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Genetic and Environm ental Inf luences onPercept ions of Self-Worth and Com pe tence inAdolescence : A S tudy of Tw ins Full Siblings

and Step-Siblings

Shirley McGuire and Jenae M. NeiderhiserThe Pennsylvania S tate University

David ReissGeorge Washington University

E Mavis Hetherington

University of Virginia

Robert Plomin

The Pennsylvania S tate University

McGuiRE, SHIRLEY; NEIDERHISER, JENAE M .; REISS, DAVID; HETHEHINGTON, E. MAVIS; and PLOMIROBERT. Genetic and Environmental Influences on Perceptions of Self-Worth and Competenin Adolescence: A Study of Twins, Full Siblings, and Step-Siblings. CHILD DEVELOPMENT, 1994,65, 785-799. Although it is generally assum ed that the origins of adolescents' perceptions oself-competence lie in shared family enviro nm ental influences, the con tributions of nOl^MUaenvironmental or genetic influences have not been explored. We investigated sibling reseblance for perceived competence and self-worth in 720 adoles cent pairs aged 10 to 18 yearusing a twin, full sibling , and step-sibling design. Our goals w ere to assess the magnitude oshared and nonsh ared environm ental influences and to disentangle resemblance due to sharegenetic heritage from that due to shared environmental experiences. Shared en vironm en wasnot significant for any of the scales. 4 of the subscaies s how ed significant genetic influescholastic, social, physical, and athletic competence. We also explored possible sources oinfluences on perceived com petence . B ivariate models revealed common gen etic varia between scholastic competence and vocabulary and social competence and sociability. These sures, however, did not account for all of the genetic variance in perceived social and scholascompetence.

The family environment has been high- thermore, sibling pairs who vary in genetichghted as a central source of children's relatedness can be used to separate siblingsense of self (Wylie, 1979). Few studies, similarity due to shared genetic heritagehowever, have explored perceived self- from that due to shared environment,worth and competence for more than onechild per family. The purpose of the present Environmental influences. —Adoles-study was to examine the link between the cence, in particular, is a time of heightenedfamily environment and adolescent per- self-awareness with the emergence of newceived self-worth and competence in sib- social roles and cognitive abilities (Elkind,lings who vary in genetic reiatedness. The 1967; Harter, 1990a). Several theoretical ori-

use of siblings makes it possible to disentan- entations emphasize the social, especiallygle environmental influences shared by sib- the parental, origins of perceived compe-lings from those specific to one child Fur- tence and self-worth (e g Bandura 1977;



78 6 Child Developm ent

C ooley, 1902; Gecas & Schwalbe, 1986;Harter, 1986, 1990b; Mead, 1934; Rogers,

1951; Sullivan, 1953). Following from thesetheoretical traditions, recent research in ado-lescent seif-perception has focused on thefamily environment Many aspects of thefamily environment have been considered,such as the general atmosphere of the home(e.g., Amato, 1989; C ooper, H olman, &B raithwaite, 1983), pare nting styles u singcombinations of warmth and control (B aum-rind, 1971, 1978; L ambom , Mounts,Steinberg, & D om busch , 1991), and specificparental behaviors (e.g., Amato & O chitree,1986; Gecas & Schwalbe, 1986; Loeb,Horsa, & Horton, 1980). A variety of meth-ods have been employed, such as question-naires and interviews (e.g.. D emo , S mall, &Savin-Williams, 1987; Felson & Zielinski,1989; G ecas & Schw albe, 1986) and observa-tions (e.g., Isberg et al., 1989; Loeb et al.,1980). Although many of these studies sup-port links between family environment andadolescent self-competence, none have in-vestigated outcomes for more than one child

in die &mily. We are not aware of any re-search that investigates the extent to whichenvironmental influences relevant to self-perception operate on a family-by-familybasis making children in the same familysimilar (shared environment), or on anindividual-by-individual basis making chil-dren in the same family different (noa sharedenvironment). In other words, by studyingonly one child per family, investigators havenot been able to explore whether or not sib-lings experience the family en viron m entsimilarly. Al&ough parental warmth andcontrol may be important factors in under-standing an individual child's self-worth,each sibling in the family may have a differ-ent experience with the parent. We are notclaiming that the results of previous studiesare invalid or unimportant. Instead, we arearguing that sibling studies can extend so-cialization research by providing informa-tion about shared and nonshared experi-ences. By examining only one child perfamily, researchers have implied that theimportant differences in environmental ex-periences occur between families, but dif-

are not shared by siblings (Dunn & P lomin,1990; Reiss et al., in press). Furthermore,

sibling resemblance is often explained bygenetic rather than environmental influ-ences, as discussed in the following section.

Genetic infiuences.—Siblings can re-semble each other not only because theyshare environments, but also because theyshare a genetic heritage. Nevertheless, pos-sible genetic contributions to perceptions ofself-worth and competence have not beenpreviously examined. It seems reasonable toconsider the possibility that perceived com-

petence and self-wortii are affected by theubiquitous genetic influence on other char-acteristics of the person, such as cojpiitiveability (Plomin & Neiderhiser, 1992), scho-lastic achievement (Thompson, Detterman,& Plomin, 1^1), personality (Plomin, Chi-puer, & Loehlin, 19^), psychopathology(Rutter et al., 1990), athletic ability (B oom-sma, van den Bree, Orlebeke, & Molenaar,1989), and physical appearance (Susanne,1975). In addition to investigating geneticinfluences on perceived competence andself-worth, we also explored the extent towhich such genetic influences are linked togenetic contributions to related vwiables.That is, we examined possible anteced-ents as a first step in understanding the na-ture of genetic influences on perceptions ofthe self. Specifically, we considered theprospect diat children's genetically iiiflu-enced personality traits and abilities may bethe basis of genetic contributions to per-ceived self-worth and competence.

Furtherm ore, H arter's (1983,1985) workon the multidimenslonality of self-eval-uation and her measure (Harter, 1^8),which assesses a number of areas importantto adolescen t self-competenc e, fdlowed us toinvestigate multiple dimensions of tiie self.C hildren distinguish betw een different ar-eas of competence (e.g., school vs. a^ le t i ccompetence) and begin to describe them-selves in more complex ways across devel-opment (Damon & H art, 1982; H arter, 1983;Rosenberg, 1979). In addition, H arter's mea-sure ( 1 9 ^ , 1988) a^ es se s general self-worth



McG uire et al. 7 8

In summuy three questions were ad-dressed conceming d ie origins of a d ^ s -cents ' self-worth and competence: (1) Towhat extent are environ me ntal influences onperceptions of self-worth and competenceshared or not shared ijy siblings? (2) Are ge-netic infiuences on children's self-eval-uations significant? (3) C an ge neti c infiu-ence on these measures be explained bygenetic contributions to related variablessuch as cognitive ability and personality?

Method

Participants. — Participants w ere 720same-sex sibling pairs taking part in theNonshared Environment and AdolescentDevelopment (NEAD) Project, a regionallyrepresentative sa mple of two-paren t familieswith a pair of adolescent siblings no morethan 4 years apart in age (Reiss et al., inpress). Nondivorced families were obtainedthrough random-digit dialing of 10,000 tele-phone numbers throughout the UnitedStates; howev er, most twin and step familieswere recruited through a national marketpanel of 675,000 households. The 720 fami-lies are primarily middle class (average fam-ily income was $25,000-$35,000) an d of Eu-ropean ancestry (94% of mothers and 93%of the fathers we re C aucasian). T he averageyears of education were 13.6 for mothers and14.0 for fathers. The nondivorced and stepfamilies did not differ with respect to demo-graphic characteristics, such as the ages ofthe two children, family size, income level,ethnicity, and religion. Father's education

level, mother's age, and number of yearsmarried were higher in nondivorced fami-lies; however, none of these variables weresignificantly correlated with the measuresused in this report.

Each family contained at least twosame-sex children between the ages of 10and 18 years of age. The sample includedchildren between 10 and 18 years (M =14.47, SD = 2.21) and their siblings be-twe en 9 and 18 years of age (M = 12.89, SD= 2.20). The present design resulted in sixsibling categories representing two familyt di d d t f ili Th

SD = 3.5). Step families included 180siblings (86 boys, 94 girls), 109 half sib(60 boys, 49 girls), and 130 unrelatedlings (74 boys, 56 girls). The three sibling groups were matched by age ofest child and age spacing in order to incthe comparability of these groups.

Zygosity of the twins was determby tester, self-reports, and parent reporphysical similarity (e.g., eye and hair cusing a modified questionnaire designeadolescents. Twins were classified as Dany of the respo nde nts rep orted differe

in physical characteristics. In additionquestionnaire asked if the twins we realike as two peas in a po d, if peo ple mixed them up, or if people used spmarks to tell them apart. Twins were cfied as D Z if respon dents reported neveing confused about each twin's identitymeasure has yielded over 90% accuwhe n com pared to tests of single-gene mers in the blood (Nichols & B ilbro, Twelve of the twins, however, could nclassified with certainty and were exclfrom these analyses.

Procedure.—The families were inviewed and videotaped twice in their happroximately 2 weeks apart. The pranalyses used only data collected durininterview session. Each interview wasducted by two testers and lasted appmately 3 hours.

Measures.— Perceived com petence

self-worth were assessed using Ha(1988) Self-Perception Profile for Adcents. Each child completed the quesnaire on their own; however, interviewere on hand to answer any questionscerning the scale. T his version contains5-item subscales including scholasticcial, athletic, physical appearance, morfriendship, romantic appeal, and job cotence, as well as global self-worth. Thelescent version was used for the entire ple, with two exceptions for children bseventh grade: (a) the word teen age r repla ced w ith ki d for each item , anthe job competence and romantic ap



788 C hild D evelopment

sistency for the child version of the moralitysubscale was .49, which warrants caution ininterpretations involving this subscale.

Personality was measured using theEAS Tem perament S urvey—C hild Form, aself-report questionnaire consisting of 20items rated on a 5-point scale (B uss &Plomin, 1984). The EAS subscales includeemotionality, activity, and sociability. C hil-dren completed the questionnaire on theirown, and interviewers were available forany questions. T he internal consistency reli-abilities using C ronbach's alpha were m od-erate: .61 for emotionality, .57 for activity,and .62 for sociability. B uss and Plomiri(1984) report 2-week test-retest correlationsof .82 for emotionality, .81 for activity, and.85 for sociability.

Vocabulary was measured using the Vo-cabulary subtest of the Wechsler Intelli-gence Scales for C hildren— R evised (WISC -R) for children un der 17 years (Wechsler1974) and the Vocabulary subtest of the'Wechsler Adult Intelligence Scales—Re-

vised (WAIS-R) for children 17 and 18 yearsold (Wechsler, 1981). During the home in-terview, each child was asked to define 32WISC -R or WAIS-R words. T he interviewerrated the acceptability of the definition.

Design and model-fitting.—The presentstudy employs a combined twin and step-sibling design to disentangle similarity dueto shared environment from that due toshared genetic heritage. B ehavioral ge neticmodels use quantitative genetic theory andquasi-experimental methods to decomposephenotypic (measured) variance into geneticand environmental components of variance(Plomin, D eF ries, & McC learn, 1990). Ac-cording to this theory, if shared environmentis important, the siblings will be correlatedacross all sibling types. In the present de-sign, the correlation between unrelated sib-lings is the most direct index of shared envi-ronment because these siblings share onlyenvironmental infiuences. If genetic infiu-ences are important, then the correlationswill show the following pattern: MZ twins> DZ twins = foil siblings > half siblings

A model-fitting approach provides amore powerful analysis of sibling resem-blance than examining patterns of correla-tions. Model-fitting analyzes the data for dif-ferent sibling types simultaneously, tests thefit of the model, makes assumptions explicitand permits tests of alternative models(Eaves, Last, Young, & Martin, 1978; Jink& Fulker, 1970; Loehlin, 1987).

In this study, maximum-likelihoodmodel-fitting analyses were performed usingLISREL VII (Joreskog & Sorbom, 1989)These modeling techniques have been ex-

plained in more detail elsew here (see B oom-sma, Martin, & N eale, 1989; B oomsma &Molenaar, 1986; Fulker, B aker, & B lock1983; N eale & C ardon, 1992). T wo modelswere used in this study: univariate and bi-variate. The univariate model is depicted asa path diagram in Figure 1. Sibling covari-ances were used to decompose the varianceof a measure into genetic and en vironmen talcomponents. Four parameters were derived:additive genetic variance (GJ, nonadditivegenetic variance (Gj), shared environmentalvariance (E,), and nonshared environmentalvariance (EJ, which includes error variance.This path diagram also indicates the assump-tions of the model: no assortative mating, noselective placement of step-siblings, nonad-ditive genetic variance is due to dominanceand not epistasis, genotype-environmentcorrelation and interaction are negligible,and shared environment is equal across sib-ling type. The last assumption, shared envi-ronment is equal across sibling types, wastested by two subm odels. T he first submodelestimated a separate shared environmentalparameter for twins and nontwins and thesecond for nondivorced and step families.

The bivariate models use cross-siblingcovariances to decompose the covariancebetween two measures into common andunique genetic and environmental compo-nents. Model-fitting was employed in orderto analyze the data for the six sibling groupssimultaneously. F igure 2 represen ts a bivari-ate model with eight latent variables (seeN eale & C ardon, 1992). Fou r latent variables(G^ Gj E andEJ refiect the genetic and



r = 1.0 MZ; .5 DZ Full Sib s; .25 Halt Sib s; .0 Unrelated Sibs

r ~ t.O MZ; .25 DZ Full Sibs: .0 Half Unrelated Sibs

FIG. 1.—A univariate behavioral genetic model. Sibj and Sib2 are measured variables fosiblings. G^ and G^ are latent variables representing, respectively, additive and nonadditive nance) genetic variance. E; is a latent variable representing shared environmental influenceto the sibling pair. E^ represents residual variance that does not result in sibling covariaincludes nonshared environmental influence and error of measurement. The curved, twarrows indicate correlations between the variables they connect, and the one-headed arrows paths, standardized partial regressions of the measured variable on the latent variable.

Variable X Variable Y



79 0 ChUd Development

two variables, then the cross-sibling correla-tions will show the following pattern men-

tioned above: MZ > DZ = full siblings >half siblings > unrelated siblings. If the as-sociation is a result of shared environmentalinfiuences, then the cross-sibling correla-tions will not dlflFer across the six groups.The four additional latent variables (g,, g^,e^, and e^) reflect g enetic a nd enviro nm entalinfiuences unique to the second measure.

ResultsAfter considering mean differences for

age and gender, w e investigated genetic andenvironmental effects on perceived compe-tence and self-worth. That is, we examinedsibling intraclass correlations and fitted uni-variate models for each subscale. Then, weexplored genetic mediation between sub-scales showing a significant heritability saida related heritable variable. We examinedgenetic mediation between (fl) perceivedscholastic comjwtence and vocabulary and(b ) perce ived social com pete nce an d socia-

bility.Mean and variance differences.—Aê

and gender differences can inflate siblingcorrelations beca use siblings are close in age(twins are identical in age) and the study in-cluded only same-sex sibling pairs. There-fore, we examined mean differences for theentire sample using a 2 (gender) x 3 (agegroup) MANOVA, dividing the sample intothree age groups: early adolescence (9 to 12years), middle adolescence (13 to 15 years),and late adolescence (16 to 18 years). Therewere significant gender, age, and gender xage effects; however, they accounted foronly a small amount of the variance, and no

effect or interaction explained more than 6%of the varian ce. A lthough ge nde r an d age ef-

fects w ere sm all, in order to pre ven t inflationof sibling correlations, scores w ere correctedfor age (as a continuous variable), gender,and gender x age interaction effects (seeMcGue & BouchfHfd, 1984). In addition, wetested for homogeneity of variances using alikelihood ratio test (Morrison, 1976) andfound no significant differences.

Genetic and environmental influenceson self-perceptions.— We next examined them^nitude of genetic and environmental in-fiuences on perceived self-worth and self-competence. Taibie 1 presents sibling in-traclass correlations for each subscale. Ingeneral, ^ e pattern of correlations acrosssibling groups indicates same genetic infiu-ence , but little sliiured environmental influ-ence. As mentioned above, correlations forunrelated sitdin^s {provide a direct estimateof shared environmea^ itidluences. Exceptfor athletic competence, unrekted siblingswere not significso^y correlcêd, su^pestingthat shared environmentai inlhiences areminimal. In addition, the sibling correla-tions for full siUings in nondivorced andstep families were similar in magnitude,suêsting that environmental infitiencesmay not dif&r by family type. For mostscales, MZ twin correlations exceeded thoseof DZ twins and fiill siblings, and DZ twinsand full siblings exceed uiose of half andunrelated siblings. This pattern, represent-ing evideace for genetic infiuence, was notevident for all subscales. The morality and

friendship subscales, in particular, showedcorrelations for MZ twins that were nogreater than correlations for the oth er siblinggroups.

TA B L E 1

SIBLING INTHACLASS COHHELATIONS FOB PERCEIVED SELF-COMPETENC E AND SEL F-WORT HBY SIBLING RELATIONSHIP CROUP

NONDIVORCED FAMILIES

MZ DZSUBSC ALE ( * 93) ( 97)

STEP FAMILIES

Full Full Half Unrelated(n = 94) (n = 179) (n = 109) (n = 129)



McG uire et al. 7 9

Univariate maximum-likelihood model-fitting confirmed the impressions gleanedfrom the patterns of sibling correlations. Weused variance/covariance matrices for theseanalyses because correlation matrices maymodify the mod el and prod uce incorrect chi-square values and standard errors (C udeck,1989; Neale & C ardon, 1992). In addition,information about variability is especiallyimportant when comparing multiple groupsand using equality constraints (C udeck,1989; Neale & C ardon, 1992). W hen theseanalyses were conducted using either corre-lations (see B oomsa et al., 1989) or bet w een -and within-pair cross products (Fulker et al.,1983), the results were the same.

Table 2 contains the chi-squares, ge-netic and environmental parameter esti-mates, and components of variance. T he chi-square values were not significant for any ofthe models, suggesting that the fit betweenthe model and the data is acceptable. In ad-dition, the Goodness-of-Fit Index (GFI) foreach analysis indicated an excellent fit(GFIs ranged from .97 for athletic compe-

tence to 1.00 for general self-worth). Our de-sign, however, was not powerful enough todifferentiate the additive (G^) and nonaddi-tive (Gj) genetic parameters for five of theseven subscales. In these cases, one of thegenetic parameters was fixed to zero. To ob-tain the va rianc e co m ponen ts (h^, es^, en'^),the parameter estimates were squared andeach value was divided by the total variance(i.e., the sum of all the squared values). Forexample, h^ for scholastic competence (.61)

was derived by squaring the gen etic parters, summing them (1.54^ + 2.01^ = 6and dividing by the total variance (1.54

^ + .26 + 2.01^ = 10.5).Finally, we tested four alternative

els: (1) no genetic influence, (2) no shenvironmental infiuence, (3) separate senvironmental parameters for twins nontwins, and (4) separate shared envmental infiuences for siblings in nvorced and step families. In order to tesignificance of the genetic variance conent, both genetic parameters (G^ anwere set to zero, and a change in x^(2calculated. Likewise, the significance oshared environmental parameter was tby setting Eg to zero and exam iningchange inx^(l)- The significance of the two models was determined by compthe change in the fit from th e stand ard mwhen separate shared environmental paeters were allowed for twins versus twins or for siblings (including twinnondivorced families versus siblings infamilies.

C oncerning shared environmentafiuence, the impression gained from thling correlations in T abl e 1 was suppby the model-fltting results in Table 2tested th e significance of the shared envmental parameter by setting the valuzero and examining the change in square. None of the subscales showednificant shared environment. Nongevariance was accounted for solely by

TA B L E 2

UNIVARIATE MOD EL-FITTING RESULTS FOR PERCEIVED SELF-C OMPETENC E AND S ELF-W

SUBSCALE

Athletic

VARIANCECOMPONENTS

h^

.. .61*

.. .54*

.. .49*

es^

.01

.04

.0 4

en^

.38*

.42*

.47*

G a

1.54(±.35)

.00

.0 0

PA R A M E T E R E S T I M AT E SAND S TA N D A R D E R R O R S

G d

2.01(±.26)

2.57(±.21)

2.14(± 63)

E s

.2 6(±•44)

.71(±•41)

.59(± 69)

E n

2.01(±.05)

2.26(±.16)

2.10(± 15)

X^(14

10.80

14.18

8,31



792 C hild D evelopmentnonshared environmental parameter, whichincludes error of measurement. As indi-

cated, we also conducted model-fitting anal-yses using submodels that test for the possi-bility of special twin environmental effects(as compared to nontwin siblings) and forspecial shared environmental effects for sib-lings in nondivorced versus step families.The chi-square changes for these submodelswere not significant; that is, these submod-els did not fit the data better than the stan-dard model. Model-fitting analyses did notprovide support for the hypothesis that twinsshare greater environmental influences thannontwins, or that siblings in nondivorcedfamilies share greater environmental infiu-ences than siblings in step families.

The results in Table 2 indicate signifi-cant genetic infiuence for the Scholastic, So-cial, A thletic, and P hysical A ppearance sub-scales. The estimated effect size of geneticinfiuence is noteworthy: Fully half of the to-tal variance of these scales appears to begenetic in origin. General sell-worth and

morality showed some, but nonsignificant,genetic infiuence. The friendship scaleshowed negligible genetic infiuence. Itshould be noted, however, that the lack ofshared and genetic influences on the moral-ity scale may be due to low reliability.

Two other issues were explored with re-gard to our design. We were concerned thatage spacing might be a significant factor fornontwin siblings, and that length of timestep-siblings spent together might infiuencestep-sibling correlations. First, we examinedthe sibling intraclass correlations of nontwinsiblings after controlling for age spacing.Second, we investigated step-siblings' in-traclass correlations after controlling for theduration of the rem arriage. In both cas es, thesibling correlations were not significantlydifferent from those reported in Table 1.

Genetic links with other variables.—Since several of the scales showed substan-

tial genetic influence, it is appropriate to askwhether other genetically infiuenced vari-ables, such as cognitive abilities and person-

.61 and was significantly correlated with tlievocabulary subscale of the WISG-R/WAIS-R

intelligence test (r = .28). Vocabulary alsoshows genetic infiuence with a heritabilityof .31. Therefore, our first bivariate analysisinvestigated the extent of common geneticinfiuence between perceived competence(scholastic) and a m easure of ability (vocabu-lary). Perceived social competence alsoyielded significant genetic infiuence with aheritability of .49. This subscale was relatedto the child's report of EAS sociability (r =•.35), which also shows significant gen etic in-fluence (h^ = .44). Although both of thesescales were self-report measures concerningchildren's social relations, the question-naires were not measuring the same con-struct. T he EA S sociability subscale assesseshow much the child likes peo ple (e.g., Iam something of a loner ). Harter's socialcompetence subscale, on the other hand,measures the child's perceptions of his orher popularity (e.g., Some kids have a lot offriends ). Thus, our second bivariate modelinvestigated genetic links between these

two variables.

In order to investigate genetic associa-tion, we first examined cross-sibling correla-tions by sibling type (see Table 3). In bothcases, the pattern indicated possible com-mon genetic influence between the twomeasures, although the result primarilyhinges on the cross-MZ correlations. B ivari-ate maximum-likelihood model-fitting usingthe cross-sibling intraclass covariances also

indicated significant common genetic influ-ence. B oth the m odel for vocabulary andscholastic competence, xH^^) = 55.35, p <.25, and for child-reported sociability and so-cial competence, x^(48) = 30.17, p < .98,fit the data. Similar to the univa riate mo del,genetic infiuences loaded on either the addi-tive or nonadditive paths. We were con-cerned that our models were not powerfulenough to make the distinction between ad-ditive and nonadditive influences. There-fore, we fltted two submodels and examinedthe change in chi-square values. First, weexamined a model that set nonadditive pa-

t (Gj d gj) t d f d ig



McG uire et al. 7

TA B L E 3

C ROSS-SIBLING COB RELATIONS FOR PERCEIVED SOCIAL C OMPETENCE AND SOCIABILITYPERCEIVED SC HOLASTIC C OMPETENC E AND VOCABULARY BY SIBLING RELATIONSHIP

INTACT FAMILIES STEP FAMILIES

MZ D Z Full Full H alf UnrelaBIVARIATE MODEL (n = 93) (n = 97) (n = 94) (n = 179) (n - 109) (n = 12

Social competence andsociability 35

Scholastic competenceand vocabulary 45

p < .05.

-.12

.13

-.02

.0 9

.0 3

.0 7

.06

.04

.05

- .05

ga) to zero and found no significant differ-ences in fit for the scholastic competencemodel, change x^(2) = -69, p < .75, or thesocial competence model, change, x^(2) ^.56, p < .90, compared to the original mod-els, indicating that additive genetic factorswere not significant.

In Figures 3 and 4, we present the stan-

dardized results of the parsimonious modelsthat included nonadditive genetic, sharedenvironment, and nonshared environmentparameters. The results revealed significantcommon genetic influence in both models(see Figs. 3 and 4) as indicated by the sig-nificant paths from G^ (common genetic la-tent variable) to each measure. Vocabularyand sociability, however, did not account forall genetic influence on perceived scholastic

and social competence. This is indicatthe significant paths from gd (unique gelatent variable) to the Harter subscale.

These path coefficients can be uscalculate the percentage of genetic varthe Harter subscale has in common wrelated measure and the percentage onetic variance that is unique to the Hmeasure. These percentages are obtain

dividing the squared path (G^) from thtent variable to the Harter subscale btotal genetic variance (the sum of thesquared paths from G j and gj to the Hmeasure). Social competence shares 37its genetic variance with sociability .42 1.42^ + .552 = .368), and 6 3% of netic variance is unique to social cotence. Likewise, scholastic compeshares 39% of its genetic variance wit

Sociability SocialCompetence



79 4 Child Development

Vocabulary

5 8

ScholasttcCompetence

FIG. 4.—B ivariate results for the association between the Weschler Vocabulary subscale and tHarter Scholastic C ompetence subscale.

cabulary, and 6 1% of its genetic variance isunique to scholastic competence.

DiscnssionTo what extent are environmental in-

fiuences on adolescent perceived self-worthand competence shared or not shared by sib-lings growing up in the same family? A clearanswer emerged: The salient environmentalinfiuences are not shared by siblings. Sib-ling correlations were low, and this modestsibling resemblance was accounted for bygenetic resemblance rather than shared en-

vironment.Modei-fitting analyses indicated that the

shared environmental parameter accountedfor only 1% to 6% of the v ariance. Moreover,shared environment did not differ signifi-cantly for twins versus nontwin siblings, orfor siblings in nondivorced families versusstep families. In contrast to these results,which indicate that environmental infiu-ences relevant to perceptions of the self arenonshared, most research has implicitly as-sumed that salient environmental infiuencesare shared by children growing up in thesame family For example previous work

dren in the same family experience similarenvironments. Our findings suggest that ex-

periences that are shared by siblings (e.g.,parental educE^on, SES, family size, familyclimate) do not contribute importantly to thedevelopm ent of Individual diffidences in ^ -olescent self-worth and perceived compe-tence. It is impo rtant to note that our samp lewas primarily middle class, and that sharedand nonshared environmental effects maybe different for children growing up in dis-advantaged or abusive families (see Scarr,19Sffi). In addition, we investigated the per-ceived co m peten ce and self-wortii of adoles-cent siblings. Shared environmental influ-ences, however, may be greater whenchildren are younger, and genetic infiuencesmay become more pronounced as childrenbecome older.

This conclusion does not imply that en-vironmental influences are inconsequential.On the contrary, perceptions of self-worthand com[wtence showed substantial non-shared environmental infiuence (i.e., experi-ences specific to the individual child). Al-though error of measurement is alsoincluded in this component of variance



understanding most domains of individualdifferences in development (Dunn &Plomin, 1990). For exam ple, paren tal differ-

ential treatment has been associated withadjustment problems in middle childhood(D unn, S tocker, & P lomin, 1990) and adoles-cence (DanieJs, Dunn, Furstenberg, &Plomin, 1985). In addition, we found thatscholastic and social competence did notshow common nonshared environmental in-fiuences with vocabulary and self-reportedsociability. It may be that achievement, per-sonality, self-competence, and other out-comes are related to different aspects of sib-lings' nonshared experiences.

The data in this article provide only in-direct support for nonshared environmentalinfiuences on self-perception (i.e., we didnot measure differential experiences di-rectly). C urrently, in the larger NE A D study,of which this investigation is a part, we areexploring a range of specific nonshared par-enting, sibling, and peer environment vari-ables for the possible influence on self-perception. Still, the findings from this

paper suggest that nonshared environmentalinfluence is an important direction for futurestudies on self-perception.

The second question addressed in thisresearch—Are genetic influences involvedin individual differences in perceived self-worth and competence?—yielded an affir-mative answer. Significant genetic influencewas found for four dimensions of perceivedcompetence: scholastic, athletic, social, andphysical appearanc e. A lthough this issue hasnot been investigated previously, these re-sults should not be surprising because suchdimensions seem likely to be related to ge-netically influenced characteristics such asmorphological characteristics, stature, andcognitive abilities.

It should be noted that most of the evi-dence for significant gen etic influence in ourcombined twin and step-sibling designemerges from the twin design that compares

correlations for MZ and DZ twins, ratherthan from the step-sibling design that com-pares full siblings, half siblings, and unre-

McGuire et al 795

order interactions among genes know epistasis. Thus, the hallmark of nonaddigenetic influence is an MZ correlation

is greater than half the correlation for fidegree relatives, which is not uncommothe realm of personality (Plomin et al., 19Lykken and colleagues (Lykken, M cTellegen, & B ouchard, 1992) have recediscussed the importance of epistasis, ephasizing the higher-order interactamong genes, which they refer to emergenesis. Nonetheless, the possibiremains that, in terms of perceived comtence, MZ twins reared together are so mumore similar than DZ twins and other silings because of some unknown envmental process. T his assimilation hypocould be addressed empirically by comping twins reared together and twins reaapart, analyses that are p ossible in the Swish Adoption/Twin Study of Aging (Peersen et ai., 1991) among other studietwins reared apart.

We explored possible genetic link btween scholastic competence and one aof cognitive ability— vocabulary. B ivmodel-fitting indicated that the phenotycorrelation between scholastic compeand vocabulary had a genetic componNonetheless, vocabulary did not account all of the genetic infiuence on perceptiof scholastic com petenc e, and significantnetic infiuence independent of vocabulremained.

We also explored genetic associabetween perceived social competence anself-reported sociability. Like vocaband scholastic competence, the two mea-sures showed significant common ginfiuence, but most of the genetic variathat related to perceived social competwas independent of sociability. Genetic fluences on sociability might contribu perceived social competence in at least twways. First, sociability and actual soccompetence (e.g., peer status) may hav similar genetic bases. Second, there may be sim

ilar genetic contributions to perceptions one's own temperament and social comtence.



79 6 ChUd Developm ent

lastic competence. Genetic influence oncomponents of athletic ability and physicalappearance seem likely to be involved inperceptions of competence in these do-mains. Perceived social competence may beinfiuenced by genetically influenced per-sonality dimensions other than those as-sessed by the EAS temperam ent question-naire. Exploration of these potential sourcesof genetic contributions to dimensions ofperceived competence represents an impor-tant direction for future research. On theother hand, genetic influence on perceivedcompetence may rem ain after oth er factors

are taken into consideration.In addition, it is possible to argue that

genetic overlap, for example, between per-sonality and perceive d com petence, doesnot necessarily imply that genetic influenceon personality leads to genetic influence onperceived competence in a causal direction.It is possibie that the effect is in the otherdirection. That is, genetic influence on per-ceived competence—and perhaps more gen-erally on perceptions of one's world'—mayeffect, rather than merely reflect, the geneticinfluence seen in personality and psychopa-thology. An ongoing-year longitudinal ex-tension ofthe NEAD study will help to ad-dress this issue of causal sequence.

It is equally interesting that perceivedglobal self-worth, morality, and friendshipdo not show signiflcant genetic influence.The pattern of genetic infiuence across thesubscaies might shed light on the nature of

genetic influences on adolescent self-perception. For example, perceived socialcompetence showed significant heritability(h = .49), but perceived friendship did not{h = .10). This difference may reflect a dis-tinction mad e in the literature between peerrelations and friendships. Po pularity refersto the extent to which adolescents are likedby their pe er groups (B iJcowski & Hoza,1989; Hartup, 1983) and has been related togenetically influenced traits such as physicalappearance (Langlois & Stephan, 1981).Friendship, on the other hand, is a dyadicrelationship based on mutuality and self-di l (B ig l 1977 Y i 1980)

tence dimensions. Global self-worth is con-ceptualized as the sum of a person's suc-cesses weighted by their expectations(Harter, 1983, 1985, 1990b; James, 1950).That is, feelings of self-worth reflect thecombination of one's accomplishments andthe importance a person attaches to eachsuccess. During this process, adolescentsmight discount areas in which they are notcompetent (Harter, 1985) and thus negatepossible genetic influences. Perceived com-petence, on the otiier hand, refiects adoles-cents ' evaluations of their performance incertain areas (e.g., athletics or academics). It

may be that their ability to perform in a do-main is genetically influenced.

Based on behavioral genetics researchon attitudes and religiosity (Loehlin & Nich-ols, 1976), we had expected that moralitywould show shared environmental infiu-ences. AlAough the subscale contains ques -tions about guilt, it may be that it focusesmore on the child's behavior than on his orher attitudes. Thus, we may have been in-correct in expecting a shared environmentalcomponent for this scale. We suspect, how-ever, that the lack of shared environmentalor genetic influences may be due to the lowreliability ofthe morality subscale, and thatsuch influences on morality m ay warrant fur-ther examination.

It is important to note that the measuresused in this study w ere prim arily self-report,with the exception of vocabulary. A direc-tion for research involves the comparison of

self-reported competence and ratings ofcompetence by others such as teachers andpeers or employing naturalistic observa-tions. Multivariate genetic analyses of thisnexus of relationsh ips wou ld fiirther our un-derstanding of the nature of environmentaland genetic infiuences on competence. Inaddition, our mu lttvariate an alyses focusedon the presum ed gen etic anteceden ts of per-ceived competence. For example, we haveshown that genetic influence on vocabularyperformance substantially accounts for ge-netic contributions to perceived scholasticcompetence, even though independent ge-netic influence remains Another direction



McGuire et al. 797

factors. Disentangling the role of genetics insuch relationships might contribute togreater precision in the prediction, interven-tion, and prevention of competence-relatedpsychopathology.

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