundations are not as straight MZ/DZ models are robust. The method is free from forward.1 The magnitude of genetic variance was not the criticisms assigned to twin models and is available necessarily greater or lesser in females for political for critics to falsify the equal environment assump- items that address those in need, evoke emotional tion on a trait-level basis. This is important as none responses, build stronger domestic support systems of the critiques (e.g., Beckwith and Morris 2008; or tap into an ‘‘ethic of care.’’ Nor was there less or Charney 2008; Suhay, Kalmoe, and McDermott 2007) more genetic influence on attitudes regarding the use provide any empirical evidence of unequal environ- of force, status, or group alliances in males or that mental bias on political traits or attempt to empiri- men have more genetic variance for most political cally test the potential for unequal environments. items. The findings are trait specific, akin to sex dif- Disregarding the overwhelming evidence that ferences on personality, and offer little recognizable necessitates the inclusion of inherent influences on pattern. human behaviors, there are also those who believe However, the remarkable difference in the genetic that the exploration of inherent mechanisms which estimates of Living Together solicits further discussion. influence social behaviors is equivalent to the sexist, In the full model (all sources of variation included), xenophobic, and racist eugenics movements of the last genetic influence on Living Together accounted for century. Beckwith maintains that exploring genetic 0% of the variance in males, but over 50% in females. influences for social traits can only lead to ‘‘absolving Though not intended to identify different genetic of society of any responsibility for its inequities’’ (1993, 332). However, this moralistic argument is highly selective, and ignores important genetic dis- 1Highly adaptive traits are expected to show smaller genetic coveries that have lead to increased social tolerance, variability if the traits were under intense selection (Falconer including the now discredited refrigerator mother hyp- 1981; Fisher 1930). If males or females were faced with certain selection pressures for political preferences unique to one sex, the othesis, which blamed autism on mother’s emotional amount of genetic variance should actually be very small. This frigidity (see Bettelheim 1967). Rather, twin studies does not undermine genetic influences; while the variance is not are credited with changing the discourse of the medi- genetic, the mean effect may be. However counterintuitive, evolutionary designs may predict lower genetic variance in the cal community and public image of mothers with sex for which the item is most likely related to selection, as the autistic children, ‘‘The heritability of autism has been traits that matter most are the most tightly regulated. genes and gender gap 271 sources, when quantitative analyses result in sizeable somes, influence political behaviors that directly or genetic influence for one sex, while none for the other indirectly influence reproduction (i.e., Divorce and (absent of model fitting), the findings offer reason to Living Together). However, it might equally be ar- suspect that different genetic influences are present gued that attitudes toward Abortion, Gay Rights, and on the trait, or that possibly gene by gene interaction X-rated are also what one might expect to be connected specific to one sex may be present. with fitness, equally evolutionarily novel, and under Furthermore, in most items (20 out of 27) the divergent selection pressures for males and females. If magnitude of common environmental influence was true, these items would also be expected to be poten- similar between males and females. In general, the tially influenced by different genes. This is apparently common environment accounts for individual differ- not the case. Thus, while not presuming to make a ences in females to the same degree as males, but general claim in the absence of overwhelming evidence affects the sexes differently for each specific item. that different genes for males and females influence Unique environmental influences accounted for the all political traits related to fitness, the findings on majority of the variance in all attitudes except Party Divorce and Living Together offer a prima facie case Identification for males and School Prayer, Abortion, for considering the evolutionary rationale for sex and Party Identification for females. Whether evalu- differences on political preferences related to fitness. ating traditional social science theories or evolu- However, the reduced DZ opposite-sex correla- tionary theories, there is no evident pattern to the tion does not by itself preclude the critical impor- quantitative variance component analyses, except to tance of sex roles, because it might be argued that say that for individual differences in political prefer- the sex limitation of genetic differences may be a ences, in general genes matter, and often influence (phylogenetically) recent response to greater social males and females to differing degrees depending on emphasis on the differentiation of sex roles. While we the trait. In many ways, this finding runs contrary to can find no evidence in our analyses to support this what is inferred by the extant literature. Any attempt claim, it is possible to consider that social gender roles to adopt a universal explanation for the magnitude of are so important that they even modify the expres- sex differences on all political behaviors is not sup- sion of genes differentially in the two sexes. ported by the data. Rather, a theory specific to each In summary, the analyses provide empirical evi- item or possibly each group of related items appears dence that the same genes influence males and females to be appropriate. differently for many political preferences. The data also support the conclusion that modern political Qualitative Results: Different Sources of preferences related to sexually dimorphic adaptive traits Genetic Influence (such as reproduction), are the most likely candidates for different sources of genetic influences in males Possibly the most intriguing aspect of sex limitation and females; a finding which supports themes long modeling is the potential that different genes may be suspected by evolutionary theorists. influencing political preferences for either sex. Table 2 presents the results of the qualitative model fitting for sex differences on the 27 social and political items and provides evidence that different genes may be Discussion influencing the preferences of males and females for one attitude, Divorce. A model in which genes specific Cultural and biological theorists agree that there are to one sex fits marginally better than a model where basic differences in the outlooks and preference for- the same genes are influencing attitudes on Divorce mation of males and females, but the source of these (3.62 Dx2 for 1 d.f., p5.057).2 differences remains an area of disagreement. The anal- If the evolutionary psychology theses are to be yses here provide evidence that differences between believed, for certain traits females and males have men and women go beyond obvious superficial anat- been under divergent natural selection pressures for omical contrasts or how they are treated by members tens of thousands of years, and it is not surprising in society. For better or worse, genetic influences are that different genes, potentially on the sex chromo- manifested in many aspects of political behaviors and these influences differ for males and females, both in 2 We found no evidence of significantly different sources of common magnitude and sources of variation. environment influence across sexes on any attitudes. However, for attitudes on Divorce and Women’s lib, no such test was possible, as The findings support the conclusion that the the common environmental estimate was zero in males. tortuous developmental pathway from genes to 272 peter k. hatemi, sarah e. medland, and lindon j. eaves

TABLE 2 Qualitative Sex Limitation Model Fitting for Political Attitudes

-2LL Model comparison Estimates of correlations p-values Genetic correlation Model Sex Specific A (1) fixed A to.5 (2) 1 vs. 2 (1df) between males and females Divorce 24250.37 24253.99 .057 .000 Living Together 21937.77 21939.81 .153 .085 Federal Housing 22572.87 22572.91 .847 .135 Womens’ Lib 24217.81 24217.81 1.000 .160 Abortion 23248.04 23249.16 .290 .288 Military Drill 21946.05 21946.16 .747 .306 Gay Rights 22456.74 22456.76 .916 .475 Moral Majority 24986.62 24986.63 .924 .435 Socialism 21453.74 21453.74 1.000 .500 Censorship 24511.31 24511.31 1.000 .500 Unions 24969.25 24969.25 1.000 .205 Death Penalty 19070.80 19073.59 .095 .141 Nuclear Power 24720.96 24720.96 1.000 .500 Capitalism 23238.28 23238.28 1.000 .245 Immigration 24915.10 24915.10 1.000 .186 Segregation 20363.50 20363.50 1.000 .257 School Prayer 18162.34 18163.85 .219 .292 Modern Art 25000.80 25000.80 1.000 .498 Astrology 24346.51 24346.51 1.000 .500 Foreign Aid 25379.29 25379.29 1.000 .500 Busing 22814.54 22814.58 .838 .430 X-rated Movies 18650.62 18650.82 .656 .407 Draft 22264.83 22264.83 1.000 .500 Property Tax 21270.39 21270.39 1.000 .500 Pacifism 22172.78 22172.78 1.000 .500

Notes: Significant qualitative sex-specific genetic effects noted in bold. Columns 2 and 3 give the - twice log likelihood (-2LL) for model (1) in which the correlation between the latent A variables are estimated for the DZ opposite sex twins, and model (2) in which the correlation between the latent A variables are set to .5 for the DZ opposite sex twins. Column 4 gives the p-values for the 1 degree of freedom chi-square test comparing the -2LL from Model 1 to that of Model 2. Column 5 provides the estimated A correlation between males and females. political preference cannot be simplified to a universal By no means do the findings infer that the social theory that explains all differences between and within environment or personal experience does not have the sexes, or even the preference on one attitude. explanatory capacity, or that it is not the strongest Rather, differences in the magnitude of genetic and determinant in behavior differences. In certain cases environmental variance between the sexes occur for such as Party Identification, it appears the social envi- some political attitudes, but not others. Thus, the data ronment is almost all that matters, for either sex. do not support the broad application of social psy- Rather, the social environment does not account for chology or evolutionary psychology theories to explain the majority of variance in most political preferences behavior differences, or even explain all behaviors in the population sampled. Environmental, familial, suggested as appropriate to either given theory. Ac- and heritable influences each account for different cordingly, we suggest it is necessary to examine poli- amounts of the variance on a trait level basis and are tical preferences on an individual item basis for each likely interactive. Genetic influences for social traits sex. The findings support the claim that the environ- may only be apparent under social conditions. More ment (social or other) cannot be used in isolation to complex models, with specific measures for environ- explain behavior differences between males and females, mental influence, tailored to each individual trait are nor can all differences in modern political behaviors required to explore this possibility. between the sexes simply be attributed to genes or The qualitative analyses provide a much clearer presumptions about primitive man. finding and possibly more radical departure from genes and gender gap 273 traditional social science beliefs. Considering that on issues related to reproductive fitness. Specifically, different genes act in a different manner for males the findings support the use of a more nuanced theory and females to influence political preferences offers that addresses the roles of genes and environment, but an explanation that addresses human biology as well also one that is embedded within a model that is con- as the social environment people live in. If the results sistent with the complex social, environmental, devel- are to be believed and replicated, the need to inves- opmental, and biological elements specific to each sex. tigate genetic systems that act differently in males and Doing so will challenge existing political behavior females for a host of political and social traits is models which only consider social determinants. apparent, with a specific focus on issues related to However, ignoring the influence of inherent sources mating and reproductive success. of political behavior renders contemporary research on The links between genes, neurological chemical sex differences incoherent at a time when the issue of pathways, emotive and cognitive processes, and social sex remains central to the U.S. electorate. behaviors remain understudied. Controlled mating and repeated experiments provide evidence that vole pair-bonding behavior is strongly influenced by in- Acknowledgments ducing or blocking vasopressin uptake in males and regulating oxytocin uptake in females. Only two atti- Data collection was conducted by Lindon Eaves, tudes, Divorce and Living Together, resulted in qual- Nicholas Martin, Andrew Heath, and Kenneth Kendler itative sex differences for genetic influence. The finding and supported by AA-06781 and MH-40828 from is provocative, as these two attitudes are also the issues the National Institutes of Health. Data analysis was most related to pair bonding in our 27-item index. supported by MH-068521 and NHMRC 443036. Model Existing studies on humans are limited, and the development was supported by MH-068521.We thank findings here justify the need for additional research Hermine Maes for assistance with data management on the relationship between these neuropeptides, and Michael Neale for access to the Mx program human affiliative behaviors, and political preferences. for structural modeling. We thank John Hibbing, Human behavior is influenced partly by bio- Matthew Keller, Nicholas Martin, Katherine Morley, logical factors; the precise amounts and mechanisms and the anonymous reviewers for their thoughtful are partly dependent upon sex. This does not mean discussions and insightful comments. genetic sex differences predetermine behavior; quite the contrary, the effect of the environment is equally strong or stronger and significantly different between men and women. Nor should we consider variance Manuscript submitted 19 July 2007 components estimates as absolute mutually exclusive Manuscript accepted for publication 19 July 2008 values. Genetic and environmental influences are inter- active. Here we report findings from an initial univariate References classical twin model; future models may provide more clarity on how these components interact with impor- Alford, John, Carolyn Funk, and John Hibbing. 2005. ‘‘Are tant covariates. Genetic differences in social behavior Political Orientations Genetically Transmitted?’’ American can only be elucidated in a social context, and it could Political Science Review 99 (2): 153–67. be that social roles are modulating the expression of Applegate, B., Cullen, F., and Fisher, B. 2002. ‘‘Public Views genetic sex differences. toward Crime and Correctional Policies: Is There a Gender We are only in the early stages of employing gen- Gap?’’ Journal of Criminal Justice 30 (2): 89–100. etic models to explore political preferences, and more Beckwith, John. 1993. ‘‘A Historical View of Social Responsibility sophisticated analyses with broader environmental in Genetics.’’ BioScience 43 (5): 327–33. and neurological measures, specific to each sex, are Beckwith, John, and Corey A. Morris. 2008. ‘‘Twin Studies of needed to explore the heuristic value and implications Political Behavior: Untenable Assumptions?’’ Perspectives on Politics 6 (4): 785–791. of these initial findings. However, it is clear that the Bettelheim, B. 1967. The Empty Fortress: Infantile Autism and the etiology of political behavior is not homogeneous for Birth of the Self. New York: The Free Press. the sexes. Genes matter and they matter differently for Bouchard, Thomas J. 1997. ‘‘Experience Producing Drive Theory: each sex. The evidence implies that sex differences How Genes Drive Experience and Shape Personality.’’ Acta for political preferences are best explained using an Paediatrica 422: 60–64. approach which includes a combination of factors and Brandes, J. L. 2006. ‘‘The Influence of Estrogen on Migraine: A theories unique to each topic, with a particular focus Systematic Review.’’ JAMA 295 (15): 1824–30. 274 peter k. hatemi, sarah e. medland, and lindon j. eaves

Buss, David. 1999. Evolutionary Psychology: The New Science of Eaves, Lindon J., and Peter K. Hatemi. 2008. ‘‘Transmission of Mind. Needham Heights, MA: Allyn and Bacon. Attitudes toward Abortion and Gay Rights: Parental Social- Bussey, Kay, and Albert Bandura. 1999. ‘‘Social Cognitive Theory ization or Parental Mate Selection?’’ Behavior Genetics 38 (3): of Gender Development and Differentiation.’’ Psychological 247–56. Review 106 (4): 676–713. Falconer, Douglas S. 1981. Introduction to Quantitative Genetics. Campbell, Anne. 2002. A Mind of Her Own. New York: Oxford Edinburgh: Oliver and Boyd. University Press. Fehr, Ernst, and Urs Fischbacher. 2003. ‘‘The Nature of Human Carroll, Susan. 1988. ‘‘Women’s Autonomy and the Gender Altruism.’’ Nature 425: 785–91. Gap.’’ In The Politics of the Gender Gap: The Social Con- Fehr, Ernst, and Simon Gachter. 2000. ‘‘Cooperation and Punish- struction of Political Influence, ed. C. Mueller. Newbury Park, ment.’’ American Economic Review 90 (4): 980–94. CA: Sage, 236–57. Finkel, Deborah, and Matt McGue. 1997. ‘‘Sex differences and Caspi, Avshalom, Karen Sugden, Terrie E. Moffitt, Alan Taylor, nonadditivity in heritability of the Multidimensional Person- Ian W. Craig, Honalee Harrington, Joseph McClay, Jonathan ality Questionnaire Scales.’’ Journal of Personality and Social Mill, Judy Martin, Antony Braithwaite, and Richie Poulton. Psychology 72 (4): 929–38. 2003. ‘‘Influence of Life Stress on Depression: Moderation by Fisher, Ronald A. 1930. The Genetical Theory of Natural Selection. a Polymorphism in the 5-HTT Gene.’’ Science (18 July): 386–89. Oxford: Clarendon. Charney, Evan. 2008. ‘‘Genes and Ideologies.’’ Perspectives on Fowler, James H., Laura A. Baker, and Christopher T. Dawes. Politics 6 (2): 292–319. 2008. ‘‘The Genetic Basis of Political Participation.’’ American Chen, C., M. Burton, E. Greenberger, and J. Dmitrieva. 1999. Political Science Review 102 (2): 233–48. ‘‘Population Migration and the Dopamine Receptor (DRD4) Fowler, James H., and Christopher T. Dawes. 2008. ‘‘Two Genes Allele Frequencies around the Globe.’’ Evolution and Human Predict Voter Turnout.’’ Journal of Politics 70 (3): 579–94. Behavior 20 (5): 309–24. Gangestad, S. W., and J. A. Simpson. 2000. ‘‘The Evolution of Chodorow, Nancy. 1978. The Reproduction of Mother. Psycho- Human Mating: Trade-offs and Strategic Pluralism.’’ Behav- analysis and the Sociology of Gender. Berkeley: University of ioral and Brain Sciences 23 (4): 573–87. California Press. Gangestad, S. W., and R. Thornhill. 1997a. ‘‘The Evolutionary Chodorow, Nancy 1995. ‘‘Gender as a Personal and Cultural Psychology of Extrapair Sex: The Role of Fluctuating Asym- Construction.’’ Signs 20: 516–44. metry.’’ Evolution and Human Behavior 18 (2): 69–88. Cook, Elizabeth Adell, and Clyde Wilcox. 1991. ‘‘Feminism and Geary, David C. 1998. Male, Female the Evolution of Human Sex the Gender Gap-A Second Look.’’ Journal of Politics 53 (4): Differences. Washington: American Psychological Association. 1111–22. Gilligan, Carroll. 1982. In a Different Voice: Psychological Theory Cohen, Elizabeth. 2008. ‘‘Are Your Politics Rooted in your and Women’s Development. Cambridge, MA: Harvard Uni- Genes?’’ CNN http://www.cnn.com/2008/HEALTH/02/11/ versity Press. politics.genes/index.html (February 11, 2008). Gordon, I., O. Zagoory-Sharon, I. Schneiderman, J. F. Leckman, Corning, Peter A. 2004. ‘‘The Evolution of Politics.’’ In Handbook A. Weller, and R. Feldman. 2008. ‘‘Oxytocin and Cortisol in of EvolutionVol I, ed. F. Wuketits and C. Antweiler. Weinheim: Romantically Unattached Young Adults: Associations with Bond- Wiley-VCH, 191–252. ing and Psychological Distress.’’ Psychophysiology (3): 349–52. Correll, Shelley J. 2004. ‘‘Constraints into Preferences: Gender, Hammock, Elizabeth A. D., and Larry J. Young. 2005. ‘‘Micro- Status and Emerging Career Aspirations.’’ American Socio- satellite instability generates diversity in brain and socio- logical Review 69 (1): 93–113. behavioral traits.’’ Science 308 (5728):1630–34. Cosmides, Leda, and John Tooby. 2000. ‘‘Evolutionary Psychol- Hariri, Ahmad, Venkata Mattay, Alessandro Tessitore, Bhaskar ogy and the Emotions.’’ In Handbook of Emotions, 2nd Kolachana, Francesco Fera, David Goldman, Michael F. Egan, Edition, ed. M. Lewis and J. M. Haviland-Jones. New York: and Daniel R. Weinberger. 2002. ‘‘Serotonin Transporter Guilford, 91–115. Genetic Variation and the Response of the Human Amyg- Cosmides, L., J. Tooby, and J. Barkow. 1992. ‘‘Evolutionary dala.’’ Science 297 (5580): 400–403. Psychology and Conceptual Integration.’’ In The Adapted Harpending, Henry, and Gregory Cochran. 2002. ‘‘In Our Genes.’’ Mind: Evolutionary Psychology and the Generation of Culture, Proceedings of the National Academy of Sciences 99 (1): 10–12. ed. J. Barkow, L. Cosmides, and J. Tooby. New York: Oxford Hatemi, Peter K. 2007. The Genetics of Political Attitudes. Ph.D University Press, 3–15. Dissertation. University of Nebraska-Lincoln. Deupree, J. D., S. D. Smith, C. J. Kratochvil, D. Bohac, C. R. Ellis, Hatemi, Peter K., Cary L. Funk, Sarah E. Medland, Hermine H. J. Polaha, and D. B. Bylound. 2006. ‘‘Possible Involvement of Maes, Judy Silberg, Nicholas G. Martin, and Lindon J. Eaves. Alpha-2a Adrenergic Receptors in Attention Deficit Disorder: 2008. ‘‘Genetic and Environmental Transmission of Political Radioligand binding and polymorphism studies.’’ American Attitudes Across the Life-course.’’ Presented at the annual meet- Journal of Medical Genetics 141B (8): 877–84. ing of the International Society of Political Psychology, Paris. Eaves, Lindon J. 1977. ‘‘Inferring the Causes of Human Varia- Hatemi, Peter K., John Hibbing, John Alford, Nicholas Martin, tion.’’ Journal of the Royal Statistical Society 140 (3): 324–55. and Lindon Eaves. 2007b. ‘‘We Get Opinions from Our Eaves, Lindon J., Hans J. Eysenck, and Nicholas G. Martin. 1989. Parents, but Not How We Think We Do: Genetic and Social Genes, Culture and Personality; an Empirical Approach. London: Components of the Familial Transmission of Political Atti- Academic Press. tudes’’ Presented at the annual meeting of the Midwest Eaves, Lindon, Andrew Heath, Nicholas Martin, Hermine Maes, Political Science Association. Michael Neale, Kenneth Kendler, Katherine Kirk, and Linda Hatemi, Peter K., Sarah E. Medland, Katherine I. Morley, Andrew Corey. 1999. ‘‘Comparing the Biological and Cultural Inher- C. Heath, and Nicholas G. Martin. 2007. ‘‘The Genetics of itance of Personality and Social Attitudes in the Virginia 30,000 Voting: An Australian Twin Study.’’ Behavior Genetics 37 (3): Study of Twins and Their Relatives.’’ Twin Research 2: 62–80. 435–48. genes and gender gap 275

Hatemi, Peter K., John R. Hibbing, John R. Alford, Nicholas G. Lumsden, Charles. J., and Edward O. Wilson. 1981. Genes, Mind, Martin, and Lindon J. Eaves. N.d. ‘‘Is There a Party in Your and Culture: The Coevolutionary Process. Cambridge, MA: Genes?’’ Political Research Quarterly. Forthcoming. Harvard University Press. Heath, A. C., R. Jardine, and N. G. Martin. 1989. ‘‘Interactive Maccoby, Eleanor, and Carol Jacklin. 1974. The Psychology of Sex Effects of Genotype and Social Environment on Alcohol Differences. Stanford, CA: Stanford University Press. Consumption in Female Twins.’’ Journal of Studies on Alcohol Manza, J., and Brooks, C. 1998. ‘‘The Gender Gap in U.S. 50 (1): 38–48. Presidential Elections: When? Why? Implications?’’ American Hill, Elisabeth L., and Uta Frith. 2003. ‘‘Understanding Autism: Journal of Sociology 103 (5): 1235–66. Insights from Mind and Brain.’’ Philosophical Transactions of Martin, N. G., L. J. Eaves, A. C. Heath, R. Jardine, L. M. Feingold, the Royal Society of London B 358 (1430): 281–89. and H. J. Eysenck. 1986. ‘‘Transmission of Social Attitudes.’’ Howell, Susan E., and Christine L. Day. ‘‘Complexities of the Proceedings of the National Academy of Sciences 83 (12): 4364–68. Gender Gap.’’ Journal of Politics 62 (3): 858–74. Mather, K. 1966. ‘‘Breeding Systems and Response to Selection.’’ Horwitz, Allan, Tami Videon, Mark Schmitz, and Diane Davis. In Reproductive Biology and Taxonomy of Vascular Plants, ed. 2003. ‘‘Rethinking Twins and Environments: Possible Social J. G. Hawkes. New York: Pergamon, 13–19. Structures for Assumed Genetic Influences in Twin Research.’’ McBurney, Donald H., Daniel J. Zapp, and Sybil A. Streeter. Journal of Health and Social Behavior 44 (2): 111–29. 2005. ‘‘Preferred Number of Sexual Partners: Tails of Dis- Huberman, Bernardo A., Christoph H. Loch, and Ayse O¨ nculer. tributions and Tales of Mating Systems.’’ Evolution and 2004. ‘‘Status as a Valued Resource.’’ Social Psychology Quarterly Human Behavior 26 (3): 271. 67 (1): 103–14. McDermott, Rose, James H. Fowler, and Oleg Smirnov. 2008. Hurwitz, Jon and Shannon Smithey. 1998. ‘‘Gender Differences ‘‘On the Evolutionary Origin of Prospect Theory Preferences.’’ on Crime and Punishment.’’ Political Research Quarterly 51 Journal of Politics 70 (2): 335–50. (1): 89–115. Medland, Sarah E., and Peter K. Hatemi. ‘‘Political Science, Jang, K. L., W. J. Livesley, and P. A. Vernon. 1996. ‘‘Heritability Behavior Genetics and Twin Studies: a Methodological Primer.’’ of the Big Five Personality Dimensions and Their Facets: A Political Analysis. Forthcoming. Twin Study.’’ Journal of Personality 64 (3): 577–91. Money, John. 2002. A First Person History of Pediatric Psychoen- Johnson, Gary R. 1993. ‘‘The Evolutionary Origins of Govern- docrinology. New York, London: Kluwer Academic, Plenum ment and Politics.’’ In Human Nature and Politics, ed. J. Losco Publishers. and A. Somit. Bronx, NY: JAI Press. Money, John, and Anke A. Ehrhardt. 1972. Man and Woman, Boy Kanazawa, Satoshi. 2001. ‘‘Where Do Social Structures Come and Girl: Differentiation and Dimorphism of Gender Identity From?’’ Advances in Group Processes 18: 161–83. from Conception to Maturity. Baltimore: Johns Hopkins Univer- Keller, Matthew C. 2007. ‘‘The Role of Mutation in Human sity Press. Mating.’’ In Mating Intelligence: Theoretical, Experimental, and Norrander, Barbara. 1999. ‘‘The Evolution of the Gender Gap.’’ Differential Perspectives, ed. G. Geher and G. Miller. Mahwah, The Public Opinion Quarterly 63 (4): 566–76. NJ: Lawrence Erlbaum, 173–92. Oota, Hiroki, Brigitte Pakendorf, Gunter Weiss, Arndt von Kendler, Kenneth S., and Charles O. Gardner. 1998. ‘‘Twin Haeseler, Surin Pookajorn, Wannapa Settheetham-Ishida, Studies of Adult Psychiatric and Substance Dependence Danai Tiwawech, Takafumi Ishida, and Mark Stoneking. Disorders: Are They Biased by Differences in the Environ- 2005. ‘‘Recent Origin and Cultural Reversion of a Hunter– mental Experiences of Monozygotic and Dizygotic Twins in Gatherer Group.’’ PLoS Biology 3 (3): e71 Childhood and Adolescence.’’ Psychological Medicine 28 (3): Pitkow, Lauren J., Catherine A. Sharer, Xianglin Ren, Thomas R. 625–33. Insel, Ernest F. Terwilliger, andLarryJ.Young.2001.‘‘Facili- Lake, Robert I., Lindon J. Eaves, Hermine H. Maes, Andrew C. tation of Affiliation and Pair-Bond Formation by Vasopressin Heath, and Nicholas G. Martin. 2000. ‘‘Further Evidence Receptor Gene Transfer into the Ventral Forebrain of a Monog- Against the Environmental Transmission of Individual Differ- amous Vole.’’ The Journal of Neuroscience 21 (18): 7392–96. ences in Neuroticism from a Collaborative Study of 45,850 Pratto, F. 1996. ‘‘Sexual Politics: The Gender Gap in the Twins and Relatives on Two Continents.’’ Behavior Genetics Bedroom, the Cupboard, and the Cabinet.’’ In Sex, Power, 30 (3): 223–33. Conflict: Evolutionary and Feminist Perspectives, ed. D. Buss Lambert, E. 2003. ‘‘The Views of Women and Men on Terrorism and N. Malamuth. New York: Oxford University Press, 179–230. and the Punishment of Terrorists: A Preliminary Study among Prichard, Z. M., A. J. Mackinnon, A. F. Jorm, and S. Easteal. Midwestern College Students.’’ Criminal Justice Studies: A 2007. ‘‘AVPR1A and OXTR Polymorphisms Are Associated Critical Journal of Crime, Law, and Society 16 (3): 217–321. with Sexual and Reproductive Behavioral Phenotypes in Lerner, I. M. 1954. Genetic Homeostasis. Edinburgh: Oliver and Humans.’’ Human Mutation (11): 1150. Boyd. Shapiro, Robert Y., and Harpreet Mahajan. 1986. ‘‘Gender Lerner, J. S., and D. Keltner. 2000. ‘‘Beyond Valence: Toward a Differences in Policy Preferences: A Summary of Trends From Model of Emotion-Specific Influences on Judgment and the 1960s to the 1980s.’’ Public Opinion Quarterly 50 (1): 42–61. Choice.’’ Cognition and Emotion 14 (4): 473–93. Smith, Tom W. 1984. ‘‘The Polls: Gender and Attitudes Toward Ligthart, Lannie, Dorret I. Boomsma, Nicholas G. Martin, Janine Violence.’’ Public Opinion Quarterly 48 (1): 384–96. H. Stubbe, and Dale R. Nyholt. 2006. ‘‘Migraine with Aura Suhay, Elizabeth, Nathan Kalmoe, and Christa McDermott. 2007. and Migraine without Aura Are Not Distinct Entities.’’ Twin ‘‘Why Twin Studies Are Problematic for the Study of Political Research and Human Genetics 9 (1): 54–63. Ideology: Rethinking Are Political Orientations Genetically Low, Bobbi. 2005. ‘‘Women’s Lives There, Here, Then, Now: Transmitted?’’ Presented at the International Society of Political A Review of Women’s Ecological and Demographic Con- Psychology. straints Cross-Culturally.’’ Evolution and Human Behavior Togeby, Lise. 1994. ‘‘The Gender Gap in Foreign Policy Atti- 26 (1): 64. tudes.’’ Journal of Peace Research 31 (4): 375–92. 276 peter k. hatemi, sarah e. medland, and lindon j. eaves

Tooby, John, and Leda Cosmides. 1990. ‘‘On the Universality of Visscher, P. M., S. E. Medland, M. A. R. Ferreira, K. I. Morley, G. Human Nature and the Uniqueness of the Individual: The Zhu, B. K. Cornes, G. W. Montgomery, and N. G. Martin. Role of Genetics and Adaptation.’’ Journal of Personality 58 (1): 2006. ‘‘Assumption-Free Estimation of Heritability from 17–67. Genome Wide Identity by Descent Sharing between Full Trevor, Margaret C. 1999. ‘‘Political Socialization, Party Identi- Siblings.’’ PLoS Genetics 2 (3): e41. fication, and the Gender Gap.’’ Public Opinion Quarterly 63 (1): Young, Larry J. Miranda M. Lim, Brenden Gingrich, and Thomas 62–89. R. Insel. 2001. ‘‘Cellular Mechanisms of Social Attachment.’’ Winslow, James, Nick Hastings, Sue Carter, Carroll Harbaugh, Hormones and Behavior 40 (2): 133–38. and Thomas Insel. 1993. ‘‘A Role for Central Vasopressin in Pair Bonding in Monogamous Prairie Voles.’’ Nature 365: 545–48. Williams, J. R., T. R. Insel, C. R. Harbaugh, and C. S. Carter. 1994. ‘‘Oxytocin Administered Centrally Facilitates Formation of a Partner Preference in Female Prairie Voles (Microtus Peter K. Hatemi is an assistant professor of ochrogaster).’’ Journal of Neuroendocrinology 6 (3): 247–50. political science, Virginia Commonwealth University, Wilson, G. D., and J. R. Patterson. 1968. ‘‘A New Measure of Richmond, VA 23298. Sarah E. Medland is a Ph.D. Conservatism.’’ British Journal of Social and Clinical Psychol- candidate in genetic epidemiology, Queensland In- ogy 7 (December): 264–69. stitute of Medical Research, Brisbane, QLD 4029, Wrangham, R., J. Jones, G. Laden, D. Pilbeam, and N. Conklin- Australia. Lindon J. Eaves is a distinguished professor Brittain. 1999. ‘‘The Raw and the Stolen: Cooking and the Ecology of Human Origins.’’ Current Anthropology 40 (5): of human genetics and psychiatry, Virginia Common- 567–94. wealth University, Richmond, VA 23298.