Sandra Scarr and Kathleen Mccartney Source: Child Development, Vol

How People Make Their Own Environments: A Theory of Genotype →Environment Effects Author(s): Sandra Scarr and Kathleen McCartney Source: Child Development, Vol. 54, No. 2 (Apr., 1983), pp. 424-435 Published by: Wiley on behalf of the Society for Research in Child Development Stable URL: http://www.jstor.org/stable/1129703 . Accessed: 01/04/2013 01:32

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This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions How People Make Their Own Environments: A -- Theory of Genotype Environment Effects

Sandra Scarr and Kathleen McCartney Yale University

SCARR, SANDRA, and McCARTNEY, KATHLEEN.How PeopleMake Their Own Environments:A Theory of Genotype -- Environment Effects. CHILDDEVELOPMENT, 1983, 54, 424-435. We propose a theory of development in which experience is directed by genotypes. Genotypic differences are proposed to affect phenotypic differences, both directly and through experience, via 3 kinds of genotype -- environmenteffects: a passive kind, through environmentsprovided by biologically related parents; an evocative kind, through responses elicited by individuals from others; and an active kind, through the selection of different environmentsby different people. The theory adapts the 3 kinds of genotype-environment correlations proposed by Plomin, DeFries, and Loehlin in a developmental model that is used to explain results from studies of deprivation, intervention, twins, and families.

Introduction evolutionary theory that individual differences depend in part on genotypic differences. We Theories of behavioral have development argue that genetic differences prompt differ- ranged from genetic determinism to naive ences in which environments are environmentalism. Neither of these radical experienced and what effects they may have. In this view, views nor interactionism has ex- adequately the genotype, in both its the of or the species specificity plained process development and its individual variability, largely deter- role of experience in development. In this mines environmental effects on we a of environmental development, paper propose theory because the genotype determines the organ- effects on human development that empha- ism's to environmental sizes the role of the in responsiveness op- genotype determining portunities. not only which environments are experienced by individuals but also which environments A theory of behavioral development individuals seek for themselves. To show how must explain the origin of new psychological this theory addresses the process of develop- structures. Because there is no evidence that ment, the theory is used to account for seem- new adaptations can arise out of the environ- ingly anomalous findings for deprivation, ment without maturational changes in the adoption, twin, and intervention studies. organism, genotypes must be the source of new structures. For the species, we claim that human and its experience effects on development Maturational sequence is controlled pri- on depend primarily the evolved nature of the marily by the genetic program for develop- human genome. In evolutionary theory the ment. As Gottlieb (1976) said, there is two essential concepts are selection and vari- evidence for a role of environment in (1) ation. Through selection the human genome has maintaining existing structures and in (2) evolved to program human development. elaborating existing structures; however, there Phenotypic variation is the raw material on is no evidence that the environment has a which selection works. Genetic variation must role in (3) inducing new structures. In de- be associated with phenotypic variation, or velopment, new adaptations or structures can- there could be no evolution. It follows from not arise out of experience per se. We thank Emily Cahan, Jerome Kagan, KatherineNelson, Robert Plomin, and Theodore D. Wachs for their critical and helpful comments on several drafts of this paper. Their disagree- ments with us were stimulating and always constructive. Much of the family researchreviewed here was done in collaborationwith Richard A. Weinberg and supported by the W. T. Grant Foundation and the National Institute of Mental Health. The day-care studies have the collab- oration of J. Conrad Schwarz, Susan Grajek, and Deborah Phillips and were supported by the W. T. Grant Foundation and the Bermuda Government. Requests for reprintsshould be sent to Sandra Scarr, Department of Psychology, Yale University, Box 11-A Yale Station, New Haven, Connecticut 06520. [ChildDevelopment, 1983, 54, 424-435. @ 1983 by the Society for Researchin Child Development, Inc. All rights reserved. 0009-3920/83/5402-0026$01.00]

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The most widely accepted theories of "grasped" by the person. As we all know, the development are vague about how new struc- relevance of environments changes with de- tures arise; for example, Piaget (1980) fails velopment. The toddler who has "caught on" to make the connection between organism to the idea that things have names and who and environment clear in his references to demands the names for everything is experi- interaction. Nor is development well described encing a fundamentally different verbal en- by maturation alone (see Connolly & Prechtl, vironment from what she experienced before, 1981). Neither Gesell and Ilg (1943) nor even though her parents talked to her exten- contemporary nativists (e.g., Chomsky, 1980) sively in infancy. The young adolescent who appreciate the inextricable links of nature and played baseball with the boy next door and nurture in a hierachically organized system now finds herself hopelessly in love with him of development. is experiencing her friend's companionship in a new way. We suggest that the problem of new structures in development has been extraor- A model of genotypes and environments. dinarily difficult because of a false parallel -Figure 1 presents our model of behavioral between genotype and environment, which, development. In this model, the child's pheno- we argue, are not constructs at the same level type (P,), or observable characteristics, is a of The analysis. dichotomy of nature and function of both the child's genotype (Ge) nurture has always been a bad one, not only and her rearing environment (Ec). There will for the oft-cited reasons that both are required be little disagreement on this. The parents' for development, but because a false parallel genotypes (G,) determine the child's geno- arises between the two. We propose that de- type, which in turn influences the child's velopment is indeed the result of nature and phenotype. Again, there should be little con- nurture but that genes drive experience. troversy over this point. As in most develop- Genes are components in a system that orga- mental theories, transactions occur between nizes the organism to experience its world. the organism and the environment; here they The organism's abilities to experience the are described by the correlation between world change with development and are in- phenotype and rearing environment. In most dividually variable. A good theory of the en- models, however, the source of this correla- vironment can only be one in which experi- tion is ambiguous. In this model, both the ence is guided by genotypes that both push child's phenotype and rearing environment and restrain experiences. are influenced by the child's genotype. Because the influences both Behavioral on child's genotype development depends both the and the a and a suitable environment phenotype rearing environment, genetic program their correlation is a function of the for the of the geno- expression human, species- The is for Differences type. genotype conceptually prior typical program development. to both the and the environ- can arise from phenotype rearing among people both genetic ment. and environmental differences, but the process by which differences arise is better de- It is an unconventional shorthand to -- scribed as genotype environment effects. suggest that the child's genotype can directly Like Chomsky and Fodor (1980), we pro- affect the rearing environment. What we want pose that the is the force genotype driving G- behind development, because, we argue, it is the discriminator of what environments are actually experienced. The genotype determines the of the to responsiveness person Pp those environmental opportunities. Unlike c Chomsky and Fodor, we do not think that development is precoded in the genes and with merely emerges maturation. Rather, we Ec stress the role of the genotype in determining which environments are actually experienced and what effects they have on the developing person. We distinguish here between environments to which a person is exposed and environments that are actively experienced or Fic. 1.-A model of behavioral development

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to represent is developmental changes in the with attempts to separate environmental from genetic program that prompt new experiences, genetic effects and their combinations is that before the full phenotype is developed. An people evoke and select their own environments example could be found in the development to a great extent. There may appear to be arbi- of productive speech; the child becomes atten- tary events of fate, such as being hit by a truck tive to the language environment receptively (did you look carefully in both directions?), months before real words are produced. Our falling ill (genetic differences in susceptibility, argument is that changes in what is "turned or a life-style that lowers resistance to on" in the genotype affect an emerging pheno- disease?), but even these may not be entirely type both directly through maturation (Go to divorced from personal characteristics that Pc) and through prompting new experiences. have some genetic variability. Please under- stand that we do not mean that one's environ- The model could just as well specify mental fate is determined one's intermediate phenotypes, such as receptive entirely by that some are language in the example of productive speech, genotype--only genotypes more to receive and select certain en- but the idea that genetic differences (both likely vironments than others. A that stresses developmental changes for an individual over theory either or environmental time and differences among individuals) affect genetic differences se cannot account for the experiential differences could be lost in a web per processes by which come to be the are. of path diagrams. The model is designed to people way they At one in time, behavioral present our ideas, not for analysis of any point differ- variance. ences may be analyzed into variances that can be attributed more or less to genetic and Also clouded by an endless regress of environmental sources (see Plomin, DeFries, & intermediate would be the idea phenotypes Loehlin, 1977; Scarr & Kidd, in press). A that the correlation or transaction between quantitative genetic approach to estimating and environment is determined phenotype by variances, however, does not attempt to spec- in the We developmental changes genotype. ify the processes by which individuals de- that this is not a recognize popular position, veloped their phenotypes. but we propose it to account for data to be discussed in the final sections of the paper. Genotype-environmentcorrelations.--Plo- min et al. (1977) have described a model Thus, we intend the from to path G. E. of variation that estimates the to the idea that phenotype represent developmental amount of variance that arises from in are genetic changes phenotypes prompted both by and environmental in the effective and differences. Genotype- changes genotype by environment correlation is a nonlinear com- in the salience of changes environments, in the additive variance which are then correlated. ponent model, included to account for situations in which The path from the G, to P, represents "genotypes are selectively exposed to different maturation, which is controlled primarily by environments." They did not intend to de- the genetic program. New structures arise out scribe developmental processes, as we are of maturation, from genotype to phenotype. doing here. Rather, Plomin and his colleagues Behavioral development is elaborated and were responding to the question, How much of maintained, in Gottlieb's sense, by the trans- the variation in a phenotype is due to actions of phenotype and environment, but it differences among genotypes, differences cannot arise de novo from this interaction. among environments, dominance effects, geno- Thus, in this model, the course of develop- type-environment interactions, and genotype- ment is a function of genetically controlled environment correlations? Their model ad- maturational sequences, although the rate of dresses sources of individual differences in a maturation can be affected by some environ- population of phenotypes at one point in time. mental such as circumstances, the effects of By contrast, our use of the term, genotype nutrition on -- physical growth (Watson & environment effects, is to describe develop- Lowrey, 1967). Behavioral examples include mental processes over time, not to estimate cultural differences in rates of development sources of variance in phenotypes. We seek through the sequence of cognitive stages de- to answer the questions, How do genotypes scribed by and other theoretical se- Piaget and environments combine to produce human quences (see Nerlove & Snipper, 1981). development? and How do genetic and en- Separationof genetic and environmental vironmental differences combine to produce effects on development.-The major problem variation in development?

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An Evolving Theory of Behavioral in reading; parents who read well and enjoy Development reading are likely to provide their children with books; thus, the children are more likely Plomin et al. described (1977) three to be skilled readers who enjoy reading, both kinds of genotype-environment correlations for genetic and environmental reasons. The that we believe form the basis for a develop- children's rearing environment is positively mental The of theory. theory genotype -+ correlated with the parents' genotypes and environment effects we has three propose therefore with the children's genotypes as well. propositions: An example of a negative passive geno- 1. The which children process by develop type-environment correlation can also be is best described three kinds of by genotype found in reading. Parents who are skilled environment effects: a -> passive kind, readers, faced with a child who is not learning the related whereby genetically parents pro- to read well, may provide a more enriched vide a environment that is correlated rearing reading environment for that child than for with the of the child genotype (sometimes another who acquires reading skills quickly. positively and sometimes negatively); an The more enriched environment for the less evocative the child receives kind, whereby able child represents a negative genotype responses from others that are influenced by -, environment effect (see also Plomin his genotype; and an active kind that repre- et al., 1977). There is, thus, an unreliable, sents the child's selective attention to and but not random, connection between geno- from of his environment that learning aspects types and environments when parents provide are influenced his and by genotype indirectly the opportunities for experience. correlated with those of his biological relatives. The second kind of genotype -> en- 2. The relative importance of the three vironment effect is called evocative because kinds of -- environment effects genotype it represents the different responses that with The influence of changes development. different genotypes evoke from the social and the kind declines from to ado- passive infancy physical environments. Responses to the per- lescence, and the of the active kind importance son further shape development in ways that increases over the same period. correlate with the genotype. Examples of such evocative effects can be found in the 3. The degree to which experience is in- research of the of fluenced by individual genotypes increases Lytton (1980), theory Escalona and the review of with development and with the shift from pas- (1968), Maccoby (1980). -- It is that active sive to active genotype environment effects, quite likely smiley, babies receive more social stimulation than as individuals select their own experiences. sober, passive infants. In the intellectual area, co- The first, -- environ- passive genotype operative, attentive preschoolers receive more ment effects arise in related fam- biologically pleasant and instructional interactions from ilies and render all of the research literature the adults around them than on uncooperative, parent-child socialization uninterpretable. distractible children. Individual differences in Because both and en- parents provide genes responses evoked can also be found in the vironments for their the biological offspring, physical world; for example, people who are child's environment is necessarily correlated skillful at electronics receive feedback of a with her because her are genes, genes corre- sort very different from those who fail con- lated with her and the parents' genes, parents' sistently at such tasks. genes are correlated with the rearing environ- The third kind of -- environment they provide. It is impossible to know genotype ment effect is the active, or what about the parents' rearing environment niche-picking for the child determines what about the niche-building sort. People seek out environments find child's behavior, because of the they compatible and stimulating. confounding We all select from the effect of genetic transmission of the same surrounding environment some to which to learn characteristics from parent to child. Not only aspects respond, can we not interpret the direction of effects about, or ignore. Our selections are correlated with in parent-child interaction, as Bell (1968) motivational, personality, and intellectual of our argued, we also cannot interpret the cause aspects genotypes. The active genotype - environment of those effects in biologically related families. effect, we argue, is the most powerful connection between people and their An example of a positive kind of passive environments and the most direct expression genotype-environment correlation can be found of the genotype in experience.

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Examples of active genotype -4 envi- ilarities in personal characteristics evoke ronment effects can be found in the selective similar responses from others, as shown in efforts of individuals in sports, scholarship, the case of identical twins reared apart relationships-in life. Once experiences occur, (Bouchard, Note 2). These findings are also they naturally lead to further experiences. We consistent with the third proposition. agree that phenotypes are elaborated and A probabilistic model.-The concept of maintained by environments, but the impetus -- genotype environment effects is emphasized for the experience comes, we argue, from the in this emerging theory for three major reasons: genotype. the model results in a testable set of hypotheses Developmental changes in genotype - for which disconfirmation would come from environment effects.-The second proposition random association between genotypes and en- is that the relative importance of the three kinds vironments, it describes a developmental pro- -- of genotype environment effects changes cess, and it implies a probabilistic connection over development from infancy to adolescence. between a person and the environment. It is In infancy much of the environment that more likely that people with certain genotypes reaches the child is provided by adults. When will receive certain kinds of parenting, evoke those adults are genetically related to the child, certain responses from others, and select cer- the environment they provide in general is posi- tain aspects from the available environments; tively related to their own characteristics and but nothing is rigidly determined. The idea their own genotypes. Although infants are of genetic differences, on the other hand, has active in structuring their experiences by se- seemed to imply to many that the person's lectively attending to what is offered, they developmental fate was preordained without cannot do as much seeking out and niche- regard to experience. This is absurd. By in- building as older children; thus, passive geno- voking the idea of genotype -> environment type -- environment effects are more im- effects, we hope to emphasize a probabilistic portant for infants and young children than connection between genotypes and their en- they are for older children, who can extend vironments. Although mismatches between the their experiences beyond the family's influences behaviors of parents and children certainly and create their own environments to a much exist (see Nelson, 1973), we argue that on greater extent. Thus, the effects of passive the average there are correlations of parents' genotype -> environment effects wane when characteristics and the rearing environment the child has many extrafamilial opportunities. they provide. In addition, parents can provide environ- Waddington (1962) postulated a prob- ments that are negatively related to the child's able but not determinant connection between genotype, as illustrated earlier in teaching genotypes and phenotypes through an epi- reading. Although parents' genotypes usually genetic space, in which environmental events affect the environment they provide for their deflect the course of the developing pheno- biological offspring, it is sometimes positive type. Figure 2 illustrates Waddington's theory and sometimes negative and therefore not as of the probable relationship between geno- direct a product of the young child's genotype typic and phenotypic differences. Note that as later environments will be. Thus, as stated a correlation remains between genotype and -- in proposition 3, genotype environment phenotype, even though one cannot specify effects increase with development, as active in advance what environmental events will replace passive forms. Genotype -> environ- affect phenotypic development. To this con- ment effects of the evocative sort persist ception, we add that genotypes shape many throughout life, as we elicit responses from of their own experiences through evocative and others based on many personal, genotype- active genotype -- environment correlations. related characteristics from appearance to personality and intellect. Those responses The Role of the Environment from others reinforce and extend the direc- Revisited tions our development has taken. High intelligence and adaptive skills in children from If genotypes are the driving force behind very disadvantaged backgrounds, for example, development and the determinants of what evoke approval and support from school per- environments are experienced, does this mean sonnel who might otherwise despair of the that environments themselves have no effects? child's chances in life (Garmezy, Note 1). In Clearly, environments are necessary for de- adulthood, personality and intellectual differ- velopment and have effects on the average ences evoke different responses in others. Sim- levels of development, but they may or may

This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions Scarr and McCartney 429 not cause variations among individuals driven by genotypes and may even be nega- (McCall, 1981). We argue like McCall that tively related to genotypes, as in the passive, nature has not left essential human develop- familial situation. Examples of this effect can ment at the mercy of experiences that may be found in studies of deprivation, adoption, or may not be encountered; rather, the only and day care. Studies of children reared in necessary experiences are ones that are gen- isolation (Clarke & Clarke, 1976) and children erally available to the species. Differences reared in unstimulating institutions (Dennis in experience per se, therefore, cannot be & Najarian, 1951; Hunt, 1961, 1980) have the major cause of variation among individ- demonstrated the adverse effects of deprived uals. The major features of human develop- environments on many aspects of development are programmed genetically and require ment. Such studies usually address average experiences that are encountered by the vast responses to these poor environments. In any majority of humankind in the course of living. case, studies of environments that are so ex- Phenotypic variation among individuals relies treme as to be outside of the normal range on experiential differences that are determined of rearing environments for the species have by genetic differences rather than on differ- few implications for environmental variation ences among environmental effects that occur that the vast majority of human children randomly. experience. Imposed environments.-In developmen- In contrast to the extremely poor environ- tal studies, we usually think of environments ments in the deprivation literature, the provided for a child, such as parental interac- adoption studies include only rearing environ- tion, school curricula, and various experimental ments in the range of adequate to very good. manipulations. In some cases there are passive The evidence from studies of biologically and evocative genotype-environment correla- related and adoptive families that vary in tions that go unrecognized, as in parent-child socioeconomic status from working to upper interaction and the selection of children into middle class is that most people experience school curricula. In a few cases there may be what Scarr and Weinberg (1978) have called no correlation of the child's genotype with the "functionally-equivalent" environments. That treatment afforded an experimental group of is, the large array of individual differences which she is a member. On the other hand, it is among children and late adolescents adopted impossible to ignore the attention and learning in infancy were not related to differences characteristics the child brings to the situ- among their family environments-the same ation, so that the effects of environmental array of environmental differences that were manipulations are never entirely free of in- and usually are associated with behavioral dividual differences in genotypes. Develop- differences among children born to such fami- ment is not necessarily constrained by lies (Scarr, 1981; Scarr & Kidd, in press; Scarr genotype-environment correlations, although & Weinberg, 1976, 1977, 1978). On the aver- most often genotypes and environments are age, however, adopted children profit from correlated in the real world, so that in fact, their enriched environments, and they score if not in principle, there are such constraints. above average on IQ and school achievement tests and on measures of personal adjustment. Sometimes, the influence of genotypes on environments is diminished through un- Negative genotype-environmentcorrela- usual positive or negative interventions, so tions.-Environments provided to children that that the environments experienced are less are negatively related to their genotypes can

Genotype Epigenetic Phenotype Space . Space Space

0 * S"------"-.-.. 00-0- Fitness S ------O-- Space

1 2 3 4 FIGc.2.-Waddington's epigenetic space

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have dramatic effects on average levels of de- ogy, the main effects seem not to change the velopment. Extrafamilial interventions that rank orders of children affected. The main provide unusual enrichments or deprivations effects are real, but they are also small by can alter the developmental levels of children comparison to the range of individual vari- from those that would be predicted by their ation within groups so treated or not. Some family backgrounds and estimated genotypes. children may be more responsive than others Intervention theories predict these main to the treatment, but we doubt that there effects (Caldwell & Richmond, 1968; Hunt, are many situations in which disordinal inter- 1980). actions are the rule. Very few children lose Enriched environments have developmental points by participating in day-care Headstart been shown to enhance intellectual or gain by being severely neglected develop- in ment of children from back- infancy. The search for aptitude-treatment disadvantaged interactions & & McCart- (Cronbach Snow, 1977) and grounds (Ramey Haskins, 1981; interactions Note less genotype-environment (Erlen- ney, 3). Similarly, stimulating have not environments can children's meyer-Kimling, 1972) produced day-care hamper dramatic or reliable results. intellectual and social development, even if they come from more advantaged families In studies of adoptive and biologically (McCartney, Scarr, Phillips, Grajek, & related families, the correlation of children's Schwarz, 1981; McCartney, Note 3). IQ scores with the educational level of bio- is about .35, whether or not These rather rare logical parents are, however, oppor- the rear their children & Wein- or lack of parents (Scarr tunities, same, providing negatively in this children on the correlated for In the berg, issue). Adopted experiences genotypes. have scores than their usual course of average higher IQ development beyond early as a result of the individuals biological parents influence childhood, select and evoke experi- of their ences that are influenced their above-average adoptive parents. directly by Taken these the and therefore correlated together, findings support genotypes positively claim that treatments can have main effects with their own characteristics. phenotypic without overcoming genetic differences in Environmentaleffects on averages versus children's responsiveness to those environ- individuals.--One must distinguish environments. Adopted children have IQ scores above mental events that on the average enhance or those of their biological parents, yet the cor- delay development for all children from those relations of adopted children are higher with that account for variation among children. their biological than adoptive parents (Scarr There can be "main effects" that account for & Weinberg, 1977, 1978, in this issue). The variation among groups that are naturally or average effects of treatments, such as adop- experimentally treated in different ways. With- tion, seem to increase the mean IQ scores, but in the groups of children there still remain they do not seem to affect the rank order of enormous individual differences, some of which the children's scores with respect to their arise in response to the treatment. It is rare biological parents, and it is on rank orders, that the variation between groups approaches not means, that correlations depend. These the magnitude of differences within groups, results imply that the effect of adoptive fam- as represented in the pervasive overlapping ilies is to increase the scores of adopted chil- distributions of scores. In developmental dren above those which would be predicted psychology, we have usually been satisfied if by their biological parents, but not to alter the treatment observed or implemented pro- radically the rank order of individual differ- duced a statistically reliable difference be- ences among children they rear. And so it tween groups, but we have rarely examined is, we think, with most treatments. the sources of differential responsiveness within the groups. Answering Questions from Previous Most often, the same treatments that Research on Twins and Families alter the average performance of a group Neither extreme genetic determinism nor seem to have similar effects on most members naive environmentalism can account for seem- of the group. Otherwise, we would find a ingly anomalous findings from research on of great deal variance in genotype-environ- twins and families. Three puzzling questions ment interactions; that is, what's sauce for remain, the first of which concerns the process the goose would be poison for the gander. For by which monozygotic (MZ) twins come to the kinds of deprivation or interventions be more similar than dizygotic (DZ) twins, studied most often in developmental psychol- and biological siblings more similar than

This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions Scarr and McCartney 431 adopted siblings on all measurablecharacter- served: that the hobbies, food preferences, istics, at least by the end of adolescence choices of friends, academic achievements, (Scarr & Weinberg, 1978). The second ques- and so forth of the MZ twins are very similar tion concerns the declining similarities be- (Scarr & Carter-Saltzman, 1980). Kamin tween DZ twins and adopted siblings from (1974) proposed that all of this environ- infancy to adolescence. The third question mental similarity is imposed on MZ co-twins arises from the unexpected similarities be- because they look so much alike. Theories of tween identical twins reared in different genetic resemblance do not speak to how homes. close resemblancesarise. We propose that the home environments A of provided by the parents, theory genotype-environmentcor- the that the co-twins evoke from relation can account for these responses findings by and the active choices make to the of resemblance others, they in pointing degree genetic their environmentslead to strikingsimilarities and the degree of similarity in the environments that would be through genotypicallydetermined correlations experienced by the in their co-twins and sibs. learning histories. The same of Genetic explanationapplies, course, resemblancedetermines environ- to the resemblance of mental of greater biological than similarity.-The expected degree en- The environment of vironmental for a of relativescan adopted siblings. one similarity pair sib is correlated be of as the of a own biological to the genotype thought product person's of the other as one-half the coefficient of the -- environment and the genotype path genetic sibling's environment to her own correlation of the pair. Figure 3 a genotype, presents because b = 0.50, as described in Figure 3. model of the relationshipbetween genotypes and The same is true for DZ twins. There is a very environmentsfor pairs of relatives who small correlation in genetic for intelligence be- vary genetic relatedness. G1 and G2 sym- tween in most studies bolize two adopted siblings that the genotypes, E1 and E2 their arisesfrom selective of the environments.The in the placement offspring respective similarity of similarmothers in the same home. two environments the adoptive (path a) is product of More for this the coefficientof each important theory, however, is genotype with its own the selective of children environment and placement adopted (path x) the genetic corre- to match the intellectual characteristicsof lation of the the pair (path b). On the assump- This tion that adoptive parents. practice allows adop- individuals'environments are equally tive to create a influenced their own parents positive, passive geno- by genotypes, the sim- correlation for their in the of type-environment adopted ilarity environments two individuals children in when the becomes a functionof their early childhood, theory genetic correlation. asserts that this kind of correlation is most This model can be used to answer ques- important. In fact, the selective placement tion 1 concerning the process by which MZ estimatesfrom studies by Scarrand Weinberg twins come to be more similarthan DZ twins (1977) can account for most of the resem- and biological siblings more similar than blance between adoptive parents and their adopted siblings. For identical twins, for children. In addition, adoptive parents, like whom b = 1.00, the relationshipof one twin's their biological counterparts, can provide environmentwith the other's genotype is the negative genotype-environment correlations same as the correlationof the twin's environ- that assure that their several children will not ment with her own genotype. Thus, one differ too much on important skills, such would certainly predict what is often ob- as reading. Changing similarities among siblings.- path b The second question left unanswered by research concerned the G - previous declining G2 similarities of dizygotic twins and adopted siblings from infancy to adolescence. It is pathx pathx clear from Matheny, Wilson, Dolan, and Krantz's (1981) longitudinal study of MZ ) E and DZ twins that the DZ correlationsfor El Z intelligence of .60-.75 are higher than genetic theory would in and path a predict infancy early childhood. For school age and older twins, Fro. 3.-A model of environmental similarity DZ correlationswere the usual .55. Similarly, based on genetic resemblance. the intelligence correlations of a sample of

This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions 432 Child Development late adolescent adopted siblings were zero, gives way to their own choices, they select compared to the .25-.39 correlations of the environments that are less similar than their samples of adopted children in early to previous environments and about as similar middle childhood (Scarr & Weinberg, 1978). as those of ordinary sibs. Neither environmental nor genetic the- Adopted sibs, on the other hand, move ories can effectively address these data. How from an early environment, in which mother can it be that the longer you live with some- may have produced similarity, to environ- one, the less like them you become? One ments of their own choosing. Because their could evoke some ad hoc environmental genotypes are hardly correlated at all, neither theory about sibling relationships becoming are their chosen environmental niches. Thus, more competitive, or "deidentified," but that by late adolescence, adopted siblings do not would not account for the continued, moderate resemble each other in intelligence, person- intellectual resemblance of biological siblings. ality, interests, or other phenotypic character- Genetic theory has, of course, nothing to say istics (Grotevant, Scarr, & Weinberg, 1977; about decreasing twin resemblance or any Scarr, Webber, Weinberg, & Wittig, 1981; resemblance among young adoptees. Scarr & Weinberg, 1978). The theory put forward here predicts that Biological siblings' early environments, the relative importance of passive versus active like those of adopted children, lead to trait genotype-environment correlations changes similarity as a result of passive genotype -+ with age. Recall that passive genotype- environmental effects. As biological siblings environment correlations are created by par- move into the larger world and begin to ents who provide children with both genes make active choices, their niches remain mod- and environments, which are then correlated. erately correlated because their genotypes re- Certainly in the case of DZ twins, whose main moderately correlated. There is no prenatal environment was shared and whose marked shift in intellectual resemblance of earliest years are spent being treated in most biological sibs as the process of active geno- of the -- same ways at the same time by the type environment influence replaces the same parents, the passive genotype -* en- passive one. vironment effect is greater than that for Identical twins reared third ordinary sibs. and apart.-The Biological adopted siblings concerned the of do not, of course, share the same question unexpected degree develop- resemblance between identical twins reared mental environments at the same time because mostly With the of they differ in The apart. theory genotype--* age. passive genotype- environment effects, their resemblance is not environment correlation still operates for sib- surprising. Given opportunities to attend selec- lings, because they have the same but parents, tively to and choose from varied to a lesser extent than for twins. (See Table 1.) opportunities, identical genotypes are expected to make sim- Monozygotic twin correlations for intel- ilar choices. They are also expected to evoke lectual competence do not decline when similar responses from others and from their active genotype-environment correlations out- physical environments. The fact that they weigh the importance of the passive ones, were reared in different homes and different because MZ co-twins typically select highly communities is not important; differences in correlated environments anyway. Dizygotic their development could arise only if the ex- on pairs, the other hand, are no more geneti- periential opportunities of one or both were cally related than sibs, so that as the intense very restricted, so that similar choices could of their similarity early home environments not have been made. According to previous TABLE 1

THE SIMILARITYOF CO-TWIN'S AND SIBLING'S GENOTYPESAND ENVIRONMENTSDUE To:

CORRELATIONSIN THE ENVIRONMENTSOF RELATED PAIRS Passive Genotype-+ Active Genotype-- GENETIC Environment Effects Environment Effects CORRELATION in Early Development in Early Development MZ twins...... 1.00 High High DZ twins...... 52 High Moderate Biologicalsiblings ...... 52 Moderate Moderate Adoptedsiblings ...... 01 Moderate Low

This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions Scarr and McCartney 433 studies (Juel-Nielsen, 1980; Newman, Free- Fourth, longitudinal studies of adopted man, & Holzinger, 1937; Shields, 1962) and children, such as the ongoing work of Plomin the recent research of Bouchard and col- and colleagues, can provide valuable evidence leagues at the University of Minnesota of the changing influences of family environ- (Bouchard,Note 2), the most dissimilarpairs ments on children. The theory predicts that of MZs reared apart are those in which one children'scharacteristics will be more related was severely restricted in environmentalop- to characteristicsof the adoptive parents and portunity.Extreme deprivation or unusual en- other adopted siblings in earlierthan later de- richment can diminish the influence of geno- velopment.If adoptedchildren are as similarto type and environment and therefore lessen their adoptive parents and each other in late the resemblance of identical twins reared adolescence as they were in early childhood, apart. that aspect of the theory is wrong. Fifth, studies of older adolescents and Research Strategies adults who were adopted in infancy and The theory we propose can be tested in others who were born into their families can several ways and prove unable to account for provide evidence on the long-term effects of results. First, studies of parental treatmentof passive genotype -- environment effects more than one child would be informative within families. Both evocative and active about -- passive genotype environment kinds of genotype -- environmental effects effects. In general, we expect the rearing en- can be traced through the similarities and vironment provided for the children in a dissimilaritiesof the two kinds of siblings. family to differ in ways that are related to each child's characteristics.Do treat In these ways, and others, the theory parents can be tested. It can fail to account for results all of their children alike, as so many studies of one child seem to Can obtained, or it can account for the diverse per family imply? results more than other theories. parents be authoritativewith one child and adequately with another?Our Given the various results of family studies permissive theory predicts in this we believe that its that parents will respond to individual differ- presented paper, will be fulfilled. At least, we ences in their children, in keeping with predictions research on families with hope it will encouragemore developmentalists Lytton's (1980) to more than one child twins. If parent treatment of their children study per family, is not related to children's genetically unrelated families, and individual talents, interests, differencesin and personalities,the theory is wrong. experience. Second, studies of responses that individuals evoke from otherswould test our ideas Summary about evocative genotype -- environment In summary, the theory of genotype effects. The social psychology literature on - environment correlations proposed here attractiveness (Bersheid & Walster, 1974; describes the usual course of human develop- Mursteid, 1972), for example, would seem to ment in terms of three kinds of genotype- support our view that some personal charac- environment correlationsthat posit coopera- teristics evoke differential responses from tive effortsof the nature-nurtureteam, directed others. Similarly, teachers' responses to chil- by the genetic quarterback.Both genes and dren with high versus low intelligence, hyper- environmentsare constituentsin the develop- activity versus acceptable levels of energy, mental system, but they have different roles. and so forth provide some evidence for our Genes direct the course of human experience, theory. If others do not respond differentially but experientialopportunities are also neces- to individualcharacteristics for which there is sary for development to occur. Individual genetic variability,then the theory is wrong. differences can arise from restrictionsin environmental to what Third, active niche-building is being opportunities experience studied the of the genotype would find compatible. With a by Laboratory Comparative rich of HumanCognition in their naturalisticobserva- array opportunities, however, most differences tions of children's adaptations to problem- among people arise from geneti- solving situations (Cole & The Laboratoryof cally determineddifferences in the experiences Human Note Our to which they are attracted and which they Comparative Cognition, 4). evoke from their environments. theory predicts that children select and build niches that are correlated with their talents, The theory also accounts for individual interests, and personality characteristics. If differencesin responsivenessto environments not, the theory is wrong. ----differencesthat are not primarily interac-

This content downloaded from 128.95.71.159 on Mon, 1 Apr 2013 01:32:53 AM All use subject to JSTOR Terms and Conditions 434 Child Development tions of genotypes and environments but Clarke, A. M., & Clarke, A. D. B. Early experi- roughly linear combinations that are better ence: Myth and evidence. New York: Free described as genotype-environment correla- Press, 1976. tions. In addition, the theory accounts for Connolly, K. J., & Prechtl, H. F. R. (Eds.), Matu- seemingly anomalous results from previous ration and development: Biological and psy- research on twins and families. chological perspectives.Philadelphia: Lippin- 1981. Most the addresses the cott, important, theory L. & R. E. Attitudes and in- issue of Rather than a Cronbach, J., Snow, process. presenting structional methods. New York: static view of individual differences Irvington, through 1977. variance this allocation, theory hypothesizes & P. Infant which Dennis, W., Najarian, development processes by genotypes and environ- under environmental ments combine across to make handicap. Psychologi- development cal 1951, Whole No. us both human and Monographs, 71(7, unique. 436). Erlenmeyer-Kimling,L. Gene-environmentinter- Reference Notes actions and the variability of behavior. In L. G. & E. 1. Garmezy, N. The case for the single case in Ehrman, Omenn, Caspair (Eds.), environment and behavior. New experimental-developmentalpsychology. Pa- Genetics, York: Academic 1972. per presented at the annual meeting of the Press, S. C. The roots Chi- American Psychological Association, Los Escalona, of individuality. 1968. Angeles, August 1981. cago: Aldine, & F. L. and child in the 2. Bouchard, T. The Minnesota study of twins Gesell, A., Ilg, Infant culture New York: & reared apart: Description and preliminary of today. Harper Bros., 1943. findings. Paper presented at the annual meet- The role of in the devel- ing of the American Psychological Associa- Gottlieb, G. experience of behavior in the nervous tion, August 1981. opment system. In G. Gottlieb Studies in the 3. McCartney, K. The effect of quality of day (Ed.), development behavior and the nervous care environment upon children's language of system. Vol. 3. and neural and behav- development. Unpublished doctoral disserta- Development ioral New York: Academic tion, Yale University, 1982. specificity. Press, 1976. 4. Cole, M., & The Laboratoryof Comparative H. & R. A. Human Cognition. Niche-picking. Unpub- Grotevant, D., Scarr, S., Weinberg, Patterns of interest in and lished manuscript, University of California, similarity adoptive families. and San Diego, 1980. biological Journalof Personality Social Psychology, 1977, 35, 667-676. References Hunt, J. McV. Intelligence and experience. New York: Ronald, 1961. Bell, R. Q. A reinterpretationof the direction of Hunt, J. McV. Early psychological development effects in studies of socialization. Psychologi- and experience. Worcester, Mass.: Clark cal Review, 1968, 75, 81-95. University Press, 1980. Bersheid, E., &Walster, E. Physical attractiveness. Juel-Nielsen, N. Individual and environment: In L. Berkowitz (Ed.), Advances in experi- Monozygotic twins reared apart. New York: mental social ysychology. New York: Aca- InternationalUniversities Press, 1980. demic Press, 1974. Kamin, L. J. The science and politics of IQ. Po- Caldwell, B. M., & Richmond, I. The Children's tomac, Md.: Erlbaum, 1974. Center in Syracuse. In L. Dittman (Ed.), Lytton, H. Parent-childinteraction: The socializa- Early child: The new perspectives.New York: tion process observed in twin and single fam- Atherton, 1968. ilies. New York: Plenum, 1980. Chomsky, N. On cognitive structures and their McCall, R. B. Nature-nurtureand the two realms development: A reply to Piaget. In M. Piat- of development: A proposed integrationwith telli-Palmarini (Ed.), Language and learn- respect to mental development. Child Devel- ing: The debate between Jean Piaget and opment, 1981, 52, 1-12. Noam Chomsky. Cambridge, Mass.: Harvard McCartney, K., Scarr, S., Phillips, D., Grajek, S., University Press, 1980. & Schwarz, J. C. Environmental differences Chomsky, N., & Fodor, J. Statement of the para- among day care centers and their effects on dox. In M. Piattelli-Palmarini (Ed.), Lan- children's development. In E. F. Zigler & guage and learning: The debate between E. W. Gordon (Eds.), Day care: Scientific Jean Piaget and Noam Chomsky. Cambridge, and social policy issues. Boston: Auburn Mass.: Harvard University Press, 1980. House, 1981.

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