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Why & Nurture Won't Go Away

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Citation Pinker, Steven. 2004. Why nature & nurture won't go away. Daedalus 133(4): 5-17.

Published Version doi:10.1162/0011526042365591

Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:3600799

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Why nature & nurture won’t go away

When Richard Mulcaster referred in that debates over nature and nurture 1581 to “that treasure . . . bestowed on evoke more rancor than just about any them by nature, to be bettered in them issue in the world of . by nurture,” he gave the world a eupho- During much of the twentieth century, nious name for an opposition that has a common position in this debate was to been debated ever since. People’s beliefs deny that existed at all– about the relative importance of heredi- to aver, with José Ortega y Gasset, that ty and environment affect their opinions “Man has no nature; what he has is his- on an astonishing range of topics. Do tory.” The doctrine that the is a adolescents engage in because blank slate was not only a cornerstone of the way their treated them of in and social early in life? Are people inherently ag- constructionism in the social sciences, gressive and sel½sh, calling for a market but also extended widely into main- economy and a strong police, or could stream intellectual life.1 they become peaceable and cooperative, Part of ’s appeal came allowing the state to wither and a spon- from the realization that many differ- taneous socialism to blossom? Is there a ences among people in different classes universal aesthetic that allows great art and ethnic groups that formerly were to transcend time and place, or are peo- 1 Carl N. Degler, In Search of Human Nature: ple’s tastes determined by their era and The Decline and Revival of in American culture? With so much seemingly at Social (New York: Oxford University stake in so many ½elds, it is no surprise Press, 1991); Steven Pinker, The Blank Slate: The Modern Denial of Human Nature (New York: Viking, 2002); Robin Fox, The Search for Soci- ety: Quest for a Biosocial Science and Steven Pinker, Johnstone Professor in the (New Brunswick, N.J.: Rutgers University department of psychology at Harvard University, Press, 1989); Eric M. Gander, On Our : conducts research on and cognition. A How Is Reshaping the Fellow of the American Academy since 1998, he Nature-Versus-Nurture Debate (Baltimore: is the author of six books, including “How the Johns Hopkins University Press, 2003); and , “The Psychological Mind Works” (1997), “The Language ” Foundations of Culture,” in : (2000), and “The Blank Slate” (2002). Evolutionary Psychology and the Generation of Culture, ed. Jerome H. Barkow, Leda Cosmides, © 2004 by the American Academy of Arts and John Tooby (New York: Oxford University & Sciences Press, 1992).

Dædalus Fall 2004 5 Steven thought to reflect innate disparities in that infants have a precocious grasp Pinker talent or temperament could vanish of objects, intentions, numbers, faces, on human through immigration, social mobility, tools, and language. Behavioral nature and cultural change. But another part has shown that temperament emerges of its appeal was political and moral. If early in life and remains fairly constant nothing in the mind is innate, then dif- throughout the life span, that much of ferences among races, sexes, and classes the variation among people within a cul- can never be innate, making the blank ture comes from differences in genes, slate the ultimate safeguard against rac- and that in some cases particular genes ism, sexism, and class prejudice. Also, can be tied to aspects of cognition, lan- the doctrine ruled out the possibility guage, and . Neuroscience that ignoble traits such as greed, preju- has shown that the genome contains a dice, and aggression spring from human rich tool kit of growth factors, axon nature, and thus held out the hope of un- guidance molecules, and cell adhesion limited social progress. molecules that help structure the Though human nature has been debat- during development, as well as mecha- ed for as long as people have pondered nisms of plasticity that make learning their condition, it was inevitable that the possible. debate would be transformed by the re- These discoveries not only have shown cent efflorescence of the sciences of that the innate organization of the brain mind, brain, genes, and . One cannot be ignored, but have also helped outcome has been to make the doctrine to reframe our very conception of nature of the blank slate untenable.2 No one, and nurture. of course, can deny the importance of learning and culture in all aspects of Nature and nurture, of course, are not human life. But has alternatives. Learning itself must be shown that there must be complex in- accomplished by innate circuitry, and nate mechanisms for learning and cul- what is innate is not a set of rigid in- ture to be possible in the ½rst place. Evo- structions for behavior but rather pro- lutionary psychology has documented grams that take in information from the hundreds of universals that cut across senses and give rise to new and the world’s cultures, and has shown that actions. Language is a paradigm case: many psychological traits (such as our though particular such as Jap- taste for fatty foods, social status, and anese and Yoruba are not innate, the ca- risky sexual liaisons) are better adapted pacity to acquire languages is a uniquely to the evolutionary demands of an an- human talent. And once acquired, a lan- cestral environment than to the actual guage is not a ½xed list of sentences, but demands of the current environment. a combinatorial algorithm allowing an has shown in½nite number of new thoughts to be 2 Pinker, The Blank Slate; Gary F. Marcus, The expressed. Birth of the Mind: How a Tiny Number of Genes Moreover, because the mind is a com- Creates the Complexities of Human Thought (New plex system composed of many inter- York: , 2004); , Nature acting parts, it makes no sense to ask Via Nurture: Genes, Experience, and What Makes whether humans are sel½sh or generous Us Human (London: Fourth Estate, 2003); , Michael J. Owen, and Peter or nasty or noble across the board. Rath- McGuf½n, “The Genetic Basis of Complex Hu- er, they are driven by competing motives man Behaviors,” Science 264 (1994): 1733–1739. elicited in different circumstances. And

6 Dædalus Fall 2004 if genes affect behavior, it is not by tug- there is a widespread desire that the Why nature ging on the muscles directly, but by their whole issue would somehow just go & nurture won’t go intricate effects on the circuitry of a away. A common position on nature and away growing brain. nurture among contemporary scientists Finally, questions of what people in- can be summarized as follows: nately have in common must be distin- No one today believes that the mind is a guished from questions of how races, blank slate; to refute such a belief is to tip sexes, or individuals innately differ. Evo- over a straw man. All behavior is the prod- lutionary gives to be- uct of an inextricable interaction between lieve that there are systematic species- and environment during develop- wide universals, circumscribed ways in ment, so the answer to all nature-nurture which the sexes differ, random quantita- questions is “some of each.” If people only tive variation among individuals, and recognized this truism, the political re- few if any differences among races and criminations could be avoided. Moreover, ethnic groups.3 modern biology has made the very dis- This reframing of human nature also tinction between nature and nurture ob- offers a rational way to address the polit- solete. Since a given set of genes can have ical and moral fears of human nature.4 different effects in different environ- Political equality, for example, does not ments, there may always be an environ- hinge on a dogma that people are innate- ment in which a supposed effect of the ly indistinguishable, but on a commit- genes can be reversed or canceled; there- ment to treat them as individuals in fore the genes impose no signi½cant con- spheres such as education and the crim- straints on behavior. Indeed, genes are inal justice system. Social progress does expressed in response to environmental not require that the mind be free of ig- signals, so it is meaningless to try to dis- noble motives, only that it have other tinguish genes and environments; doing motives (such as the emotion of empa- so only gets in the way of productive re- thy and cognitive faculties that can search. learn from history) that can counteract them. The attitude is often marked by words like ‘interactionist,’ ‘developmentalist,’ By now most scientists reject both the ‘dialectic,’ ‘constructivist,’ and ‘epige- nineteenth-century doctrine that biolo- netic,’ and is typically accompanied gy is destiny and the twentieth-century by a diagram with the labels ‘genes,’ doctrine that the mind is a blank slate. ‘behavior,’ ‘prenatal environment,’ ‘bio- At the same time, many express a dis- chemical environment,’ ‘family environ- comfort with any attempt to character- ment,’ ‘school environment,’ ‘cultural ize the innate organization that the mind environment,’ and ‘socioeconomic envi- does have (even in service of a better ronment,’ and arrows pointing from understanding of learning). Instead, every label to every other label. This doctrine, which I will call holistic 3 John Tooby and Leda Cosmides, “On the interactionism, has considerable appeal. Universality of Human Nature and the Unique- It is based on some unexceptionable ness of the Individual: The Role of Genetics points, such as that nature and nurture and ,” Journal of Personality 58 are not mutually exclusive, that genes (1990): 17–67. cannot cause behavior directly, and that 4 Pinker, The Blank Slate. the direction of causation can go both

Dædalus Fall 2004 7 Steven ways (for example, school can make you his culture, from the man-made part Pinker on smarter, and smart people are most en- of the environment, from other human human gaged by schooling). It has a veneer of beings.”6 Postmodernism and social nature moderation, of conceptual sophistica- constructionism, which dominate many tion, and of biological up-to-dateness. of the humanities, vigorously assert that And as John Tooby and Leda Cosmides human emotions, conceptual categories, have put it, it promises “safe conduct and patterns of behavior (such as those across the politicized mine½eld of mod- characterizing men and women or ho- ern academic life.”5 mosexuals and heterosexuals) are social But the very things that make holistic constructions. Even many humanists interactionism so appealing should also who are not postmodernists insist bio- make us wary of it. No matter how com- logy can provide no insight into human plex an interaction is, it can be under- mind and behavior. The critic Louis stood only by identifying the compo- Menand, for instance, recently wrote nents and how they interact. Holistic that “every aspect of life has a biological interactionism can stand in the way of foundation in exactly the same sense, such understanding by dismissing any which is that unless it was biologically attempt to disentangle heredity and en- possible it wouldn’t exist. After that, it’s vironment as uncouth. As Dan Dennett up for grabs.”7 has satirized the attitude: “Surely ‘every- Nor is a belief in the blank slate absent one knows’ that the nature-nurture de- among prominent scientists. Richard bate was resolved long ago, and neither Lewontin, , and Steven side wins since everything-is-a-mixture- Rose, in a book entitled , of-both-and-it’s-all-very-complicated, asserted that “the only sensible thing to so let’s think of something else, right?” say about human nature is that it is ‘in’ In the following pages I will analyze that nature to construct its own his- the tenets of holistic interactionism and tory.”8 wrote that show that they are not as reasonable or the “brain [is] capable of a full range of as obvious as they ½rst appear. behaviors and predisposed to none.”9 Anne Fausto-Sterling expressed a com- “No one believes in the extreme nurture mon view of the origin of sex differ- position that the mind is a blank slate.” ences: “The key biological fact is that Whether or not this is true among scien- boys and have different genitalia, tists, it is far from true in the rest of in- tellectual life. The prominent anthropol- 6 Ashley Montagu, ed., Man and Aggression, 2nd ogist Ashley Montagu, summing up a ed. (New York: Oxford University Press, 1973). common understanding in twentieth- century , wrote in 1973 that 7 Louis Menand, “What Comes Naturally,” The “With the exception of the instinctoid New Yorker, 25 November 2002. reactions in infants to sudden with- 8 R. C. Lewontin, , and Leon J. drawals of support and to sudden loud Kamin, Not in Our Genes: Biology, , and noises, the human being is entirely in- Human Nature (New York: Pantheon Books, stinctless . . . .Man is man because he has 1984). no , because everything he is 9 Stephen Jay Gould, “Biological Potential vs. and has become he has learned . . . from Biological ,” in Ever Since Darwin: 5 Tooby and Cosmides, “The Psychological Reflections in Natural History, ed. Stephen Jay Foundations of Culture.” Gould (New York: Norton, 1977).

8 Dædalus Fall 2004 and it is this biological difference that are women is less than 50 percent, are Why nature & nurture leads adults to interact differently with attributed entirely to prejudice and hid- won’t go different babies whom we conveniently den barriers. The possibility that, on away color-code in pink or blue to make it average, women might be less interested unnecessary to go peering into their dia- than men in people-free pursuits is simi- pers for information about gender.”10 larly unspeakable.13 The point is not that These opinions spill into research and we know that evolution or genetics are policy. Much of the scienti½c consensus relevant to explaining these phenomena, on , for example, is based on but that the very possibility is often studies that ½nd a correlation between treated as an unmentionable taboo rath- the behavior of parents and the behavior er than as a testable hypothesis. of children. Parents who spank have children who are more violent; authori- “For every question about nature and tative parents (neither too permissive nurture, the correct answer is ‘some of nor too punitive) have well-behaved each.’” Not true. Why do people in Eng- children; parents who talk more to their land speak English and people in Japan children have children with better lan- speak Japanese? The ‘reasonable com- guage skills. Virtually everyone con- promise’ would be that the people in cludes that the behavior of the England have genes that make it easier causes the outcomes in the child. The to learn English and the people in Japan possibility that the correlations may have genes that make it easier to learn arise from shared genes is usually not Japanese, but that both groups must be even mentioned, let alone tested.11 exposed to a language to acquire it at all. Other examples abound. Many scien- This compromise is, of course, not rea- ti½c organizations have endorsed the sonable but false, as we see when chil- slogan “violence is learned behavior,” dren exposed to a given language acquire and even biologically oriented scientists it equally quickly regardless of their ra- tend to treat violence as a public cial ancestry. Though people may be ge- problem like malnutrition or infectious netically predisposed to learn language, disease. Unmentioned is the possibility they are not genetically predisposed, that the strategic use of violence could even in part, to learn a particular lan- have been selected for in human evolu- guage; the explanation for why people in tion, as it has been in the evolution of different countries speak differently is other primate species.12 Gender differ- 100 percent environmental. ences in the professions, such as that the Sometimes the opposite extreme turns proportion of mechanical engineers who out to be correct. Psychiatrists common- ly used to blame psychopathology on 10 Anne Fausto-Sterling, Myths of Gender: Bio- mothers. Autism was caused by ‘refrig- logical Theories About Women and Men (New York: Basic Books, 1985). erator mothers’ who did not emotionally engage their children, by 11 David C. Rowe, The Limits of Family Influ- mothers who put their children in dou- ence: Genes, Experience, and Behavior (New York: ble binds. Today we know that autism Guilford Press, 1994); , The Nurture Assumption: Why Children Turn Out the 13 David Lubinski and Camilla Benbow, “Gen- Way They Do (New York: Free Press, 1998). der Differences in Abilities and Preferences Among the Gifted: Implications for the Math- 12 and , Homicide Science Pipeline,” Current Directions in Psycho- (New York: A. de Gruyter, 1988). logical Science 1 (1992): 61–66.

Dædalus Fall 2004 9 Steven and schizophrenia are highly heritable, chology, the possibility that heredity has Pinker on and though they are not completely de- any explanatory role at all is still inflam- human termined by genes, the other plausible matory. nature contributors (such as toxins, pathogens, and developmental accidents) have “The effects of genes depend crucially nothing to do with how parents treat on the environment, so heredity imposes their children. Mothers don’t deserve no constraints on behavior.” Two exam- some of the blame if their children have ples are commonly used to illustrate the these disorders, as a nature-nurture point: different strains of corn may grow compromise would imply. They de- to different heights when equally irrigat- serve none of it. ed, but a plant from the taller strain might end up shorter if it is deprived of “If people recognized that every aspect water; and children with phenylke- of behavior involves a combination tonuria (pku), an inherited disorder of nature and nurture, the political dis- resulting in retardation, can end up nor- putes would evaporate.” Certainly mal if given a diet low in the amino acid many strive for an in- phenylalanine. nocuous middle ground. Consider this There is an aspect of this statement quotation: that indeed is worth stressing. Genes do not determine behavior like the roll of a If the reader is now convinced that either player piano. Environmental interven- the genetic or environmental explanation tions–from education and psychothera- has won out to the exclusion of the other, py to historical changes in attitudes and we have not done a suf½ciently good job of political systems–can signi½cantly af- presenting one side or the other. It seems fect human affairs. Also worth stressing highly likely to us that both genes and en- is that genes and environments may in- vironment have something to do with this teract in the statistician’s sense, namely, issue. that the effects of one can be exposed, This appears to be a reasonable interac- multiplied, or reversed by the effects of tionist compromise that could not pos- the other, rather than merely summed sibly incite controversy. But in fact it with them. Two recent studies have comes from one of the most incendiary identi½ed single genes that are respec- books of the 1990s, Herrnstein and tively associated with violence and de- Murray’s The Bell Curve. In this passage, pression, but have also shown that their Herrnstein and Murray summed up their effects are manifested only with particu- argument that the difference in average lar histories of stressful experience.14 iq scores between American blacks and At the same time, it is misleading to American whites has both genetic and invoke environment dependence to deny environmental causes. A “some-of- 14 Avshalom Caspi, Karen Sugden, Terrie E. each” position did not protect them Mof½tt, Alan Taylor, and Ian W. Craig, “Influ- from accusations of racism and compar- ence of Life Stress on Depression: Moderation isons to Nazis. Nor, of course, did it by a Polymorphism in the 5-htt Gene,” Science establish their position was correct: as (2003): 386–389; Avshalom Caspi, Joseph with the language a person speaks, the McClay, Terrie E. Mof½tt, Jonathan Mill, Judy iq Martin, and Ian W. Craig, “Evidence that the black-white average gap could be 100 Cycle of Violence in Maltreated Children De- percent environmental. The point is that pends on ,” Science 297 (2002): 727– in this and many other domains of psy- 742.

10 Dædalus Fall 2004 the importance of understanding the matoes or pinecones. Chinese foot-bind- Why nature & nurture effects of genes. To begin with, it is sim- ing is an environmental manipulation won’t go ply not true that any gene can have any that can radically affect the shape of the away effect in some environment, with the foot, but it would be misleading to deny implication that we can always design an that the anatomy of the human foot is environment to produce whatever out- in an important sense speci½ed by the come we value. Though some genetic genes, or to attribute it in equal parts to effects may be nulli½ed in certain envi- heredity and environment. The point is ronments, not all of them are: studies not merely rhetorical. The fact that kit- that measure both genetic and environ- tens’ visual systems show abnormalities mental similarity (such as when their eyelids are sewn shut in a designs, where correlations with adop- critical period of development does not tive and biological parents can be com- imply (as was believed in the 1990s) that pared) show numerous main effects of playing Mozart to babies or hanging col- personality, , and behavior orful mobiles in their cribs will increase across a range of environmental varia- their intelligence.16 tion. This is true even for the poster In short, the existence of environmen- child of environmental mitigation, pku. tal mitigations doesn’t make the effects Though a low-phenylalanine diet does of the genes inconsequential. On the prevent severe mental retardation, it contrary, the genes specify what kinds does not, as is ubiquitously claimed, ren- of environmental manipulations will der the person ‘perfectly normal.’ pku have what kinds of effects and with what children have mean iqs in the 80s and costs. This is true at every level, from the 90s and are impaired in tasks that de- expression of the genes themselves (as pend on the prefrontal region of the I will discuss below) to large-scale at- cerebral cortex.15 tempts at social change. The totalitarian Also, the mere existence of some envi- Marxist states of the twentieth century ronment that can reverse the expected often succeeded at modifying behavior, effects of genes is almost meaningless. but at the cost of massive coercion, ow- Just because extreme environments can ing in part to mistaken assumptions disrupt a trait does not mean that the about how easily human motives ordinary range of environments will would respond to changed circum- modulate that trait, nor does it mean stances.17 that the environment can explain the Conversely, many kinds of genuine nature of the trait. Though unirrigated social progress succeeded by engaging corn plants may shrivel, they won’t grow speci½c aspects of human nature. Peter arbitrarily high when given ever-increas- Singer observes that normal humans in ing amounts of water. Nor does their dependence on water explain why they 16 John T. Bruer, The Myth of the First Three bear ears of corn as opposed to to- Years: A New Understanding of Early Brain Devel- opment and Lifelong Learning (New York: Free 15 Adele Diamond, “A Model System for Study- Press, 1999). ing the Role of Dopamine in the Prefrontal Cor- tex During Early Development in Humans: Ear- 17 Jonathan Glover, Humanity: A Moral His- ly and Continuously Treated ,” tory of the Twentieth Century (London: J. Cape, in Handbook of Developmental Cognitive Neuro- 1999); , A Darwinian Left: , science, ed. Charles A. Nelson and Monica Evolution, and Cooperation (London: Weidenfeld Luciana (Cambridge, Mass.: mit Press, 2001). & Nicolson, 1999).

Dædalus Fall 2004 11 Steven all societies manifest a sense of sympa- temperature, hormones, the molecular Pinker 21 on thy: an ability to treat the interests of environment, and neural activity. human others as comparable to their own.18 Among the environmentally sensitive nature Unfortunately, the size of the moral cir- gene-expression effects are those that cle in which sympathy is extended is a make learning itself possible. Skills and free parameter. By default, people sym- memories are stored as physical changes pathize only with members of their own at the synapse, and these changes re- family, clan, or village, and treat anyone quire the expression of genes in response outside this circle as less than human. to patterns of neural activity. But under certain circumstances the cir- These causal chains do not, however, cle can expand to other clans, tribes, render the nature-nurture distinction races, or even species. An important way obsolete. What they do is force us to to understand moral progress, then, is to rethink the casual equation of ‘nature’ specify the triggers that prompt people with genes and of ‘nurture’ with every- to expand or contract their moral circles. thing beyond the genes. Biologists have It has been argued that the circle may be noted that the word ‘gene’ accumulated expanded to include people to whom several meanings during the twentieth one is bound by networks of reciprocal century.22 These include a unit of hered- trade and interdependence,19 and that ity, a speci½cation of a part, a cause of a it may be contracted to exclude people disease, a template for protein synthesis, who are seen in degrading circum- a trigger of development, and a target of stances.20 In each case, an understand- . ing of nonobvious aspects of human na- It is misleading, then, to equate the ture reveals possible levers for humane prescienti½c concept of human nature social change. with ‘the genes’ and leave it at that, with the implication that environment- “Genes are affected by their environ- dependent gene activity proves that hu- ments, and learning requires the expres- man nature is inde½nitely modi½able by sion of genes, so the nature-nurture dis- experience. Human nature is related to tinction is meaningless.” It is, of course, genes in terms of units of heredity, de- in the very nature of genes that they are velopment, and evolution, particularly not turned on all the time but are ex- those units that exert a systematic and pressed and regulated by a variety of sig- lasting effect on the wiring and chem- nals. These signals in turn may be trig- istry of the brain. This is distinct from gered by a variety of inputs, including the most common use of the term ‘gene’ in molecular biology, namely, in refer- 18 Peter Singer, The Expanding Circle: ence to stretches of dna that code for a and (New York: Farrar, Straus & Giroux, 1981). 21 Marcus, The Birth of the Mind; Ridley, Nature 19 Robert Wright, NonZero: The Logic of Human Via Nurture. Destiny (New York: Pantheon Books, 2000). 22 Ridley, Nature Via Nurture; Richard Dawk- 20 Glover, Humanity; Philip G. Zimbardo, ins, The Extended : The Gene as the Unit Christina Maslach, and Craig Haney, “Reflec- of Selection (San Francisco: W. H. Freeman & tions on the Stanford Prison Experiment: Gen- Company, 1982); Seymour Benzer, “The Ele- esis, Transformations, Consequences,” in Obe- mentary Units of Heredity,” in A Symposium on dience to Authority: Current Perspectives on the the Chemical Basis of Heredity, ed. William D. Milgram Paradigm, ed. Thomas Blass (Mahwah, McElroy and Bentley Glass (Baltimore: Johns N.J.: Lawrence Erlbaum Associates, 2000). Hopkins Press, 1957).

12 Dædalus Fall 2004 protein. Some aspects of human nature perceptual environment are instructive Why nature & nurture may be speci½ed in information carriers in the sense that their effects are pre- won’t go other than protein templates, including dictable by the information contained in away the cytoplasm, noncoding regions of the the input. Given a child who is equipped genome that affect , to learn words in the ½rst place, the con- properties of genes other than their se- tent of her vocabulary is predictable quence (such as how they are imprint- from the words spoken to her. Given an ed), and cross-generationally consistent adult equipped to understand contin- aspects of the maternal environment gencies, the spot where he will park his that the genome has been shaped by car will depend on where the No Parking natural selection to expect. Conversely, signs are posted. But other aspects of the many genes direct the synthesis of pro- environment, namely, those that affect teins necessary for everyday metabolic the genes directly rather than affecting function (such as wound repair, diges- the brain through the senses, trigger ge- tion, and memory formation) without netically speci½ed if-then contingencies embodying the traditional notion of that do not preserve information in the human nature. trigger itself. Such contingencies are per- The various concepts of ‘environ- vasive in biological development, where ment,’ too, have to be re½ned. In most many genes produce transcription fac- nature-nurture debates, ‘environment’ tors and other molecules that set off cas- refers in practice to aspects of the world cades of expression of other genes. A that make up the perceptual input to the good example is the Pax6 gene, which person and over which other humans produces a protein that triggers the ex- have some control. This encompasses, pression of twenty-½ve hundred other for example, parental rewards and pun- genes, resulting in the formation of the ishments, early enrichment, role mod- eye. Highly speci½c genetic responses els, education, laws, peer influence, cul- can also occur when the organism inter- ture, and social attitudes. It is misleading acts with its social environment, as to blur ‘environment’ in the sense of the when a change of social status in a male psychologically salient environment of cichlid ½sh triggers the expression of the person with ‘environment’ in the more than ½fty genes, which in turn al- sense of the chemical milieu of a chro- ter its size, aggressiveness, and stress mosome or cell, especially when that response.23 These are reminders both milieu itself consists of the products of that innate organization cannot be other genes and thus corresponds more equated with a lack of sensitivity to the closely to the traditional notion of he- environment, and that responses to the redity. There are still other senses of environment are often not speci½ed by ‘environment,’ such as nutrition and the stimulus but by the nature of the environmental toxins; the point is not organism. that one sense is primary, but that one should seek to distinguish each sense “Framing problems in terms of nature and characterize its effects precisely. and nurture prevents us from under- A ½nal that the environment standing human development and mak- dependence of the genes does not vitiate 23 Russell Fernald, “How Does Behavior the concept of human nature is that an Change the Brain? Multiple Methods to An- environment can affect the organism in swer Old Questions,” Integrative Comparative very different ways. Some aspects of the Biology 43 (2003): 771–779.

Dædalus Fall 2004 13 Steven ing new discoveries.” On the contrary, but only half their variable genes); and Pinker on some of the most provocative discover- that biological (who share their human ies in twentieth-century psychology environment and half their variable nature would have been impossible if there genes) are more similar than adoptive had not been a concerted effort to dis- siblings (who share their environment tinguish nature and nurture in human but none of their variable genes). These development. studies have been replicated in large For many decades psychologists have samples from several countries, and have looked for the causes of individual dif- ruled out the most common alternative ferences in cognitive ability (as mea- explanations (such as selective place- sured by iq tests, school and job per- ment of identical in similar adop- formance, and indices of brain activity) tive homes). Of course, concrete behav- and in personality (as measured by ques- ioral traits that patently depend on con- tionnaires, ratings, psychiatric evalua- tent provided by the home or culture– tions, and tallies of behavior such as di- which language one speaks, which reli- vorce and crime). The conventional gion one practices, which political party wisdom has been that such traits are one supports–are not heritable at all. strongly influenced by parenting prac- But traits that reflect the underlying tal- tices and role models. But recall that this ents and temperaments–how pro½cient belief is based on flawed correlational with language a person is, how religious, studies that compare parents and chil- how liberal or conservative–are partially dren but forget to control for genetic heritable. So genes a role in making relatedness. people different from their neighbors, Behavioral geneticists have remedied and their environments play an equally those flaws with studies of twins and important role. adoptees, and have discovered that in At this point it is tempting to con- fact virtually all behavioral traits are clude that people are shaped both by partly (though never completely) heri- genes and by family upbringing: how table.24 That is, some of the variation their parents treated them and what among individual people within a cul- kind of home they grew up in. But the ture must be attributed to differences in conclusion is unwarranted. Behavioral their genes. The conclusion follows from genetics allows one to distinguish two repeated discoveries that identical twins very different ways in which people’s reared apart (who share their genes but environments might affect them. The not their family environment) are highly shared environment is what impinges similar; that ordinary identical twins on a person and his or her siblings alike: (who share their environment and all their parents, home life, and neighbor- their genes) are more similar than frater- hood. The unique environment is every- nal twins (who share their environment thing else: anything that happens to a 24 Plomin, Owen, and McGuf½n, “The Genet- person that does not necessarily happen ic Basis of Complex Human Behaviors”; Eric to that person’s siblings. Turkheimer, “Three Laws of Behavior Genetics Remarkably, most studies of intelli- and What They Mean,” Current Directions in gence, personality, and behavior turn up Psychological Science 9 (5) (2000): 160–164; few or no effects of the shared environ- Thomas J. Bouchard, Jr., “Genetic and Environ- mental Influences on Intelligence and Special ment–often to the surprise of the re- Mental Abilities,” Human Biology 70 (1998): searchers themselves, who thought it 257–259. was obvious that nongenetic variation

14 Dædalus Fall 2004 had to come from the family.25 First, data.26 Children of immigrants end up Why nature & nurture adult siblings are about equally correlat- with the language, accent, and mores of won’t go ed whether they grew up together or their peers, not of their parents. Wide away apart. Second, adoptive siblings, when variations in child-rearing practices– tested as adults, are generally no more day-care versus stay-at-home mothers, similar than two people from the same single versus multiple caregivers, same- culture chosen at random. And third, sex versus different-sex parents–have identical twins are no more similar than little lasting effect when other variables one would expect from the effects of are controlled. Birth order and only- their shared genes. Setting aside cases of child status also have few effects on be- extreme neglect or abuse, whatever ex- havior outside the home.27 And an ex- periences siblings share by growing up tensive study testing the possibility that in the same home in a given culture children might be shaped by unique as- make little or no difference to the kind pects of how their parents treat them (as of people they turn into. Speci½c skills opposed to ways in which parents treat like reading and playing a musical in- all their children alike) showed that dif- strument, of course, can be imparted by ferences in parenting within a family are parents, and parents obviously affect effects, not causes, of differences among their children’s happiness and the quali- the children.28 ty of family life. But they don’t seem to determine their children’s intellects, The discovery of the limits of family tastes, and in the long run. influence is not just a debunking exer- The discovery that the shared family cise, but opens up important new ques- environment has little to no lasting ef- tions. The ½nding that much of the vari- fect on personality and intelligence ance in personality, intelligence, and be- comes as a shock to the traditional wis- havior comes neither from the genes nor dom that “as the twig is bent, so grows from the family environment raises the the branch.” It casts doubt on forms of question of where it does come from. psychotherapy that seek the roots of an Judith Rich Harris has argued that the adult’s dysfunction in the family envi- phenomena known as ronment, on theories that attribute ado- –acquiring the skills and values needed lescents’ alcoholism, smoking, and de- to thrive in a given culture–take place linquency to how they were treated in in the peer group rather than the family. early childhood, and on the philosophy of parenting experts that parental micro- 26 Harris, The Nurture Assumption. management is the key to a well-adjust- ed child. The ½ndings are so counterin- 27 Ibid.; Judith Rich Harris, “Context-Speci½c tuitive that one might doubt the behav- Learning, Personality, and Birth Order,” Current ioral genetic research that led to them, Directions in Psychological Science 9 (2000): 174– 177; Jeremy Freese, Brian Powell, and Lala Carr but they are corroborated by other Steelman, “Rebel Without a Cause or Effect: Birth Order and Social Attitudes,” American 25 Rowe, The Limits of Family Influence; Sociological Review 64 (1999): 207–231. Harris, The Nurture Assumption; Turkheimer, “Three Laws of Behavior Genetics”; Robert 28 David Reiss, Jenae M. Neiderhiser, E. Mavis Plomin and Denise Daniels, “Why Are Children Hetherington, and Robert Plomin, The Relation- in the Same Family So Different from One An- ship Code: Deciphering Genetic and Social Influ- other?” Behavioral and Brain Sciences 10 (1987): ences on Adolescent Development (Cambridge, 1–60. Mass.: Harvard University Press, 2000).

Dædalus Fall 2004 15 Steven Though children are not prewired with ly similar, they are far from indistin- Pinker on cultural skills, they also are not indis- guishable: by most measures, correla- human criminately shaped by their environ- tions in their traits are in the neighbor- nature ment. One aspect of human nature hood of 0.5. Peer influence cannot directs children to ½gure out what is explain the differences, because identi- valued in their peer group–the social cal twins largely share their peer groups. milieu in which they will eventually Instead, the unexplained variance in per- compete for status and mates–rather sonality throws a spotlight on the role of than to surrender to their parents’ at- sheer chance in development: random tempts to shape them. differences in prenatal blood supply Acknowledging this feature of human and exposure to toxins, pathogens, hor- nature in turn raises questions about mones, and antibodies; random differ- how the relevant environments, in this ences in the growth or adhesion of axons case peer cultures, arise and perpetuate in the developing brain; random events themselves. Does a peer culture trickle in experience; random differences in down from adult culture? Does it origi- how a stochastically functioning brain nate from high-status individuals or reacts to the same events in experience. groups and then proliferate along peer Both popular and scienti½c explanations networks? Does it emerge haphazardly of behavior, accustomed to invoking in different forms, some of which en- genes, parents, and society, seldom trench themselves when they reach a acknowledge the enormous role that tipping point of popularity? unpredictable factors must play in the A revised understanding of how chil- development of an individual. dren socialize themselves has practical If chance in development is to explain implications as well. Teen alcoholism the less-than-perfect similarity of identi- and smoking might be better addressed cal twins, it also highlights an interesting by understanding how these activities property of development in general. One become status symbols in peer groups can imagine a developmental process in than by urging parents to talk more to which millions of small chance events their adolescents (as current advertise- cancel one another out, leaving no dif- ments, sponsored by beer and tobacco ference in the resulting organism. One companies, insist). A major determinant can imagine a different process in which of success in school might be whether a chance event could disrupt develop- classes ½ssion into peer groups with ment entirely. Neither of these happens different status criteria, in particular to identical twins. Their differences are whether success in school is treated as detectable both in psychological testing admirable or as a sign of selling out.29 and in everyday life, yet both are (usual- The development of personality–a ly) healthy human beings. The develop- person’s emotional and behavioral idio- ment of organisms must use complex syncrasies–poses a set of puzzles dis- loops rather than prespeci½ed tinct from those raised by the process of blueprints. Random events can divert socialization. Identical twins growing up the trajectories of growth, but the trajec- in the same home share their genes, their tories are con½ned within an envelope of parents, their siblings, their peer groups, functioning designs for the species. and their culture. Though they are high- These profound questions are not about . They are 29 Harris, The Nurture Assumption. about nurture versus nurture: about

16 Dædalus Fall 2004 what, precisely, are the nongenetic Why nature & nurture causes of personality and intelligence. won’t go But the puzzles would never have come away to light if researchers had not ½rst taken measures to factor out the influence of nature, by showing that correlations be- tween parents and children cannot glibly be attributed to parenting but might be attributable to shared genes. That was the ½rst step that led them to measure the possible effects of parenting empiri- cally, rather than simply assuming that parents had to be all-powerful. The everything-affects-everything diagram turns out to be not sophisticated but dogmatic. The arrows emanating from ‘parents,’ ‘siblings,’ and ‘the home’ are testable hypotheses, not obvious tru- isms, and the tests might surprise us both by the arrows that shouldn’t be there and by the labels and arrows we may have forgotten. The human brain has been called the most complex object in the known uni- verse. No doubt hypotheses that pit na- ture against nurture as a dichotomy or that correlate genes or environment with behavior without looking at the in- tervening brain will turn out to be sim- plistic or wrong. But that complexity does not mean we should fuzz up the issues by saying that it’s all just too com- plicated to think about, or that some hypotheses should be treated a priori as obviously true, obviously false, or too dangerous to mention. As with inflation, cancer, and global warming, we have no choice but to try to disentangle the mul- tiple causes.30

30 The writing of this paper was supported by nih Grant hd-18381. I thank , , Judith Rich Harris, and Matt Ridley for comments on an earlier draft.

Dædalus Fall 2004 17 This article has been cited by:

1. Matteo Mameli, David Papineau. 2007. The new nativism: a commentary on Gary Marcus’s The birth of the mind. Biology & Philosophy 21:4, 559-573. [CrossRef] 2. Robert J. Wyman. 2005. Experimental analysis of nature-nurture interactions. Journal of Experimental Zoology Part A: Comparative Experimental Biology 303A:6, 415-421. [CrossRef]