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Why are children in the same family so different fro onme another Robert Plomin Department of Individual and Family Studies Pennsylvania State University University Park Pa 16802 Denise Daniels Department of Psychiatry Stanford University Stanford Calif 94305 Abstract One of the most important findings that has emerged from human behavioral genetics involves the environment rather

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Plomin Daniels Nonshared environment, describe the evidence that leads to this conclusion and its tives such as cousins will be less similar than second. implications Our main goal however is to draw atten degree relatives such as half siblings who in turn will be. tion to this dramatic discovery and to elicit commentary less similar than first degree relatives such as full siblings. and suggestions from our peers Despite the far reaching If heredity does not affect the trait then differences in. implications of the evidence that psychologically relevant genetic similarity should not affect the resemblance of. environmental influences make children in a family dif these pairs of individuals. ferent from rather than similar to each other we are The problem is that environmental resemblance often. aware of no major criticism of these findings We expect covaries with genetic relatedness Cousins half siblings. that BBS commentary will rock this boat s smooth sailing and full siblings respectively are likely to share in. and perhaps even alter its course creasingly similar environments Because relatives share. family environment as well as heredity familial re. 1 Quantitative genetics semblance can be due to environmental influences as well. as to hereditary influences In other words a portion of. In order to understand the evidence pointing to the environmental influence could be shared by relatives. importance of nonshared environment it is necessary to making them similar to one another Nonetheless family. begin with an overview of the theory and methods of studies are useful in estimating limits of genetic and. quantitative genetics which when applied to behavioral environmental influences For example if the correlation. phenomena is referred to as behavioral genetics After for first degree relatives is zero for a particular trait then. describing the basic twin and adoption designs we shall neither shared heredity nor shared family environment. examine the implications of twin and adoption data for the affect the trait. separation of shared and nonshared environmental varia The two major designs of human behavioral genetics. tion in the three domains with the most relevant data the adoption design and the twin design were devel. personality psychopathology and cognition oped to circumvent the problem of conflating genetic and. Quantitative genetic theory began in the early part of environmental influences in studies of family members. this century as a solution to the problem of reconciling who share heredity and family environments By doing. Mendelian genetics with normal distributions As anyone so these designs partition environmental variance into. who has taken high school biology knows about a hun two components one shared by members of a family and. dred years ago the monk Gregor Mendel studied di the other consisting of the remainder of the environmen. chotomous either or characteristics such as round ver tal variance which is referred to as nonshared envi. sus wrinkled seeds in the pea plant When his work was ronment. rediscovered 30 years later it provoked controversy. among biometricians who felt that the laws of heredity 1 1 Adoption design The basic problem in family studies. described by Mendel could not apply to human charac is that resemblance among relatives could be due to. teristics because unlike discontinuous pea plant charac shared heredity or to shared environment The adoption. teristics human characteristics nearly always involve a design powerfully cleaves these two sources of familial. normal continuous distribution The resolution to the resemblance Genetically related individuals adopted. controversy came when it was understood that a normal apart and reared in uncorrelated environments will re. distribution would be observed if several genes affected a semble each other only for genetic reasons Genetically. characteristic In 1918 when Ronald Fisher put the unrelated individuals adopted together in the same fami. finishing touches on this theory and spelled out the ly will resemble each other only for reasons of shared. expectations for familial resemblance based on the theo environment. ry quantitative genetics was born The simplest adoption design to understand is the rare. The theory uses the covariance or correlation among but dramatic situation in which identical twins are. relatives on normally distributed traits to estimate the adopted separately at birth and reared apart in uncorre. role of heredity Although the theory and its methods are lated environments The resemblance of these pairs of. usually presented in a sophisticated algebraic manner twins expressed as a correlation is a direct estimate of. the basic idea which is all that is needed to understand the proportion of phenotypic variance that is clue to. the way in which environmental variation is partitioned in genetic variance a descriptive statistic known as herit. quantitative genetics is very simple Details such as ability A correlation of 50 for identical twins reared. the distinction between additive and nonadditive genetic apart implies that half of the phenotypic variance is. variance can be found in textbooks on the topic e g genetic in origin 1. Falconer 1981 Hay 1985 Plomin DeFries McClearn A technical point that has some bearing on the estima. 1980 The fundamental tenet of the theory is that indi tion of nonshared environment concerns the distinction. viduals in a population differ for both genetic and non between additive and nonadditive genetic variance. genetic reasons How can we assess the extent to which Identical twins share all sources of genetic variance no. phenotypic observed variability is due to genetic varia matter how complex the interactions among genes Thus. tion among individuals or to nongenetic differences In an estimate of heritabilitv derived from the correlation for. studies of human beings for whom selection studies or identical twins reared apart is referred to as broad. comparisons among inbred strains cannot be conducted heritability it includes all sources of genetic variance In. the only way is to study pairs of individuals who differ in contrast first degree relatives primarily share only ad. genetic resemblance If heredity is important for a partic ditive genetic variance genetic effects that add up line. ular characteristic pairs of individuals who are more. arly in their effect on the phenotype estimates of. similar genetically ought to be more similar for the. heritability based on first degree relatives adopted apart. measured characteristic For example third degree rela. are thus primarily limited to additive genetic variance. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. and are thus referred to as narrow heritability This ment For example a correlation of 25 for a trait. distinction is important to the extent that nonadditive measured in pairs of adoptees reared in the same adoptive. genetic variance is important if nonadditive genetic vari homes suggests that 25 of the phenotypie variation in. ance affects a trait behavioral genetic designs that assess the trait can be explained by shared environment A. narrow heritability will misread this genetic variance as correlation of zero for pairs of adoptees on the other. nonshared environment Although most behavioral ge hand implies that shared environment contributes noth. neticists discount the importance of nonadditive genetic ing to phenotypie variance which implies that all of the. variance some recent work suggests that it contributes to environmental variation is nonshared. certain characteristics Lykken 1982 Plomin 1986 It should be mentioned that the distinction between. Phcnotypic variance not explained by genetic variance shared and nonshared environment is not limited to. is ascribed to environmental sources More properly this family relationships in which relatives are the same age. component of variance is nongenetic that is it is broader such as twins or relatives who are nearly the same age. than the usual way psychologists think about the environ such as siblings We can also consider shared and non. ment in that it includes accidents and illnesses prenatal shared environmental factors that affect the resemblance. influences cytoplasmic changes and even DNA changes between parents and their offspring In this case shared. that are not transmitted hereditarily Data for relatives environment refers to environmental influences that in. adopted apart as in the case of separately adopted identi crease resemblance between parents and offspring It. cal twins cannot by themselves separate shared and does not involve all parental influences on offspring only. nonshared environmental components of nongenetic those environmental influences that increase phenotypie. variance similarity between parents and their children. Other adoptions designs can assess shared and non, shared environment Comparisons between relatives 1 2 Twin design The twin design compares the re. adopted apart and relatives reared together permit an semblance of identical twins with that of same sex frater. indirect assessment Relatives adopted apart share he nal twins Both types of twins are born at the same time. redity but not environment whereas relatives reared share the same womb and home and are of the same sex. together are similar for reasons both of shared heredity One major difference distinguishes the two types Identi. and shared environment If relatives reared together are cal twins are twice as similar genetically on the average. no more similar than relatives adopted apart we can as fraternal twins If heredity affects a trait the twofold. conclude that growing up in the same family does not add greater genetic similarity of identical twins will make. to relatives resemblance beyond the similarity induced them more similar than fraternal twins with respect to a. by heredity In other words environmental influence particular trait The difference between the correlations. operates in a nonshared manner For example if for a for identical twins and fraternal twins is an estimate of. particular trait identical twins reared together are no roughly half of the genetic variance in the population. more similar than identical twins reared in uncorrelated because the coefficient of genetic relationship is 1 0 for. environments shared environment is unimportant for identical twins and 50 for fraternal twins Thus for a trait. that trait and all of the environmental variance must be completely determined by heredity the expected cor. nonshared On the other hand if the correlation for relations are 1 0 for identical twins and 50 for fraternal. identical twins reared together is 75 and the correlation twins If the pattern of twin correlations were 75 and 50. for identical twins reared apart is 50 25 of the phe for identical and fraternal twins respectively heredity. notypic variance could be attributed to shared environ would be estimated to explain half of the phenotypie. ment and the remaining 25 to nonshared environment variance for the trait If heredity does not affect the trait. We have included this concrete example of partitioning the twofold greater genetic similarity of identical twins. only for purposes of clarification We do not mean to will not make them more similar than fraternal twins for. convey that such estimates will be particularly precise the particular trait. The accuracy of the estimates depends on all of the usual This discussion has oversimplified the twin method for. statistical issues such as sample size as well as on the didactic purposes For example assortative mating. assumptions of behavioral genetic designs The estimates would raise the fraternal twin correlation and nonadditive. of nonshared environment described later in our review genetic variance would lower it Also even though twin. come from large samples are replicated in many studies partners of both types live in the same family it is. and are based on quite different designs such as adoption possible that identical twins experience more similar. as well as twin studies Moreover our estimate of non family environments than do fraternal twins If this were. shared environment would have to be very substantially the ease some of the greater observed similarity of. wrong before it would seriously affect our conclusion that identical twins might be due to greater similarity of their. nonshared environment is responsible for most environ experience This possible confounding effect has been. mental variation relevant to psychological phenomena examined and in research to date does not appear to. A direct test of the importance of shared environment represent a major problem for the twin design Plomin. comes from the other side of the adoption design in which DeFries McClearn 1980 Finally genotype environ. genetically unrelated individuals are adopted into the ment interaction and correlation can affect these esti. same family These adoptive family members share major mates as discussed later. features of their environment the same parents home If genetic variance accounts for 50 of the phenotypie. social class community schools and so forth but they variance the rest of the phenotypie variance is attributed. do not share heredity The correlation for pairs of unrelat to nongenetic variance which includes shared and non. ed children adopted together directly estimates the pro shared environment as well as error of measurement The. portion of phenotypie variance due to shared environ twin method can be used to partition nongenetic variance. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. into its shared and nonshared components Consider two due to nonshared environment and error are removed. patterns of identical vs fraternal twin correlations 75 Environmental variance not due to shared environment. vs 50 and 50 vs 25 Doubling the difference between is called nonshared environment this portion of environ. the twin correlations suggests a heritability of 50 for mental variance makes family members different from. both patterns of correlations Thus for both patterns the one another This variance component is usually esti. proportion of phenotypic variance due to environmental mated as the remainder of phenotypic variance once. variance is 50 In the first case however the one with variance due to heredity shared environment and error. correlations of 75 and 50 for identical and fraternal of measurement is removed Differences within pairs of. twins respectively half of the environmental variance is identical twins reared together provides a direct estimate. shared by the twins making them resemble each other of nonshared environment as experienced by identical. and the other half of the environmental variance makes twins. them different In the case of identical and fraternal twin Because we are developmentalists we feel compelled. correlations of 50 and 25 all of the environmental to make the point that all components of variance can. variance contributes to differences within pairs change during development Estimates of genetic and. The reasoning behind this conclusion is as follows environmental components of variance depend upon the. Differences within pairs of identical twins are due only to age of the subjects sampled Genetic change during. nongenetic factors not shared by twins because members development is the focus of a new subdiscipline develop. of identical twin pairs do not differ genetically Thus mental behavioral genetics Plomin 1986 Nonshared. when identical and fraternal twin correlations are 50 and shared environmental components can also change. and 25 respectively 50 of the phenotypic variance is during development Research is needed to trace the. genetic and 50 is nongenetic Because identical twins developmental course of shared and nonshared environ. are identical genetically and yet their phenotypic correla mental variance For example there may be a general. tion is only 50 all of the nongenetic variance specifical trend for nonshared environmental variance to increase. ly nonshared environment and error of measurement with age as individuals expand their social and environ. leads to differences within pairs Variance clue to error of mental networks beyond the family On the other hand. measurement can be assessed as the difference between as this happens there may be fewer forces contrasting. the reliability coefficient e g test retest correlation children in the same family Research throughout the. and 1 0 For example if a test retest correlation is 90 lifespan especially research past adolescence will be. error variance is 10 the 50 nongenetic variance thus needed to resolve such developmental issues One strik. consists of 40 nonshared environmental variance and ing example of developmental change in the relative. 10 error variance When the identical and fraternal twin influence of shared and nonshared environmental vari. correlations are 75 and 50 half of the phenotypic vari ance serves to indicate the potential usefulness of a. ance is again environmental but in this case only half of lifespan perspective For 1Q the shared environment. the environmental variance 25 of the total phenotypic component of variance diminishes dramatically after. variance 1 0 75 25 is due to nonshared environ childhood as discussed in the next section. ment and error and the other half is shared The shared. environment component of variance can be estimated as 2 Evidence for the importance of nonshared. twice the fraternal twin correlation minus the identical environmental effects on behavior. twin correlation, In summary the twin design provides a direct estimate This section provides a brief summary of behavioral.
of nonshared environment the component of phe genetic research in personality psychopathology and. notypic variance that is not shared by members of identi cognition that leads to the conclusion that the most. cal twin pairs In addition the twin design provides an important source of environmental variance is nonshared. indirect estimate of shared family environment It is the environment This material is based on a recent review of. component of phenotypic variance that remains after behavioral genetic research throughout the lifespan. accounting for genetic variance and nonshared environ which can be consulted for additional studies and details. mental variance The generalizability of twin results con Plomin 1986 Although readers might take issue with. cerning shared family environment to the population of the precise magnitude of one or another of the estimates. nontwin siblings is questionable however because it the forest should not be overlooked for the trees Our. seems likely that twins share family environments to a point one that to our knowledge has not been disputed is. greater extent than do siblings who are not twins as will that nonshared environment is responsible for most en. be discussed later vironmental variation relevant to psychological develop. Thus adoption and twin studies can separate environ ment Thus our goal in the following section is not to. mental variance for behavioral traits into two compo provide an encyclopedic review of behavioral genetic. nents One component called shared environment in studies but rather to summarize the results to the extent. cludes all environmental influences that make children in needed to understand their message regarding the im. a family similar to one another This component of vari portance of nonshared environment. ance can be estimated in three ways 1 from the correla. tion for genetically unrelated children reared together in 2 1 Personality The importance of nonshared environ. the same adoptive families 2 from the difference in ment was first highlighted by Loehlin and Nichols 1976. correlations for relatives reared together and relatives whose twin analyses of personality data led to the follow. adopted apart and 3 from twin studies as the remainder ing conclusion. of phenotypic variance when genetic variance variance Thus a consistent though perplexing pattern is. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. emerging from the data and it is not purely idiosyn These twin studies used self report questionnaires. cratic to our study Environment carries substantial Perhaps some artifact exists so that identical twins always. weight in determining personality it appears to rate themselves as 50 similar when asked about their. account for at least half the variance but that environ personality Other assessment procedures however. ment is one for which twin pairs are correlated close to yield similar results For example in recent years sever. zero In short in the personality domain we seem al twin studies using parental ratings of children s person. to see environmental effects that operate almost ran ality have been reported reviewed by Buss Plomin. domly with respect to the sorts of variables that psy 1984 The average identical twin correlation is about 50. chologists and other people have traditionally again suggesting that about half of the variance is due to. deemed important in personality development nonshared environment The few twin studies that have. Loehlin Nichols 1976 p 92 used objective observations of personality yield some. Loehlin and Nichols reached this conclusion because what less ubiquitous evidence for nonshared environ. identical and fraternal twin correlations were consistently mental variance than do paper and pencil questionnaires. about 50 and 30 respectively within their large study Plomin Foch 1980 Nonetheless estimates of non. of high school aged twins that used self report person shared environmental influence from these studies are. ality questionnaires This pattern of correlations suggests still substantial usually greater than estimates of shared. 40 genetic variance and 60 environmental variance environmental variance even when error variance is. and that over 80 of the environmental component of taken into account. variance is due to nonshared environment plus error Studies of nontwin siblings and other family rela. Error accounts for about 20 of the variance tionships confirm the hypothesis that shared family en. These results are not peculiar to Loehlin and Nichols s vironment accounts for a negligible amount of environ. study of high school twins In a review of 10 recent twin mental variance relevant to personality development. studies of personality Goldsmith 1983 the average twin For example one of the earliest studies found an average. correlations were 47 for identical twins and 23 for fra sibling correlation of 12 Crook 1937 a recent large. ternal twins This pattern of twin correlations suggests family study Ahem Johnson Wilson McCleam Van. that heredity accounts for 50 of the phenotypic variance denberg 1982 yielded an average sibling correlation. and that nonshared environment and error of measure of 16 for three widely used personality questionnaires. ment explain the rest The average parent offspring correlations in this study. It might seem odd to report average correlations across were also low 12 for father son 10 for father. a domain as diverse as personality Nonetheless the twin daughter 13 for mother son and 14 for mother. results are generally similar across the dozens of traits daughter. measured by self report questionnaires Consider extra Four recently reported adoption studies of personality. version and neuroticism the two super factors in per indicate that this modest familial resemblance is not due. sonality which are associated with Eysenck e g 1967 to shared family environment the average adoptive. but also emerge as major second order factors from other sibling correlation is 04 and the average adoptive par. personality questionnaires such as Cattell s Sixteen Per ent adopted child correlation is 05 Loehlin Horn. sonality Factor Questionnaire Cattell Eber Tatsuoka Willerman 1981 Loehlin Willerman Horn 1985. 1970 See also Zuckerman Sensation Seeking BBS Scarr Weber Weinberg Wittig 1981 Scan Wein. 7 3 1986 A study of over 12 000 adult twin pairs in berg 1978a Adoptive sibling correlations are also low in. Sweden Floderus Myrhed Pedersen Rasmuson 1980 the first report of infant adoptive siblings involving 61. revealed twin correlations of 51 and 21 for identical and pairs at 12 months and 50 pairs at 24 months tested as part. fraternal twins respectively for extraversion and correla of the Colorado Adoption Project Daniels 1985 Parental. tions of 50 and 23 for neuroticism ratings of temperament yielded average adoptive sibling. Similar results emerge for less central dimensions of correlations of 11 at 12 months and 05 at 24 months. personality as well For example Loehlin and Nichols s tester ratings on the Infant Behavior Record Bayley. study used the California Psychological Inventory which 1969 yielded average adoptive sibling correlations of. includes diverse scales such as Sense of Weil Being 1 4 at 12 months and 05 at 24 months. Tolerance and Good Impression The identical and fra. ternal twin correlations respectively for these scales 2 2 Psychopathology Behavioral genetic data on psy. were 50 and 30 53 and 35 and 48 and 30 Another chopathology are also consistent with the conclusion that. example involves twin results for a new personality ques environmental variation is preponderantly of the non. tionnaire the Differential Personality Questionnaire shared variety Research on schizophrenia is difficult to. which assesses nontraditional dimensions of personality summarize briefly because concordance rates vary widely. A twin study of over 200 identical twin pairs and over 100 depending on the following whether or not age correla. fraternal twin pairs yielded the following sampling of tions are used the type of diagnostic criteria used and. correlations for identical and fraternal twins respec the selection and severity of probands Nonetheless. tively 50 and 36 for Danger Seeking 61 and 37 for relying on a recent book length review Gottesman. Authoritarianism and 58 and 25 for Alienation Lyk Shields 1982 familial concordance rates for schizo. ken Tellegen DeRubeis 1978 The only personality phrenia in a dozen studies found about 10 concordance. trait that appears to show significant shared environmen rates for schizophrenia for first degree relatives The. tal influence is masculinity feminity which one might concordance rate for fraternal twins is also about 10. argue falls more in the category of attitudes than person Concordance rates for identical twins are substantially. ality Loehlin 1982 higher than those for fraternal twins indeed higher. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. than would be expected on the basis of a simple additive relation in over 30 studies is 85 for identical twins. genetic model in which identical twins would be about and 58 for fraternal twins Bouchard McGue 1981. twice as similar as fraternal twins For example Got which suggests again that about 30 of the variance of IQ. tesman and Shields review five recent studies that yield scores can be accounted for by shared environment. an average case wise concordance of 45 for identical Although these data appear to converge on the reason. twins Regardless of the complications this pattern of twin able conclusion that shared environment accounts for a. concordance causes for estimates of genetic influence the substantial portion of environmental variance relevant to. results indicate that most schizophrenic identical twins IQ doubts have begun to arise For fraternal twins who. do not have an affected cotwin Because these are genet share environment to a greater extent than do nontwin. ically identical pairs of individuals nonshared environ siblings the IQ correlation is about 60 whereas the. ment must be the reason for these striking differences correlation for nontwin siblings is about 40 which. within pairs of identical twins means that the twin method overestimates the impor. This conclusion is confirmed in Gottesman and tance of shared environmental in comparison to family. Shields s review of recent adoption studies in Denmark studies. in which the same concordance of about 10 is found for The crucial piece of evidence in support of substantial. individuals adopted apart from a first degree schizo shared environmental variance is the correlation of 30. phrenic relative Thus sharing the same family environ for adoptive siblings reared together These studies have. ment with a schizophrenic relative does not increase included adoptive siblings still living at home with two. familial concordance exceptions The first exception is a study of postadoles. Gottesman and Shields 1982 also review attempts to cent adoptee pairs by Scarr and Weinberg 1978a which. isolate environmental sources of variance and conclude found a correlation of 03 for IQ This unsettling finding. So far no specific environmental source of liability is implies that shared environment is important for IQ. known the most likely environmental contributor during childhood when children are living at home and. stress may come from many sources and apparently then fades in importance after adolescence when children. may come during any stage of development Prenatal have left home. or birth complications early deprivations broken The hypothesis that shared environmental influences. homes censuring parents the death of someone close have no lasting impact on IQ is supported by results of a. failures in school poor work or social relationships recent study of adoptive and nonadoptive siblings Kent. childbirth a bad drug trip as well as all kinds of good 1985 The study included 52 pairs of adoptive siblings. fortune may have effects on a predisposed individual and 54 pairs of nonadoptive siblings ranging from 9 to 15. that are obvious only in retrospect In prospect it will years of age with the average age of 13 years A battery of. be impossible to prophesy the events themselves let cognitive ability measures was developed for administra. alone their effects Gottesman Shields 1982 pp tion over the telephone this battery correlated with face. 241 42 to face testing near the reliabilities of the tests An unro. We suggest however that until more systematic re tated first principal component used as an index of IQ. search on nonshared environmental variance sources is yielded a reasonable correlation of 38 for nonadoptive. conducted it is too early to conclude that the large siblings however the IQ correlation for adoptive siblings. environmental component of variance in schizophrenia is was 16 not significantly different from zero A similar. brought about by idiosyncratic experiences pattern of results emerged for specific cognitive abilities. Research on manic depressive psychosis yields results The adoptive sibling correlations for verbal spatial per. similar to those for schizophrenia Plomin DeFries ceptual speed and memory abilities were 06 07. McClearn 1980 Environmental influences on less se 10 and 16 respectively. vere forms of psychopathology such as neuroses and Thus this study leads to the conclusion that shared. alcoholism also appear to be predominantly nonshared environmental influence on IQ and specific cognitive. Sibling concordances are generally less than 20 and abilities is of negligible importance by the end of early. when twin and adoption studies have been conducted adolescence Because these estimates of shared environ. most of this familial resemblance has been found to be mental influences were obtained directly from adoptive. genetic in origin Fuller Thompson 1978 Rosenthal sibling correlations reasonable confidence can be at. 1970 In other words the most important influences on tached to this conclusion For example the sample of 52. psychopathology lie in the category of nonshared en pairs of adoptive siblings permits detection of a true. vironment Much more often than not affected children correlation of 30 with 70 power the standard error of. in families with more than one child will have unaffected the estimates of shared environment were found to be. siblings between 10 and 14 when a multiple regression model. fitting approach suggested by DeFries and Fulker 1985. 2 3 Cognition Until recently environmental variance was used. that affects individual differences in IQ was thought to fall In summary nonshared environmental influence is a. primarily in the category of shared environment In 11 major component of variance for personality psycho. studies the average IQ correlation for adoptive siblings pathology and IQ after childhood We conclude that. is 30 suggesting that 30 of the variance in IQ scores is nonshared environment explains perhaps as much as 40. due to shared environmental influences Bouchard to 60 of the total variance for these domains Although. McGue 1981 Adoptive parent adopted child IQ correla one can quibble with the magnitude of our estimates. tions are lower about 20 but still suggest substantial they would have to be substantially in error before they. influence of shared environment on parent offspring would affect our argument that most of the environmental. resemblance Twin studies agree The average IQ cor variance is nonshared. 6 BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. 3 Shared and nonshared environmental variance specific environmental factors are responsible for the. components of variance it seems to be a reasonable first. The purpose of this section is to consider some conceptual step to ask about components of variance without this. details of the distinction between shared and nonshared tack we would not have discovered that nearly all en. environment before discussing sources of nonshared en vironmental variance is of the nonshared variety It is a. vironment These details include other labels for shared major strength of the approach that it can reveal the. and nonshared environment the distinction between presence of genetic and environmental influences even. environmental components of behavioral variance and when these are not assessed directly. the relationship between specific environmental mea Attempts to isolate specific environmental factors will. sures and behavior the impact of nonshared environ be presented later A related issue however should be. mental influence on the development of singletons gen mentioned at this time Traditional environmental re. otype environment correlation and interaction and search attempts to relate measures of family environment. model fitting to measures of behavior of one child per family The yield. from such research has been disappointing especially if. 3 1 Other labels Shared and nonshared environmental one considers the amount of variance explained Maccoby. influences were named by Rowe and Plomin in 1981 Martin 1983 Knowing this research one might ask. although the distinction between environmental influ why such environmental factors as parental affection. ences that contribute to the resemblance between rela should be important within families when they account. tives and those that do not has been implicit in quan for little variance in behavior across families That is if it. titative genetics since its inception Many labels have makes little difference that some parents love their chil. been used to refer to these two components of environ dren more than other parents love their children why. mental variance Shared environmental influence has should parental love make a difference within families if a. been called E2 between family and common environ parent loves one child more than another The answer is. mental variance labels that have been used to refer to that there is no necessary relationship between the causes. nonshared environmental include E l within family in of differences between families and the causes of dif. dividual unique and specific environmental variance ferences within families That is environmental factors. Rowe and Plomin suggested that the symbols E l and E2 that create differences within families can act indepen. Jinks Fulker 1970 are probably best in that they carry dently of factors that cause differences between families. no connotations although they have the distinct disad For example a child really knows only his own parents. vantage that they provide no mnemonic to remember the child does not know if his parents love him more or. which is which Within and between family environ less than other parents love their children A child is. ment are the terms most often used They are useful for likely to be painfully aware however that parental affec. those familiar with the terminology of analysis of variance tion toward him is less than toward his sibling. which considers variance within and between groups. Variance within families refers to differences among fami 3 3 Singletons Because over 80 of U S families have. ly members and variance between families describes more than one child it is important to understand why. resemblance among family members The term within children in a family are so different from one another. family environment however connotes factors that oc How does nonshared environment relate to singletons. cur within the confines of the family whereas nonshared In general reasons why two children in the same family. influences are those that cause family members to differ differ are likely to yield clues as to the environmental. regardless of whether the locus of influence is the family source of variance for singletons as well The easiest. such as differential treatment by parents or outside the example involves nonsystematic events such as accidents. family such as different experiences at school or with and illnesses which are just as likely to befall singletons. peers For these reasons we suggest that the most However systematic nonshared influences may also be. descriptive and straightforward terms to use are shared found to affect singleton variance For example if certain. and nonshared characteristics of peer groups differ within pairs of sib. lings and contribute importantly to behavioral differences. 3 2 Components of variance versus specific measures It within sibling pairs it is likely that these characteristics. should be noted that this discussion pertains to environ also contribute to variance for singletons. mental components of behavioral variance not to the Obviously singletons do not have siblings with whom. relationship between specific environmental measures they interact thus this potential source of nonshared. and behavioral measures In this sense quantitative environment cannot contribute variance for singletons. genetic analyses describe the bottom line of genetic Although it might seem at first that differential parental. and environmental influence That is the total impact of treatment of two children in the same family is irrelevant. genetic variability on phenotypic variability will be de to singletons it is possible that once identified such. tected regardless of the complexity of the genetic effects factors might contribute to the variance of singletons. for example whether the genetic effects arise from vari There is evidence that parents with more than one child. ability in structural genes that code for polypeptides or treat the children similarly if we look at the children at the. from regulatory genes Similarly quantitative genetics same age which suggests that parental treatment is not. estimates the bottom line of environmental influence an important source of nonshared environment Dunn. regardless of the specific mechanisms by which environ Plomin Nettles 1985 Except for twins however. mental factors affect behavior Although this components siblings are not the same age and when we examine. of variance approach may be unsatisfying for those who contemporaneous parental treatment of children of dif. would like to know which specific genes and which ferent ages we find that parents treat the children differ. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. ently Dunn Plomin Daniels 1986 Differences in correlation for adoptive siblings is significant For this. parental behavior during development can also affect reason and because of the relative inaccessibility of most. singletons in that parents will treat their singleton chil models we have emphasized the basic correlational data. dren differently during the course of development and merely note that model fitting approaches confirm. Thus studies of differences within pairs of siblings are our conclusions. likely to illuminate factors responsible for singleton vari. ance as well as sibling variance The important point in. the present context is the obvious one that the study of 4 Categories of nonshared environmental. singletons cannot isolate factors that make two children in influence. the same family different from one another Because this. is the best clue we have as to the source of environmental What is happening environmentally to make children in. variance relevant to psychological development it makes the same family so different from one another One. sense to focus on environmental sources of differences gloomy prospect is that the salient environment might be. between children in the same family unsystematic idiosyncratic or serendipitous events such. as accidents illnesses and other traumas as biographies. 3 4 Genotype environment correlation and interaction often attest In his autobiography Darwin noted one. Two complicating factors in the estimation of quantitative example. genetic parameters are genotype environment correla The voyage of the Beagle has been by far the most. tion and genotype environment interaction Plomin important event in my life and has determined my. DeFries Loehlin 1977 Genotype environment cor whole career yet it depended on so small a circum. relation refers to an increase in phenotypic variance that stance as my uncle offering to drive me thirty miles to. occurs when children experience environments corre Shrewsbury which few uncles would have done and. lated with their genetic propensities Phenotypic vari on such a trifle as the shape of my nose Darwin 1892. ance can also be due to genotype environment interac p 28. tion when children respond differently to the same Darwin s comment about his nose refers to the quixotic. environment because of genetic differences among them captain of the Beagle Captain Fitz Roy who nearly. What are the effects of genotype environment correla rejected Darwin for the trip because the shape of his nose. tion and interaction on estimates of shared and nonshared indicated to Fitz Roy that Darwin would not possess. environment Consider a direct estimate of nonshared sufficient energy and determination for the voyage Dar. family environment the extent to which the correlation win wrote that during the voyage Fitz Roy became. for identical twins reared together is less than 1 0 This convinced that my nose had spoken falsely p 27. estimate will not include either genotype environment It is possible that nonshared environmental influences. correlation or interaction because identical twins are could be unsystematic in the sense of stochastic events. identical genetically thus in terms of genetic propen that when compounded over time make children in the. sities identical twins will correlate and interact with the same family different in unpredictable ways Such ca. environment in a similar manner Similarly the direct pricious events however are likely to prove a dead end. estimate of shared family environment the correlation for research More interesting heuristically are possible. for unrelated children reared together will not include systematic sources of differences within families. genotype environment correlation or interaction be Table 1 describes categories of environmental factors. cause these children are genetically uncorrelated thus that could lead to observed differences between children. in terms of their genetic propensities they will correlate in the same family These include such systematic sources. and interact with the environment in ways that do not add of nonshared influence in the family as birth order and. to their resemblance However estimates of nonshared gender differences of siblings interactions between sib. or shared environment derived as the remainder of phe lings differential treatment by parents and extrafamilial. notypic variance after other components of variance are influences such as peers. taken into account can be affected by genotype environ In one sense thinking about environmental influences. ment correlation and interaction because of their effects that create differences between children in the same. on estimates of genetic variance Plomin et id 1977 family represents a dramatic reconceptualization of psy. chological environments On the other hand this recon. 3 5 Model fitting Fitting models to adoption and twin ceptualization need not involve mysterious elements in. data is a powerful way to estimate quantitative genetic the environment Any environmental factor can be. parameters Loehlin 1978 Although model fitting tech viewed in terms of its contribution to nonshared environ. niques differ in their specifics they all express family mental variance For example parental affection can be. resemblance in terms of an underlying model consisting easily construed as a source of differences among children. of several unobserved genetic and environmental param in the same family because parents may be more affec. eters The approach is powerful because it makes assump tionate toward one child than another. tions explicit it tests a specific model and it can incorpo In this sense our conceptualization of nonshared en. rate into a single analysis different types of data such as vironmental influence is not new and exciting Although. family and adoption data rather than analyzing each type any traditional environmental factor can be viewed in. of data separately Model fitting procedures however terms of its contribution to nonshared environmental. only find significant parameters when they are implicit in variance it is important to emphasize the point men. the basic data for example in a study of adoptive siblings tioned earlier There is no necessary relationship be. a reasonable model fitting analysis will estimate signifi tween environmental factors that contribute to dif. cant shared family environmental influences onlv if the ferences between families and those that affect differ. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment.
Table 1 Categories of environmental influences that cause compare their relative effectiveness in predicting sibling. children in the same family to differ differences A first attempt to assess differences in per. ceived environments of siblings is discussed in the follow. Categories Examples ing section,Error of measurement Test retest unreliability. 5 Attempts to identify nonshared environmental,Nonshared environment. influences,Nonsystematic Accidents illnesses trauma. Systematic This section explores attempts to assess specific factors. Family composition Birth order gender differences within these categories that may be responsible for non. Sibling interaction Differential treatment shared environmental variance Family constellation var. Parental treatment Differential treatment iables especially birth order have been studied exten. Extrafamilial Peer groups teachers sively Other categories of possible nonshared environ. networks television mental influence such as differential parental treatment. differential sibling interaction and differential extra. Source Adapted from Rowe and Plomin 1981 familial experiences have not yet received much. 5 1 Birth order The only specific source of nonshared. ences between siblings within a family In some cases it family environment to receive considerable attention is. seems likely that there is no relationship Socioeconomic birth order For example over 1 000 entries for birth. status SES for example is an important factor that order appear in Psychological Abstracts Birth order is a. operates between families but even though the SES of prototype of nonshared environmental influence in that it. families changes it is unlikely that SES is an important is different for children in the same family and yet cannot. source of differences between siblings Conversely an originate in genetic differences among siblings Paradox. environmental factor that makes only a slight difference ically however most studies have analyzed its effect. between families may be critical within families For across families rather than within families and most of the. reasons such as these what is needed more than specula relationships are weak for IQ Galbraith 1982 and for. tion about the most relevant nonshared environmental personality Ernst Angst 1983. influences is research identifying relevant factors This. research can at the same time provide insights into 5 2 Other systematic nonshared environmental influ. theoretical issues such as the relationship between non ences Although birth order has received considerable. shared influences and traditional environmental factors attention studies of differential parental treatment sib. studied across families ling interaction and extrafamilial influences are more. The perspective of nonshared environment does how promising In exploring possible nonshared influences. ever suggest some new ways to study environmental the first step is to ask whether siblings in a family have. influences For example we must focus on measures of different experiences If siblings do not differ in their. experience specific to each child That is one implication experience for a particular aspect of the environment. of our conclusion concerning the importance of non then that environmental factor cannot be a source of. shared environment is that environmental factors shared differences between them For birth order this first step. by both children in a family are unlikely to be important is unnecessary because siblings obviously differ in birth. sources of environmental influence Environmental mea order Experiential differences however cannot be as. sures are needed that capture the major sources of differ sumed to affect behavioral differences within pairs of. ential experience of siblings Another strategy for re siblings therefore demonstrating that nonshared experi. search is exemplified by the emphasis of family therapists ences are related to differences in sibling behavior is the. on systems theory in which the child is viewed as part of second step The third step is to describe the direction of. an organized family system creating and maintaining effects when associations are found between differential. patterns of behavior Minuchin 1985 Another strategic experience and differences in their behaviors Do sibling. suggestion for the study of nonshared environment is to differences in experience affect or merely reflect dif. explore environmental sources of developmental dif ferences in sibling behavior. ferences within individuals McCall 1983 An environ Thus there are three steps in research on nonshared. mental factor that is responsible for change in a child from environmental influences identifying experiences that. early childhood to school age is also likely to make are not shared by family members relating such non. children in the same family different from one another shared environmental factors to differences in sibling. Finally another even more speculative meth behavior and determining the causal direction of such. odological lead for research is that subjective perceived relationships Because the topic of nonshared environ. experiences may prove to be important e g Jessor ment is so new only a few relevant studies have been. 1981 For example even if during home observations reported and most of these address the first step. children in the same family appear to receive the same. environmental treatment this does not mean that the 5 3 Sibling inventory of differential experience One sys. children experienced the treatment similarly We do not tematic approach to the topic is the Sibling Inventory of. mean to suggest that objective assessment of the environ Differential Experience SIDE Daniels Plomin 1985. ment is not also needed it would be best to use objective The 73 item self report SIDE asks each sibling to com. and subjective measures in the same study in order to pare his experiences to those of a sibling in the domains of. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1 9,Plomin Daniels Nonshared environment. sibling interaction parental treatment peer charac college oriented delinquent and popular respectively. teristics and events specific to the individual For all Siblings also agree substantially as to which sibling was. items siblings are asked to compare their relative experi more jealous r 56 and which sibling displayed more. ences rather than to make absolute judgments about their caretaking r 56 Siblings agree to a lesser extent on. experience For example rather than asking the extent to differences in parental treatment r 26 and 28 for. which my sibling and I show understanding for each maternal and paternal affection The median sibling. other the SIDE asks Who has shown more under agreement correlation over the 11 SIDE scales is 49. standing for the other A 5 point scale is used for the which is above typical interrater agreement on person. siblings ratings 1 My sibling has been much more this ality and environmental paper and pencil measures The. way than I have 2 My sibling has been a bit more this high sibling agreement found for some of the SIDE scales. way than I have 3 My sibling and I have been the same may be due to the fact that siblings are asked to make a. in this way 4 1 have been a bit more this way than my relative and specific comparison to their sibling rather. sibling and 5 1 have been much more this way than my than an absolute judgment in comparison to all other. sibling This provides relative scores indicating for exam children of that age Because the SIDE intentionally. ple the extent to which one sibling feels he is understood assesses siblings perceptions of their differential experi. by the other Although somewhat unusual these relative ence sibling agreement is not an important criterion for. judgments have several advantages First they should be the usefulness of the measure as long as the measure is. easier to make than absolute judgments for example on reliable Other substantive findings from the SIDE are. a 5 point scale how much do you understand your sib interwoven throughout the following discussion on the. ling compared to what Second relative judgments do major categories of systematic nonshared environment. not require that a sibling difference be calculated in order. to assess nonshared environment Third they can be 5 4 Parental treatment Environmental research has tra. used when data are available from only one member of a ditionally focused on parental treatment because parents. sibling pair The SIDE can also be coded to indicate the appear first and foremost in young children s lives It has. absolute rather than relative amount of differential sibling not been easy however to document parental effects on. experience by disregarding the direction of the differen children s development A recent review of the rela. tial experience i e 0 no difference in sibling experi tionship between parental treatment and children s de. ences 1 some difference 2 much difference velopment concludes that in most cases the rela. The 11 scales of the SIDE see Table 2 were devised tionships that have appeared are not large if one thinks. using the results of factor analyses of data on a sample of in terms of the amount of variance accounted for Mac. 396 12 to 28 year old siblings from the Denver metro coby Martin 1983 p 82 Indeed these findings led. politan area The word differential precedes the label the authors to argue for the need to examine intrafamilial. for each scale to emphasize that all items involve relative variation in the parent child relationship It should be. differential ratings The 2 week test retest reliabilities reiterated that the importance of nonshared environ. are reasonable with a mean of 84 and a range from 70 ment does not denigrate the importance of environmen. to 94 The scales are virtually independent of siblings tal influence Environmental influence is important but. age birth order and gender Also included in the table it operates differently from the way we thought it oper. are sibling agreement correlations which indicate that ated In the case of parental influences the effect that. siblings agree quite substantially especially in the areas parents have on their children has little to do with those. of differential sibling interaction and peer group charac aspects of parenting that are experienced similarly by. teristics The sibling agreement correlations are 55 73 two children in their family Whatever these parental. and 60 concerning which sibling s peer group was more influences might be they differentiate rather than inte. Table 2 Scales of nonshared environmental influence from the Sibling Inventory of. Differential Experience SIDE,Test retest Sibling,Category Scale reliability agreement.
Sibling Differential Sibling Antagonism 83 39,interaction Differential Sibling Jealousy 93 56. Differential Sibling Caretaking 89 56,Differential Sibling Closeness 70 23. Parental Differential Maternal Affection 82 26,treatment Differential Maternal Control 77 25. Differential Paternal Affection 77 28,Differential Paternal Control 85 49. Peers Differential Peer College Orientation 88 55,Differential Peer Delinquency 94 73.
Differential Peer Popularity 84 60,Source Adapted from Daniels and Plomin 1985. 10 BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. grate the children Parenting is likely to be an important Table 3 Sibling intraclass correlations for environmental. source of environmental variance only if parents differ measures in the national survey of children sample. entiate their children, Mow similarly or differently do parents treat their Parental Sibling. offspring The SIDE data indicate that siblings perceive Environ men till measure ratings ratings. their parents to treat them quite similarly Only 9 of. siblings report much difference and 35 report a bit of Familv cooperation 17. difference in their parents treatment on the average Familv stress 29. across parental treatment items For the four SIDE scales Parental rule expectations 18. that assess parental treatment the mean absolute score Parental chore expectations 49 21. is 50 0 refers to no reported difference in sibling experi Maternal closeness 38 19. ences 1 indexes some difference and 2 indicates much Paternal closeness 49 26. difference Other categories of nonshared environmen Child s sav in decisions 65 18. tal influence show greater differentiation within sibling. pairs and are thus more likely to be important sources of Xott X 299 348 sibling pairs p 05. nonshared environmental influence Nonetheless it is Source Adapted from Daniels Dunn Furstenberg and. possible that small differences in siblings perceptions of Plomin 1985. their parents treatment lead to large differences in their. development, Another study of adolescent siblings found similar. results not just for adolescents reports of their parents Two analyses of sibling data in the Colorado Adoption. treatment but also for the parents reports of their treat Project include the first objective data concerning. ment of their children Daniels Dunn Furstenberg differential parental behavior towards siblings Sibling. Plomin 1985 The 1981 follow up of the longitudinal correlations were reported for the interview observation. National Survey of Children Furstenberg Winquist measure Home Observation for Measurement of the. Nord Peterson Zill 1983 included 348 families with Environment HOME Caldwell Bradley 1978 for 133. two siblings 11 to 17 years of age mean age 13 7 years sibling pairs in which both members of each pair were. from a nationally representative sample of 1 077 families studied at 12 months of age and 103 sibling pairs were. In telephone interviews each sibling and mother was studied at 24 months Daniels 1985 The average sibling. interviewed individually concerning family cooperation spacing was nearly 3 years nonetheless the sibling. family stress parental rule and chore expectations close correlations for the HOME were nearly as great as the. ness to mother and father and child s say in decisions In stability of the HOME measure for all individuals from 12. contrast to the SIDE study the ratings of environment in to 24 months Using the HOME data when each sibling. this study are absolute in that parents and siblings were was 12 months old the sibling correlation for a general. not asked to rate parental treatment as it differed for the factor of the HOME was 42 at 24 months it was 43. two siblings Sibling intraclass correlations for the mea Sibling correlations at 12 months for the Family Environ. sures of parental treatment as rated by parents and by ment Scales FES Moos Moos 1981 also approached. the siblings themselves are listed in Table 3 The sibling the 1 year stability of the measure The FES is not at all. correlations indicate the extent to which parents and specific to a particular child however because it assesses. siblings themselves perceive that siblings share similar the general social climate of the home the HOME is only. parental treatment These data indicate that parents somewhat specific to each child some items such as. perceive that they treat their two children quite similarly number of books present and visible are likely to be. the sibling correlations range from 38 to 65 In con similar for all children in the family. trast the siblings do not perceive that their parents The most impressive results suggesting that parents. treatment of them is highly similar the sibling correla treat their several children similarly comes from a longi. tions average about 20 tudinal study of 50 families in which mothers were vid. Two twin studies using absolute ratings of adoles eotaped while interacting individually with each of two. cents perceptions of parental treatment Rowe 1981 siblings when each child was 12 months old Dunn et al. 1983 have found substantial correlations within twin 1985 The children were nearly 3 years apart in age. pairs for parental treatment Different measures of paren which means that the observations of maternal behavior. tal affection and control were used in the two twin toward the two children were separated by nearly three. studies and twin correlations of about 45 emerged years Maternal behavior was reliably assessed and factor. Parents appear to treat children less similarly in this analysis yielded three factors affection verbal attention. study which used absolute ratings than in the SIDE and control The results indicate that the mothers were. research which used relative ratings It is reasonable remarkably consistent in their behavior toward their two. however that sibling correlations for absolute ratings of children at the same age Corrected for unreliability the. parental treatment are lower than those for relative rat average correlation for maternal behavior toward two. ings because the absolute rating procedure asks each siblings was 70 These data suggest that differential. sibling to rate his parents treatment in relation to all maternal treatment of their children in infancy does not. other parents differences between the siblings re appear to be a major source of the marked individual. sponses are used to compute a correlation The relative differences within pairs of siblings Other longitudinal. approach is more direct for assessing differences in sib studies on this topic agree that mothers are quite con. lings experiences because it asks them about parental sistent in their behavior toward two of their children. treatment specifically in comparison to their sibling when the children are studied at the same age Abra. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1 11,Ploinin Daniels Nonshared environment.
movitch Pepler Corter 1982 Dunn Kendrick 1982 siblings reported much difference in their parents. Jacobs Moss 1976 treatment on the average and the mean absolute score. Nontwin siblings are in fact different in age Subse is 50 In contrast 19 of the siblings report much. quent work by Dunn and her colleagues has indicated difference and 40 report a bit of difference in their. that even though mothers treated their two children siblings treatment of them the mean absolute score. quite similarly when the children were the same age is 80 Daniels Plomin 1985. longitudinal analyses from 12 to 24 months showed little In summary although a few relevant studies have been. stability for maternal behavior to the same child The reported some data suggest that each member of a. authors suggest that rank order of the mothers on these sibling pair may provide a substantially different environ. dimensions changes from 12 to 24 months because differ ment for the other member of the pair especially when. ent mothers respond differently to the new developmen the data are based on adolescents self reported percep. tal advances of children Analyses from a study using a tions In terms of components of variance one might. very different methodology extensive and intensive predict that to the extent that siblings affect one another. unstructured home observations of a sample of 80 British the variance of individuals who are siblings should exceed. families support the same interpretation Dunn 1977 the variance of individuals who are singletons We are not. Individual differences in maternal responsiveness that aware of any tests of this prediction However there may. were highly stable during the first year of life changed be other factors diluting this variance difference between. markedly with the developments in the children s com siblings and singletons for example it is not implausible. municative abilities in the second year Similarly in to suggest that parents of siblings have less of an effect on. another study correlations between 12 and 24 months in each of their children than do parents of singletons. measures of maternal physical affectionate verbal visu. al and responsive behavior were very low 05 05 5 6 Peer characteristics Even less is known about extra. 05 04 and 17 respectively Clarke Stewart Hevey familial sources of nonshared environment such as peers. 1981 The only report of peers as a possible source of differential. The implication of these results is that in a cross experience for siblings is based on the SIDE Daniels. sectional slice of time siblings differ in age and are treated Plomin 1985 For the 26 peer characteristic items 20. quite differently Thus the possible effect of differential of the siblings report much difference and 42 report. parental treatment on siblings of different age needs a bit of difference in their peer groups characteristics. further exploration The mean absolute score is 83 for the three peer scales of. In summary sibling reports parental self reports and the SIDE which suggests that siblings experience peer. observational studies yield no clear conclusion concern differences as great as the differences they experience in. ing differential parental treatment To the extent that their interaction with each other. parents treat their children similarly we would not ex. pect parental treatment to be a major source of nonshared 6 Relationships between nonshared factors and. environmental influence although as mentioned earlier sibling differences in behavior. it is possible that small differences in parental treatment. lead to large differences in development The first question in studies of nonshared environmental. influences is whether such factors exist The answer is. 5 5 Sibling interaction The possiblity that siblings in clearly affirmative Siblings in the same family experience. teractions with each other are a source of nonshared different environments perhaps with respect to parental. environmental influences has not been studied nearly as treatment and probably in their interaction with each. much as parental treatment It is noteworthy however other and in characteristics of their peer groups The next. that the results of intensive observational studies of moth question is whether these differences in experience are. er sibling sibling triads emphasize the importance of related to differences in behavioral development. sibling sibling interactions Dunn 1983 Dunn Ken The study of adolescent siblings from the National. drick 1982 Twin data on sibling interaction have been Survey of Children Daniels et al 1985 related differen. reported for 88 pairs of high school twins Rowe Ploinin tial parental treatment to differences in sibling adjust. 1981 For a Liking scale the correlation for all 88 pairs ment As in all studies of personality and psycho. was 61 indicating that twins liking and disliking of each pathology the siblings were only moderately similar for. other is mutual Twins generally like each other though adjustment with correlations of about 20 which means. the average response was 4 4 on a 5 point scale which that the great majority of reliable variance is not shared by. means that this result involves only a small amount of siblings Table 4 lists multiple regression coefficients. variance Two other scales Respect and Understanding when sibling differences in adjustment measures are. yielded more variance than the Liking scale and twin regressed on several of the sibling differences in environ. correlations of 35 and 30 respectively indicated con ment listed in Table 3. siderable differences within pairs of twins These twin Most of the multiple regressions are significant and. correlations for twins respect for and understanding of adjusted R2 values of about 10 on the average indicate. each other are lower than those found in Rowe s studies of that nonshared environmental influences are systemat. twins perceptions of their parents treatment thus sug ically related to differences in the siblings adjustment. gesting that siblings might provide more nonshared en For example the last row of Table 4 shows associations. vironment than do parents between nonshared environment and an aggregate mea. The SIDE explores sibling interaction with scales that sure of disobedience based on parent sibling and teach. assess differential sibling antagonism caretaking jeal er ratings The significant regressions indicate that for. ousv and closeness As indicated earlier onlv 9 of 396 both parental and sibling ratings of sibling experience. 12 BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. Table 4 Multiple regressions of differences in sibling adjustment on. differences in sibling environments,Multiple R s,Parental ratings Sibling ratings. of sibling differences of sibling differences,Adjustment measure in experience in experience. Parental report of 38 25,emotional distress,Parental report of 37 25. delinquency,Parental report of 37 26,disobedience,Self report of 12 28.
emotional distress,Self report of 29 37,delinquency. Self report of 35,dissatisfaction,Teacher report of 35. disobedience,Parent sibling teacher 40 34,aggregate score of. disobedience, The multiple regressions involve sibling difference scores for each adjustment. Not available, Source Adapted from Daniels Dunn Furstenberg and Plomin 1985.
differential experiences of siblings are related to dif 1968 The direction of effects issue is just as relevant to. ferences in disobedience It is noteworthy that some the study of nonshared environmental influences as it is in. significant relationships emerge when different indi traditional studies For example differential parental. viduals rate the siblings adjustment and the siblings affection might be related to differences in siblings so. environment For example parental perceptions of sib ciability because preexisting differences in the siblings. ling differences in environment are related to differences sociability elicit differences in their parents affection. in the siblings own perception of delinquency and sib toward them. ling perceptions of environmental differences are related Behavioral genetic designs can be profitably applied to. to teacher ratings of disobedience With regard to the this issue because one possible explanation for a child to. specific environmental differences that relate to sibling environment direction of effects is genetic differences. differences in the adjustment measures both the parent between the siblings That is siblings might report dif. and sibling reports of the environment converge on the ferences in treatment that occur as a result of genetic. finding that the sibling who experiences more maternal differences between them Finding genetic influence on. closeness more sibling friendliness more say in family a nonshared environmental measure suggests that genet. decision making and more parental chore expectations ic differences between the siblings underlies at least in. as compared to the other sibling is better adjusted part their experiential differences There are two sub. psychologically sidiary issues Do measures of nonshared environment. Other studies that relate differential sibling experience show genetic influence Are relationships between mea. to differences in siblings behavior have been reported sures of nonshared environment and measures of behav. however these studies have used twin and adoption ior mediated genetically. designs to test the possibility that such relationships are It should be noted that failure to find genetic influence. mediated genetically For this reason these studies are does not prove that the measured nonshared environ. described in the following section mental influence causes behavioral differences within. pairs It is possible for example that behavioral dif. ferences within pairs of siblings originate from prior. 7 Direction of effects experiences with which the contemporaneous measure of. nonshared environment is correlated, Once relationships are identified between any environ. mental factor and behavior one can address the issue of 7 1 Do measures of nonshared environment show genet. direction of effects Does the environmental factor affect ic influence One study Daniels Plomin 1985 exists. or merely reflect differences among individuals Bell that explored the origins of differential sibling experi. BEHAVIORAL AND BRAIN SCIENCES 1987 10 1 13,Plomin Daniels Nonshared environment. ence SIDE data from 222 adoptive siblings were com pairs of fraternal twins are due to genetic differences as. pared to data from 174 biological siblings If the SIDE well as nonshared environmental influences Although. reflects genetic differences mean SIDE differences this approach has not been used systematically a study by. should be greater for adoptive than for biological pairs Rowe and Plomin 1981 examined the relationship be. because adoptive siblings are uncorrelated genetically in tween differences in interpersonal treatment of the twins. the absence of selective placement whereas biological and differences in self reported personality The authors. siblings correlate 50 genetically Samples of this size noted that the relationships between twin differences in. have 80 power to detect mean differences in experience the measures of nonshared environment and twin dif. that account for as little as 2 of the variance In general ferences in the measures of personality were generally. the SIDE measures of differential experience were sim weak for both identical and fraternal twins The fact that. ilar for adoptive and nonadoptive siblings Average cor the fraternal twin correlations were no greater than the. relations were 76 for adoptive siblings and 69 for bio identical twin correlations suggests that what little rela. logical siblings Thus the SIDE scales on average suggest tionship exists between nonshared environment as mea. negligible genetic influence which implies that the ori sured in this study in terms of the twins perceptions of. gins of perceived differential experience are indeed en their interpersonal relationship and personality does not. vironmental However 4 of the 11 SIDE scales yielded appear to be mediated by heredity. significantly greater differences for adoptive siblings than As mentioned earlier twins probably share more en. for biological siblings that accounted for 4 12 of the vironmental influences than do nontvvin siblings For this. variance thus suggesting slight genetic influence for reason the twin method is not a powerful approach to the. some of the SIDE scales study of relationships between nonshared environmental. Finding little genetic influence on the SIDE measure influences and behavioral differences That is twins may. is both surprising and interesting because behavioral experience more similar environments and be more sim. genetic studies of most behavioral traits and of shared ilar behaviorally than nontwin siblings Another method. environmental measures do show considerable genetic that is less direct but might prove to be more generaliza. influence reviewed by Plomin 1986 Although replica ble to the nontwin situation is to compare correlations. tion of this finding is necessary it may be that differential. between nonshared environmental differences and be. experiences of siblings are in fact insensitive to genetic. havioral differences for pairs of adoptive and nonadoptive. differences between the siblings Because siblings are. siblings Behavioral differences within pairs of nonadop. asked to make relative comparisons to their other sibling. tive siblings could be either genetic or environmental in. on the SIDE this micro analysis may go beyond the, origin because first degree relatives are 50 similar ge. genetic make up of family members Regardless of the. explanation it is noteworthy that this first study of the netically In the absence of selective placement howev. etiology of nonshared environment as assessed by the er adoptive sibling pairs do not resemble each other. SIDE shows little evidence of genetic influence genetically Thus if the relationship between nonshared. environmental measures and differences in sibling be. haviors reflects genetic differences within pairs of sib. 7 2 Are relationships between nonshared environment lings we would expect correlations for sibling differences. and behavior mediated genetically Thus one set of data in environment and in behavior to be greater for adoptive. has implied that nonshared environmental influences siblings than for nonadoptive siblings. may be virtually uncontaminated by hereditary influ A recent study of adoptive and nonadoptive infant. ences If a measure of nonshared environment is not siblings in the Colorado Adoption Project explored this. influenced by heredity its relationship to behavioral issue Daniels 1985 Although no measure designed. differences is unlikely to be mediated genetically None specifically to assess differential sibling experiences was. theless because so little work has been done in this area used the HOME and FES were included As mentioned. it is important to ask the next question whether genetic earlier the results suggested little differential experience. differences on measures of nonshared environment as for two siblings when each sibling s environment was. sessed directly by the SIDE or indirectly through sibling assessed separately at the time the child was 12 months of. difference scores on shared environment measures are age However even these slight differential experiences. translated into behavioral differences between siblings of the siblings as assessed by the HOM E and FES showed. The possibility of genetic mediation of relationships be some association r s 2 3 with behavioral dif. tween environment and behavior has recently been dis ferences between the infant siblings For hundreds of. cussed Plomin Loehlin DeFries 1985 although not comparisons between sibling differences on the HOME. in the context of nonshared environmental influences and FES and various sibling behavioral differences over. One way to study nonshared environment free of 13 were significant For example at 12 months dif. genetic bias is to relate experiential differences within ferences in the extent to which mothers consciously. pairs of identical twins to behavioral differences within encouraged developmental advance as measured by the. the twin pairs Because identical twins share exactly the HOME correlated 31 with differences in the siblings. same heredity environmental and behavioral differences activity level as assessed by the tester using the Infant. within pairs cannot be explained by genetic differences Behavior Record More to the point this study showed. Twin studies can also assess possible genetic influences only environmental mediation in that no differences were. by comparing the relationship between experiential and found between correlations for adoptive and nonadoptive. behavioral differences within identical twin pairs to the sibling pairs. relationship within fraternal twin pairs If heredity is In the only other study examining nonshared environ. influential the correlations will be greater for fraternal ment behavior relationships the SIDE scales were re. twins than for identical twins because differences within lated to adolescent sibling personality differences. 14 BEHAVIORAL AND BRAIN SCIENCES 1987 10 1,Plomin Daniels Nonshared environment. Daniels in press In this study of adoptive and nonadop vironmental influences will be aided by theories of the. tive siblings no genetic influence was detected even processes by which nonshared environment can lead to. though the SIDE accounted for 6 26 of the variance developmental differences between siblings Nearly. of sibling personality difference scores For example the every psychological theory including learning psycho. sibling who experienced more sibling closeness and peer analytic Piagetian ethological biopsychological family. popularity also reported more sociability as compared to system and social psychological theories has some. his sibling the sibling who reported more sibling jealousy thing to offer when viewed from the perspective of. and peer delinquency also reported more emotionality as nonshared environment To mention but a few examples. compared to his sibling Although longitudinal work is learning theory offers sibling conditioning and modeling. necessary to address the direction of effects in these as processes by which nonshared environment may leave. relationships it can at least be said that genetically its mark sibling deidentification and split parent identifi. influenced personality differences between the siblings cation have emerged from psychoanalytic theory Schach. do not lead to differences in their interactions with ter 1982 social psychology could offer contrast effects. siblings and peers and attribution differences as possible mediators of non. In summary the results of these two studies suggest shared experience Developing a coherent theory of the. that at least in infancy heredity does not importantly processes by which nonshared experience lead to dif. mediate relationships between siblings experiential dif ferences between children in the same family is a high. ferences and differences in their behavior A reasonable priority for the area. priority for research would be to identify relationships Our new knowledge concerning the importance of. between nonshared environment and sibling differences nonshared environment may have its deepest implica. in behavior using nontwin siblings in nonadoptive fami tions for intervention The data are descriptive not. lies and to worry about the direction of effects only after prescriptive That is they indicate that of the variability. such relationships are found that exists in children s environments the portion of the. environmental variability that affects children s psycho. logical development is nearly exclusively of the non. 8 Implications and conclusions shared variety It does not mean that shared environmen. tal factors cannot or should not affect the development of. In this target article we have presented evidence that children Nonetheless it is critical for interventionists to. converges on the conclusion that children in the same know for example that what parents do that is experi. family experience practically no shared environmental enced similarly by their children does not have an impact. influence that makes them similar for behavioral traits In on their behavioral development If the effects of parents. other words the effective environments of siblings are on their children lie in the unique environments they. hardly any more similar than are the environments of provide for each child childrearing books need to be. strangers who grow up in different families This conclu rewritten and early childhood education and interven. sion has been put particularly forcefully by Scarr and tions aimed at the prevention of psyehopathology need to. Grajek 1982 p 361 be rethought The importance of nonshared environ. Lest the reader slip over these results let us make ments as it works both systematically and stochastically. explicit the implications of these findings Upper mid implies that the environmental impact on children works. dle class brothers who attend the same school and through the power of differentiation within the family. whose parents take them to the same plays sporting The possibly subtle differenees experienced or perceived. events music lessons and therapists and use similar by children in the same family are the environmental. child rearing practices on them are little more similar factors that drive behavioral development. in personality measures than they are to working class In conclusion although it was less than a decade ago. or farm boys whose lives are totally different Now that the importance of nonshared environmental influ. perhaps this is an exaggeration of the known facts but ences was brought to the attention of behavioral scien. not by much Given the low correlations of biological tists the results of research in this area have led to the. siblings and the near zero correlations of adopted following conclusions. siblings it is evident that most of the variance in 1 Behavioral genetic studies consistently point to. personality arises in the environmental differences nonshared environment as the most important source of. among siblings not in the differenees among families environmental variance for personality psyehopath. This unsettling fact is rich in implications for research ology and IQ after childhood. theory and application In terms of research implica 2 When more than one child is studied per family it is. tions studies of the family environment and socialization apparent that siblings in the same family experience. can take advantage of the key of nonshared environment considerably different environments in terms of their. by studying more than one child per family in order to treatment of each other in their peer interactions and. identify environmental factors that make children in a perhaps in terms of parental treatment. family so different from one another Recent studies 3 Family composition variables such as birth order. presented in this review indicate that this is a promising and gender differences account for only a small l 5. area for research portion of the variance of sibling differences in devel. The importance of nonshared environment also sug opment. gests the need for a theoretical reconceptualization of 4 Differences in siblings experiences relate signifi. environmental influences in development Most impor cantly to siblings differences in behavior implying that. tant the child rather than the family must be considered nonshared environmental influences are at least in part. the unit of socialization The search for nonshared en systematic.

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