Baker & Daiels 1990 Nonshared Environmental Influences and Personality Differences in Adult Twins

7/30/2019 Baker & Daiels 1990 Nonshared Environmental Influences and Personality Differences in Adult Twins

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Journal of Personality and Social Psychology1990, Vol. 58, No. 1,103-110 Copyright 1990 by the American Psychological Association, Inc.0022-3514/90/S00.75

Nonshared Environmental Influences andPersonality Differences in Adult TwinsLaura A. BakerUniversity of Southern California Denise DanielsStanford University

The twin design was used to examine the importa nce of different experiences of siblings within thefamily and to identify relations between twins' personality differences and their differential experi-ences. A sample of 161 monozygotic and 74 dizygotic twin individuals between the ages of 18 and75 years retrospectively reported on their different experiences when growing up . The Sibling Inven-tory of Differential Experience (SIDE) was used for thefirst ime w ith a sample of twin siblings. Inaddition, the twins provided self-report measures of affect and personality. In contrast to results froma sibling adoption design, this study of twins showed greater evidence for genetic variance in theSIDE scales. Nevertheless, the SIDE showed significant associations with differences in personalityand affect for monozygotic twins, which reflect p ure environm ent-behavior relations.

Behavioral genetic research p oints to new approaches for thestudy of development and socialization. Instead of remainingfocused on similarities and shared experiences of membersgrowing up in the same family, attention has turned to within-family differences in both experience and behavior. Also, thereis increasing awareness that socialization influences can be bothenvironmentally and genetically m ediated and, in turn , that theinterface between socialization influences and behavioral devel-opment may be explained by both genetic and environmentalfactors (Plomin, Loehlin, & DeFries, 1985).The three main areas of psychological development investi-gated in behavioral geneticscognition, personality, and psy-chopathologyeach show both genetic influence and a consid-erable amo unt of environmental influence. However, the envi-ronmental influences practically all vary within the family (i.e.,they are not shared by relatives in the same family) and thusserve to make family members different from one another. Evi-dence has shown that, especially in personality and psychopa-thology and in cognition after ado lescence, the differential expe-riences of family memb ers are invariably the most influentialin the socialization process (Plomin & D aniels, 1987; Plomin,DeFries, & McC learn, 1980).In addition to studying behavioral outcome and its generalsources of variation, some behavioral geneticists have investi-

This research was supported by a Biomedical Research Grant toLaura A. Baker at the University of Southern California. D enise Danielswas supported by National Institute of Mental Health G rant MH16744and by the William T. Grant Foundation.We wish to thank the twin participants who generously volunteeredtheir time and effort to disclose such personal information about theirlives. A great deal of appreciation is also expressed to the m any under-graduate research assistants for their contributions to this study, espe-cially David Caliguiri, Elvira Garcia , David Gosse, Yvette Lamar, Sun-ghi Park, Christine Suh, and Mary Villanueva.Correspondence concerning this article should be addressed to LauraA. Baker, Depa rtment of Psychology, SGM 501, University of SouthernCalifornia, Los Angeles, California 90089-1061.

gated psychometric measures of the environm ent. For example,in his studies of adolescent twins, Rowe (1983) has found sig-nificant genetic variance in a measure of perceived parentalaffection derived from Moos and Moos' (1981) Family Environ-ment Scale (FES). Another study of the FES also found moder-ate genetic influence for perceptions of warmth from parents ina joint analysis of twins reared together and twins reared sepa-rately (Plomin, McC learn, Pedersen, Nesselroade, & Bergeman,1988). Thus, the m ere labeling of a scale as an environmentalmeasure does not necessarily mean that it is uninfluenced bygenetic factors. Even for an instrum ent such as the FE S, whichis typically assumed to assess some aspects of the home thatare common to all family members, there may be importantdifferences from one sibling to the next that are a function ofboth genetic and environmental variations.

It is possible that the genetic differences between siblingswithin the family could lead them to experience the environ-ment differently. Daniels and Plomin (1985) and Daniels,Dunn, Furstenberg, and Plomin (1985) have reported that dy-nam ic different experiences of siblings, including nonm utua lityof sibling interaction, paren tal treatm ent, and peer-group char-acteristics, can be systematically tapped. In a sibling-adoptiondesign, these dynam ic variables were shown to be primarily en-vironmentally rather than genetically influenced (Daniels &Plomin, 1985; Daniels, 1986), in contrast to the previouslymentioned results from studies of the FES in twins. Further,Daniels et al. (1985) and Daniels (1986) have found relationsbetween the dynamic systematic differential experience withinthe family and persona lity and adjustment differences of adoles-cents and young adults. For example, the sibling who reportsexperiencing a more popular peer group and more sibling close-ness shows more sociability as compared with his or her sibling(Daniels, 1986), and the more well-adjusted sibling experiencesmore parental closeness and peer and sibling congeneality(Daniels et al., 1985).

If environmental measures are somewhat genetically influ-enced, they can relate to behavioral outcome for both geneticand environmental reasons (Plomin et al., 1985). Because ge-103


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104 LAURA A. BAKER AND DENISE DANIELSnetic variance was negligible in the SIDE, Daniels (1986) con-cluded that the SIDE-behavior associations are purely environ-mentally mediated. However, to the extent that genetic effectsare present in environmental indices, such as in the FES, theseeffects may be correlated with genetic effects in behavioral mea-sures. One preliminary investigation of twin siblings, unlike thesibling design, supported a genetic influence on differential ex-perience/behavior associations (Loehlin & Nichols, 1976).Thus, results concerning genetic effects in environmentalmeasures and their relations to behavior are somewhat variedacross the few studies investigating these effects. It is unclearwhether the discrepancies are due to different designs, differentinstruments, or both. More research is needed in this relativelynew and interesting area.

The present report describes a twin study utilizing a broadassessment of the differential experiences of twin siblings asthey were growing up . The Sibling Inventory of Differential Ex-perience (SIDE; Daniels& Plomin, 1985) was used to assess theextent to which the twin and adoption findings converge whenthe same measure is used. We posed two sets of questions: First,to what extent do monozygotic (MZ) twins perceive less differ-ential experience than dizygotic (DZ) twins, and is any suchdifference comparable to that in biological versus adoptive sib-lings' differential experience? Greater differential experience inDZ as compared with MZ twins might suggest a possible roleof genetic factors in the SIDE measures, but if the MZ-DZcomparison is much larger than the biological-adopted siblingcomparison, epistasis or special twin environmental factors (orboth) could be considered as well. Second, what are the system-atic relations of the SIDE scales to personality and affect in asample of MZ twin pairs? Regardless of any genetic effects thatmay be present in the SIDE, any significant associations be-tween the SIDE and MZ personality differences must be due tononshared environmental effects.

MethodSubjects

Twins were recruited throug h newspaper and radio advertisements aspart of the Southern California Twin Project in Los Angeles and thesurrounding community. In a study ofpersonality and lifestyles, twinpairs over the age of 18 years were sent a questionnaire packet, whichwas completed and returned by mail. Completed packets were returnedby 235 individuals representing 161 MZ twins and 74 DZ twins. In thissample there were 75 complete MZ and 29 complete DZ pairs for co-twin analyses. Average age of the respondents was 35.24 years (range =18-75).Zygosity determ ination for same-sex twin pairs was achieved prima r-ily using a questionnaire similar to that used by Nichols and Bilbrow(1966). Based on self-reports ofphysical similarities and differences,such methods have been found to correctly classify MZ and D Z twinswith over 90% accuracy. High-quality p hotographs were also obtainedfrom several participants, so that our own judgments of similarity couldbe checked for agreement with these tw ins' self-reports. Zygosity resultsfrom questionnaires and photographs were consistent for all pairs wheredata were available for both.

Measures and Preliminary AnalysesThe SIDE was used to assess twins ' past experiences. It was adminis-tered to both MZ and DZ twins, along with a set of questionnaires as-

Table 1Within-Pair Agreement for SIDE Scales

SIDE scaleSibling intera ctions

Sibling AntagonismSibling CaretakingSibling JealousySibling ClosenessParental treatmentMaternal AffectionMaternal ControlPaternal AffectionPaternal ControlPeer-group characteristicsPeer College-OrientationPeer DelinquencyPeer PopularityN (pairs)

All pairs

- .13**- .28**- .24**- .10- .28**- .52**- .43**-.47**- .43**- .64**- .47**83-104

Correlations'Age 34

.04- .13- .2 1*- . 2 1 *- .07-.56**- .34**-.64**- .54**- .63**-.47**40-54

Note. Relative scoring was used for SIDE scales.* Double-entry calculations were used for twin correlations.* p < . 1 0 . * * p < . 0 5 .

sessing dimensions of personality, affect, and health. Brief descriptionsof those measures used in the present analyses are provided here, inaddition to some preliminary analyses concerning twin correlations forthe different measures.SIDE: Retrospective reporting. This is the instrument of primaryfocus in this report. It measures individuals' self-reported perceptionsof their environm ents while growing up, as compared w ith their percep-tions of their siblings' environments. Psychometric properties of the

SIDE include high test-retest reliability and substantial sibling agree-ment validity.The SIDE contains 63 statements or adjectives describing differencesin parental treatment, sibling interactions, and peer group characteris-tics. The subjects respond to each of these items on a 5-point scale indi-cating the amo unt and direction of difference between themselves andtheir siblings (e.g., for sibling interaction items: 1 = My sibling ha s beenmuch more this way than me, 2 = My sibling ha s been a bit more thisway than me, 3 = My sibling and I have been the same for this, 4 = /have been a bit more this way than my sibling, and 5 = 7 have been muchmore this way than my sibling.) By averaging ratings for several relateditems, subscales may be derived reflecting perceived sibling differencesin four aspects of sibling interaction (Sibling Antagonism SA, SiblingJealousySJ, Sibling CaretakingSC T, and Sibling ClosenessSC),four aspects of parental tre atmen t (Maternal AffectionMA , Maternal

ControlMC, Paternal AffectionPA, and Paternal ControlPC),and three aspects ofpeer-group c haracteristics (Peer College-Orienta-tionPCO, Peer DelinquencyPD, and Peer PopularityPP).The SIDE was revised in the present study for use with twin siblingssimply by substituting every occurrence of the word sibling with twin.It was emphasized th at each individual should consider his or her expe-riences when growing up and living at home.Table 1 summarizes the SIDE scales and presents twin agreement forretrospective re ports of differential experience in the total sam ple of 104MZ and DZ pairs where both members completed the revised SIDE. Ifthe twins are in agreement about the ir retrospective reports of differen-tial experience relative to each other when growing up, these corre-lations should be negative and large in absolute value. This would indi-cate, for example, that one's own report of greater maternal affection


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DIFFERENTIAL EXPERIENCE IN TWINS 105Table 2Personality an d Affect Measures: Within-PairCorrelations for MZ and DZ Twins

MeasureEysenck Personality Questionn aireExtroversion

PsychoticismNeuroticismBern Sex-Role InventoryMasculinityFemininityAffect Intensity MeasureSelf-Rating Depression ScaleAffect Balance ScaleN (pairs)

.51*

.62*.41*

.35*.52*

.36*

.35*

.22*73-75

.34*

.20.35*

.18.28*

.13

.29*

.1129

Note. MZ = monozygotic; DZ = dizygotic.*p


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106 LAURA A. BAKER AND DENISE DANIELSdifference between my twin an d me) before forming the 11 SIDEscores. The averages for these absolute amounts of differentialexperience are presented in Table 3 separately for MZ and DZtwin individuals. (A separate examination of the relative scoresrevealed that men reported significantly more paternal controlthan did their twin sisters. Otherwise, same-sex and opposite-sex DZ twins reported co mparable am ounts of differential ex-perience, so that the DZ averages in Table 3 are based on thepooled sample of opposite-sex and same-sex twins.) Averagesare reported for all persons completing the SIDE, includingthose whose co-twins did not participate in the study. This wasjudged appropriate because no significant differences in SIDEscales were found for singletons as compared with complete-pair respondents.

To test for significant differences between MZ and DZ tw ins'retrospective reports of differential experience, multivariateanalyses of variance (MANOVAS) were performed separately forscales of sibling interaction, paren tal treatm ent, and peer-groupcharacteristics. Univariate F ratios were also compu ted for eachscale, and these significance levels are included in Table 3. Al-though there was some skewness in many of the scales, particu-larly for parental treatment indices such that fewer twins re-ported "a lot of differences" than those reporting little or nodifferences, MANOVAS and analyses of variance (ANOVAS) werestill deemed appropriate because F ratios are known to be quiterobust with respect to typical deviations from non-normality(Tan, 1982). Of greater importance is the assumption that thevariances for the SIDE scales do not differ between the two twintypes (see Wilcox, 1987), which was found to be the case inthese data.

Multivariate analyses of variance revealed significant differ-ences between MZ and all DZ twin respondents for scales ofsibling interaction , F(4,157) = 113.54, p < .001; parental treat-ment, .F(4, 133) = 24.68, p < .001; and peer-group characteris-tics, .F(3,147) = 46.97, p< .001. As shown in Table 3, MZ pairsconsistently reported less difference than DZ pairs on every sub-scale, these differences being statistically significant (p < .05)for 10 of the 11 scales.

Because the subject sample consisted of twins, the assum p-tion in ANOVA and MANOVA that observations are indepen-dently sampled was clearly not met. Since most respondents'co-twins were included in the sample, the significance tests justdescribed may be biased. Hence, analyses were repeated for twoseparate subsamples, each containing one randomly selectedmember of each twin pair, in order to check for consistenciesacross the two twins' responses. Although the two samples werenot independent from one another, observations within eachsample were, so that their respective F ratios should providetwo unbiased (albeit not independent) tests of the MZ-DZdifferences. The MZ twins reported significantly fewer differ-ences than DZ twins in both subsamples for peer-group charac-teristics, F(3, 115) = 2.51, p < .001, for Sample 1 and, F(3,98) = 3.21, p < .05 for Sample 2; and for parental treatment,F(A, 99) = 4.69, p < .01 for Sample 1, and, F(4, 87) = 2.49, p


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DIFFERENTIAL EXPERIENCE IN TWINS 107Table 4Adjusted Multiple Correlations ofMZ Within-Pair Personalityan d Affect Differences With SIDE Scales

Personality ofaffect differenceExtraversionPsychoticismNeuroticismMasculinityFemininityAffect intensityZung depressionAffect Balance Scale

TwinInteraction

.10

. 4 3 * "

.28**

SIDE scaleParentalTreatment

.20

.14

.26*

.28**

.40**

Peer Group. 4 1 " *.20.20.20

.28**

Note. SIDE relative scores are averaged within pairs, using reverse scor-ing for one twin. Dashes indicate that adjusted R2 is less than zero.*p


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108

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JO

UJi s(0

LAURA A. BAKER AND DENI SE DANI ELS

SIBLING INTERACTION

MZ twinsfS DZ twinsB Biological SibsE3 Adopted Sibs

SC

PARENTAL TREATMENT

MZ twinsDZ twinsBiological SibsAdopted Sibs

PEER-GROUP CHARACTERISTICS

IeQ"omSI

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Baker & Daiels 1990 Nonshared Environmental Influences and Personality Differences in Adult Twins