Nonshared Environmental Influences and Personality Differences in Adult Twins

Transcription

1 Journal of Personality and Social Psychology 1990, Vol. 58, No. 1, Copyright 1990 by the American Psychological Association, Inc /90/S00.75 Nonshared Environmental Influences and Personality Differences in Adult Twins Laura A. Baker University of Southern California Denise Daniels Stanford University The twin design was used to examine the importance of different experiences of siblings within the family and to identify relations between twins' personality differences and their differential experiences. A sample of 161 monozygotic and 74 dizygotic twin individuals between the ages of 18 and 75 years retrospectively reported on their different experiences when growing up. The Sibling Inventory of Differential Experience (SIDE) was used for the first time with a sample of twin siblings. In addition, the twins provided self-report measures of affect and personality. In contrast to results from a sibling adoption design, this study of twins showed greater evidence for genetic variance in the SIDE scales. Nevertheless, the SIDE showed significant associations with differences in personality and affect for monozygotic twins, which reflect pure environment-behavior relations. Behavioral genetic research points to new approaches for the study of development and socialization. Instead of remaining focused on similarities and shared experiences of members growing up in the same family, attention has turned to withinfamily differences in both experience and behavior. Also, there is increasing awareness that socialization influences can be both environmentally and genetically mediated and, in turn, that the interface between socialization influences and behavioral development may be explained by both genetic and environmental factors (Plomin, Loehlin, & DeFries, 1985). The three main areas of psychological development investigated in behavioral genetics cognition, personality, and psychopathology each show both genetic influence and a considerable amount of environmental influence. However, the environmental influences practically all vary within the family (i.e., they are not shared by relatives in the same family) and thus serve to make family members different from one another. Evidence has shown that, especially in personality and psychopathology and in cognition after adolescence, the differential experiences of family members are invariably the most influential in the socialization process (Plomin & Daniels, 1987; Plomin, DeFries, & McClearn, 1980). In addition to studying behavioral outcome and its general sources of variation, some behavioral geneticists have investi- This research was supported by a Biomedical Research Grant to Laura A. Baker at the University of Southern California. Denise Daniels was supported by National Institute of Mental Health Grant MH16744 and by the William T. Grant Foundation. We wish to thank the twin participants who generously volunteered their time and effort to disclose such personal information about their lives. A great deal of appreciation is also expressed to the many undergraduate research assistants for their contributions to this study, especially David Caliguiri, Elvira Garcia, David Gosse, Yvette Lamar, Sunghi Park, Christine Suh, and Mary Villanueva. Correspondence concerning this article should be addressed to Laura A. Baker, Department of Psychology, SGM 501, University of Southern California, Los Angeles, California gated psychometric measures of the environment. For example, in his studies of adolescent twins, Rowe (1983) has found significant genetic variance in a measure of perceived parental affection derived from Moos and Moos' (1981) Family Environment Scale (FES). Another study of the FES also found moderate genetic influence for perceptions of warmth from parents in a joint analysis of twins reared together and twins reared separately (Plomin, McClearn, Pedersen, Nesselroade, & Bergeman, 1988). Thus, the mere labeling of a scale as an environmental measure does not necessarily mean that it is uninfluenced by genetic factors. Even for an instrument such as the FES, which is typically assumed to assess some aspects of the home that are common to all family members, there may be important differences from one sibling to the next that are a function of both genetic and environmental variations. It is possible that the genetic differences between siblings within the family could lead them to experience the environment differently. Daniels and Plomin (1985) and Daniels, Dunn, Furstenberg, and Plomin (1985) have reported that dynamic different experiences of siblings, including nonmutuality of sibling interaction, parental treatment, and peer-group characteristics, can be systematically tapped. In a sibling-adoption design, these dynamic variables were shown to be primarily environmentally rather than genetically influenced (Daniels & Plomin, 1985; Daniels, 1986), in contrast to the previously mentioned results from studies of the FES in twins. Further, Daniels et al. (1985) and Daniels (1986) have found relations between the dynamic systematic differential experience within the family and personality and adjustment differences of adolescents and young adults. For example, the sibling who reports experiencing a more popular peer group and more sibling closeness shows more sociability as compared with his or her sibling (Daniels, 1986), and the more well-adjusted sibling experiences more parental closeness and peer and sibling congeneality (Daniels et al., 1985). If environmental measures are somewhat genetically influenced, they can relate to behavioral outcome for both genetic and environmental reasons (Plomin et al., 1985). Because ge-

2 104 LAURA A. BAKER AND DENISE DANIELS netic variance was negligible in the SIDE, Daniels (1986) concluded that the SIDE-behavior associations are purely environmentally mediated. However, to the extent that genetic effects are present in environmental indices, such as in the FES, these effects may be correlated with genetic effects in behavioral measures. One preliminary investigation of twin siblings, unlike the sibling design, supported a genetic influence on differential experience/behavior associations (Loehlin & Nichols, 1976). Thus, results concerning genetic effects in environmental measures and their relations to behavior are somewhat varied across the few studies investigating these effects. It is unclear whether the discrepancies are due to different designs, different instruments, or both. More research is needed in this relatively new and interesting area. The present report describes a twin study utilizing a broad assessment of the differential experiences of twin siblings as they were growing up. The Sibling Inventory of Differential Experience (SIDE; Daniels & Plomin, 1985) was used to assess the extent to which the twin and adoption findings converge when the same measure is used. We posed two sets of questions: First, to what extent do monozygotic (MZ) twins perceive less differential experience than dizygotic (DZ) twins, and is any such difference comparable to that in biological versus adoptive siblings' differential experience? Greater differential experience in DZ as compared with MZ twins might suggest a possible role of genetic factors in the SIDE measures, but if the MZ-DZ comparison is much larger than the biological-adopted sibling comparison, epistasis or special twin environmental factors (or both) could be considered as well. Second, what are the systematic relations of the SIDE scales to personality and affect in a sample of MZ twin pairs? Regardless of any genetic effects that may be present in the SIDE, any significant associations between the SIDE and MZ personality differences must be due to nonshared environmental effects. Subjects Method Twins were recruited through newspaper and radio advertisements as part of the Southern California Twin Project in Los Angeles and the surrounding community. In a study of personality and lifestyles, twin pairs over the age of 18 years were sent a questionnaire packet, which was completed and returned by mail. Completed packets were returned by 235 individuals representing 161 MZ twins and 74 DZ twins. In this sample there were 75 complete MZ and 29 complete DZ pairs for cotwin analyses. Average age of the respondents was years (range = 18-75). Zygosity determination for same-sex twin pairs was achieved primarily using a questionnaire similar to that used by Nichols and Bilbrow (1966). Based on self-reports of physical similarities and differences, such methods have been found to correctly classify MZ and DZ twins with over 90% accuracy. High-quality photographs were also obtained from several participants, so that our own judgments of similarity could be checked for agreement with these twins' self-reports. Zygosity results from questionnaires and photographs were consistent for all pairs where data were available for both. Measures and Preliminary Analyses The SIDE was used to assess twins' past experiences. It was administered to both MZ and DZ twins, along with a set of questionnaires as- Table 1 Within-Pair Agreement for SIDE Scales SIDE scale Sibling interactions Sibling Antagonism Sibling Caretaking Sibling Jealousy Sibling Closeness Parental treatment Maternal Affection Maternal Control Paternal Affection Paternal Control Peer-group characteristics Peer College-Orientation Peer Delinquency Peer Popularity N (pairs) All pairs -.13** -.28** -.24** ** -.52** -.43** -.47** -.43** -.64** -.47** Correlations' Age < ** -.49** -.30** ** -.49** -.54** -.33** -.33** -.64** -.48** Note. Relative scoring was used for SIDE scales. * Double-entry calculations were used for twin correlations. *p<.10. **p<.05. Age > * -.21* ** -.34** -.64** -.54** -.63** -.47** sessing dimensions of personality, affect, and health. Brief descriptions of those measures used in the present analyses are provided here, in addition to some preliminary analyses concerning twin correlations for the different measures. SIDE: Retrospective reporting. This is the instrument of primary focus in this report. It measures individuals' self-reported perceptions of their environments while growing up, as compared with their perceptions of their siblings' environments. Psychometric properties of the SIDE include high test-retest reliability and substantial sibling agreement validity. The SIDE contains 63 statements or adjectives describing differences in parental treatment, sibling interactions, and peer group characteristics. The subjects respond to each of these items on a 5-point scale indicating the amount and direction of difference between themselves and their siblings (e.g., for sibling interaction items: 1 = My sibling has been much more this way than me, 2 = My sibling has been a bit more this way 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 much more this way than my sibling.) By averaging ratings for several related items, subscales may be derived reflecting perceived sibling differences in four aspects of sibling interaction (Sibling Antagonism SA, Sibling Jealousy SJ, Sibling Caretaking SCT, and Sibling Closeness SC), four aspects of parental treatment (Maternal Affection MA, Maternal Control MC, Paternal Affection PA, and Paternal Control PC), and three aspects of peer-group characteristics (Peer College-Orientation PCO, Peer Delinquency PD, and Peer Popularity PP). The SIDE was revised in the present study for use with twin siblings simply by substituting every occurrence of the word sibling with twin. It was emphasized that each individual should consider his or her experiences when growing up and living at home. Table 1 summarizes the SIDE scales and presents twin agreement for retrospective reports of differential experience in the total sample of 104 MZ and DZ pairs where both members completed the revised SIDE. If the twins are in agreement about their retrospective reports of differential experience relative to each other when growing up, these correlations should be negative and large in absolute value. This would indicate, for example, that one's own report of greater maternal affection

3 Table 2 Personality and Affect Measures: Within-Pair Correlations for MZ and DZ Twins Measure Eysenck Personality Questionnaire Extroversion Psychoticism Neuroticism Bern Sex-Role Inventory Masculinity Femininity Affect Intensity Measure Self-Rating Depression Scale Affect Balance Scale N (pairs) Note. MZ = monozygotic; DZ = dizygotic. *p<.05. DIFFERENTIAL EXPERIENCE IN TWINS *.62*.41*.35*.52*.36*.35*.22* *.35*.18.28*.13.29* toward oneself is associated with one's twin's report of less maternal affection toward his or herself. Overall, there was moderate agreement within pairs for the SIDE scales reflecting differential parental treatment and peer group characteristics, with twin correlations ranging from -.10 to -.64 (median r = -.43). Agreement was generally lower for sibling interaction scales, although all were negative and three out of four were statistically significant. These correlations are comparable to Daniels and Plomin's (1985) correlations between adoptive and biological sibling pairs, except for those among the sibling interaction scales. The reduced agreement for these scales probably reflects changes in twins' relationship over the years as compared with relationships with their parents and school-aged peers. In fact, when agreement correlations were compared for twin pairs older and younger than 34 years (see Table 1), greater agreement was found among younger twins for three of the four sibling interaction scales. However, older pairs were in at least as much agreement as younger pairs for most of the remaining scales. Overall, then, retrospective reports in the SIDE, from over 40 years past, are in substantial agreement between co-twins' reports. Personality questionnaires and twin differences. The Eysenck Personality Questionnaire (EPQ; Eysenck & Eysenck, 1975) was included to measure three broad dimensions of personality: Extraversion (E), psychoticism (P), and neuroticism (N). Gender-related personality characteristics, masculinity (M) and femininity (F), were measured through the Bern Sex Role Inventory (BSRI; Bern, 1974). The Affect Intensity Measure (AIM; Larsen & Diener, 1987) assesses the characteristic intensity with which one experiences both positive and negative emotions. It contains statements about affective experiences, such as "When I am happy it is a feeling of exuberance and excitement rather than calm contentment" and "Sad movies deeply touch me," to which the individual responds on a 6 point scale (1 = never, 6 = always). Correlations among items regarding positive and negative emotions are moderately high and positive, and thus a single score (average across items) is computed to reflect emotional intensity irrespective of type or frequency of emotion. Additional measures related to stateaffect were included to assess each person's average levels of positive and negative mood in the 2 weeks prior to participating in the study. The participants were asked to consider their negative feelings on Zung's (1965) Self-Rating Depression Scale, which contains items regarding appetite loss, insomnia, negative mood, and other indicators of depression. An indication of overall well-being, or happiness, was obtained from Bradburn's (1969) Affect Balance Scale. This score is based on the difference between amounts of positive and negative moods in the past 2 weeks. The various personality and affect measures are summarized in Table 2, along with the intraclass correlations for 75 MZ and 29 DZ twin pairs where both twins participated in the study. Each of the MZ correlations is statistically significant (p <.05) and representative of typical correlations for personality variables in identical twins (usually ). Further, DZ correlations are invariably lower than corresponding values in MZ pairs, a pattern that is consistent with findings of genetic variance in most dimensions of adult personality (e.g., Loehlin, 1986). However, the MZ-DZ differences are not of major concern in the present study because the heritable aspects of adult personality have been well-researched elsewhere (e.g., Loehlin, 1986). Of most importance here are the MZ correlations and their deviations from unity, indicating the importance of the nonshared environmental influences that serve to make twin siblings different from one another. The SIDE was utilized to specify what some of these nonshared influences might be for MZ twins as they grew up, particularly with respect to parents, peers, and their twin sibling. Results Results examining possible genetic influence on nonshared growing-up experiences are described first. Then analyses focusing on associations between nonshared experience and personality differences for the MZ twin pairs are presented. Differential Twin Experience MZ and DZ twin pairs may differ in amounts of differential experience as a function of their varying degrees of genetic similarity. Mean differences between MZ and DZ twins on the SIDE were examined by recoding the relative 5-point items into absolute 3-point items (0 = no difference between my twin and me, 1 = a bit of difference between my twin and me, and 2 = much Table 3 Means and Standard Deviations of Absolute SIDE Scales for MZ and DZ Individuals SIDE scale Sibling interaction Sibling Antagonism Sibling Caretaking Sibling Jealousy Sibling Closeness Parental treatment Maternal Affection Maternal Control Paternal Affection Paternal Control Peer-group characteristics Peer College-Orientation Peer Delinquency Peer Popularity («= M MZ ) SD DZ («= 61-74) M.76**.76*.81***.74***.41***.40***.50***.46***.65***.64***.77*** SD Note. Significant differences between means for MZ and DZ twins are indicated as follows. *p<. 10. **p<.05; ***p<.0\.

4 106 LAURA A. BAKER AND DENISE DANIELS difference between my twin and me) before forming the 11 SIDE scores. The averages for these absolute amounts of differential experience are presented in Table 3 separately for MZ and DZ twin individuals. (A separate examination of the relative scores revealed that men reported significantly more paternal control than did their twin sisters. Otherwise, same-sex and oppositesex DZ twins reported comparable amounts of differential experience, so that the DZ averages in Table 3 are based on the pooled sample of opposite-sex and same-sex twins.) Averages are reported for all persons completing the SIDE, including those whose co-twins did not participate in the study. This was judged appropriate because no significant differences in SIDE scales were found for singletons as compared with completepair respondents. To test for significant differences between MZ and DZ twins' retrospective reports of differential experience, multivariate analyses of variance (MANOVAS) were performed separately for scales of sibling interaction, parental treatment, and peer-group characteristics. Univariate F ratios were also computed for each scale, and these significance levels are included in Table 3. Although there was some skewness in many of the scales, particularly for parental treatment indices such that fewer twins reported "a lot of differences" than those reporting little or no differences, MANOVAS and analyses of variance (ANOVAS) were still deemed appropriate because F ratios are known to be quite robust with respect to typical deviations from non-normality (Tan, 1982). Of greater importance is the assumption that the variances for the SIDE scales do not differ between the two twin types (see Wilcox, 1987), which was found to be the case in these data. Multivariate analyses of variance revealed significant differences between MZ and all DZ twin respondents for scales of sibling interaction, F(4,157) = , p <.001; parental treatment,.f(4, 133) = 24.68, p <.001; and peer-group characteristics,.f(3,147) = 46.97, p<.001. As shown in Table 3, MZ pairs consistently reported less difference than DZ pairs on every subscale, these differences being statistically significant (p <.05) for 10 of the 11 scales. Because the subject sample consisted of twins, the assumption in ANOVA and MANOVA that observations are independently sampled was clearly not met. Since most respondents' co-twins were included in the sample, the significance tests just described may be biased. Hence, analyses were repeated for two separate subsamples, each containing one randomly selected member of each twin pair, in order to check for consistencies across the two twins' responses. Although the two samples were not independent from one another, observations within each sample were, so that their respective F ratios should provide two unbiased (albeit not independent) tests of the MZ-DZ differences. The MZ twins reported significantly fewer differences than DZ twins in both subsamples for peer-group characteristics, 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 <.05, for Sample 2. However, for sibling interaction scales, MZ- DZ differences were less robust in that MZ twins' scores were significantly lower only in Sample 1, F(4, 120) = 2.51, p <.05. The only difference in retrospective reports of differential sibling interaction that replicated across both samples of co-twins was for sibling closeness, F( 1,23) = 5.09, p <.05 in Sample 1; F(l, 108) = 4.50,p <.05 in Sample 2. The failure ofthe remaining SIDE scales in this domain to show consistent differences between MZ and DZ twins may be due in part to the reduced reliability these scales have (see Table 2). On the other hand, the repeated finding of greater differential experiences for DZ twins in reports of parental treatment and peer-group characteristics provides encouragement that real MZ-DZ differences do exist in these areas. Judging from the nonzero means for MZ pairs in Table 3, many MZ twins did report some differences in their past experiences, albeit less than DZ twins reported. Although the MZ- DZ difference may be a function of genetic similarities, as will be discussed later, these nonzero means for MZ pairs indicate that the SIDE nonetheless taps at least some aspects ofthe nonshared environment with respect to parental, co-twin, and peergroup influences. In turn, some of this nonshared experience may be associated with MZ personality differences, as examined in the next section. Relations ofthe SIDE With Personality and Affect The present twin design provided the unique opportunity to examine "pure" environment-behavior associations (i.e., unconfounded with genetic influences) by studying correlations of the SIDE scales with MZ within-pair differences in personality and affect variables. Multiple correlations were computed for personalitydifference scores between the two twins in each pair (Twin 1- Twin 2) with relative scores in each ofthe three domains in the SIDE scales. To increase reliability of SIDE scales, scores were reversed for Twin 2 and averaged across the two co-twins. A large relative-score average for a given pair would indicate a lot of difference in their perceived experiences, with Twin 1 scoring higher (e.g., being more antagonistic, receiving more maternal affection, or having more popular peers while growing up). However, because co-twins were not in perfect agreement on their SIDE reports, scores in the middle range (i.e., close to 3.0) could result from two different patterns of responses. These may indicate either no difference between the twins (in cases where they were in agreement), or that the two twins gave completely opposite answers (i.e., each twin reported some difference, but in opposite directions from their co-twins). Because ofthe moderately correlated co-twin reports, the latter response pattern should occur less often than the first pattern. However, due to the less than perfect agreement, multiple regression analyses were also performed separately for the two samples of cotwins used in the MANOVAS, using individual SIDE relative scores rather than twin averages. The two sets of results were then examined for consistencies in patterns of significance. The adjusted multiple correlations of personality differences with SIDE co-twin averages based on the entire sample are first presented in Table 4. For MZ pairs, significant relations were few but more than expected by chance alone. Specifically, sibling-interaction scales appear related to BSRI Masculinity, R =,43 ; F(4, 68) = 5.15, p <.05, and Affect Intensity, R =.28, F(A, 70) = 2.54, p <.05. Parental interaction scales were significantly related to twin differences in well-being measured by the Affect Balance Scale, R =.40, F(4, 54) = 3.75, p <.05)

5 DIFFERENTIAL EXPERIENCE IN TWINS 107 Table 4 Adjusted Multiple Correlations ofmz Within-Pair Personality and Affect Differences With SIDE Scales Personality of affect difference Extraversion Psychoticism Neuroticism Masculinity Femininity Affect intensity Zung depression Affect Balance Scale Twin Interaction.10.43*".28** SIDE scale Parental Treatment.14.26*.28**.40** Peer Group.41"*.28** Note. SIDE relative scores are averaged within pairs, using reverse scoring for one twin. Dashes indicate that adjusted R 2 is less than zero. *p<.\0. **/><.05. ***p<.01. and the Zung Depression scale, R =.28,.F(4, 53) = 2.42, p <.05. Differential peer-group characteristics were significantly related to twin differences in Extraversion, R =.41, F(3, 60) = 5.23,/> <.01, and Affect Balance, R =.28, ^3,60) = 2.82,p <.05. When analyses were performed on the two co-twin samples, each of these relations was found to be significant in both samples, with the exception of the multiple correlation between affect intensity differences and sibling-interaction scales. In that case, a significant correlation was obtained only in one subsample, indicating a less robust relation than the others. Finally, several additional marginal associations were found in the total sample (see Table 4), although these did not replicate across cotwin samples and are thus treated with less importance here. Examination of the standardized partial regression (beta) coefficients in each of the analyses that yielded significant multiple correlations helped further clarify the relations between differential experiences and personality/affect differences. Significant beta coefficients (0) are thus presented in Table 5. As shown, a positive relation was found for MZ within-pair differences in Masculinity with Sibling Caretaking, 0 =.252, /(68) = 2.281, p <.05) and Sibling Antagonism, 0 =.247, f(68) = 2.073, p <.05. Thus, the twin who was more antagonistic and who acted more as the caretaker of the two while growing up appeared to be more masculine than the other as an adult. These associations are not surprising because the BSRI Masculinity scale reflects leadership, aggressiveness, and other instrumental attributes. Of course, it is impossible to attach any causal interpretation to these associations, as will be discussed further later. It is nonetheless interesting that differences in self-reported adult masculinity between the twins do correlate with retrospective reports of their behaviors while growing up, which for most twins was many years past. There were also several interesting relations between reported differences in adult affect with various SIDE scales, particularly those concerning parental treatment. The twin perceived to have received greater maternal control reported higher Zung Depression scores, 0 =.397, /(58) = 2.257, p <.05, and less well-being on the Affect Balance Scale, 0 = -.564, t(59) = , p <.01. Moreover, these relations were in the opposite direction for paternal control with Zung Depression, 0 = -.467, r(58) = , p <.02, and Affect Balance, 0 =.397, /(59) = 2.198, p <.03, such that the twin with whom the father was more strict reported less depression and greater well-being as an adult. This different pattern of results for maternal and paternal control may stem from varying expectations from mothers and fathers with respect to discipline. Often the ideal father is perceived to be the controlling authority figure, while mothers are ideally understanding and warm (Lamb, 1987). Thus, receiving more maternal strictness than one's co-twin may be viewed as negative, thereby being associated with relatively poorer adjustment or affect as an adult. In addition to relations of SIDE scales with levels of affect in adulthood, there were some associations of twin differences in affect intensity with differential sibling interaction. The twin showing more jealousy toward the other also reported greater Affect Intensity scores, 0 =.305, f(70) = 2.607, p <.02. However, because this relation was not replicated in both co-twins' SIDE reports, it should be treated with greater caution than the other betas reported in Table 5. Finally, differences in peer popularity were found to be predictive of twin differences in both extraversion, ft =.376, t(67) = 2.988, p <.01, and Affect Balance, 0 =.360, f(67) = 2.720, p <.01. That is, the twin with more popular friends while growing up also reported higher levels of well-being as well as more outgoing personalities than their co-twins. Following the suggestion of Cliff (1987), significance levels for individual beta coefficients were not considered in analyses where nonsignificant or marginally significant multiple correlations were obtained to help prevent the identification of spurious relations between individual SIDE scales and personality differences. There were, in fact, other significant betas in analyses with nonsignificant multiple correlations, although these are not presented or discussed here. Table 5 Significant Standardized Partial Regression Coefficients (0) From Analyses With Significant Multiple Correlations Among MZ Personality/Affect Differences and Relative Scores on SIDE Scales SIDE scale Sibling interaction Sibling Antagonism Sibling Caretaking Sibling Jealousy Parental treatment Paternal Control Paternal Control Maternal Control Maternal Control Peer-group characteristics Peer Popularity Peer Popularity Personality/affect measure" Masculinity Masculinity Affect Intensity measure Zung Depression Affect Balance scale Zung Depression Affect Balance scale Extraversion Affect Balance scale Note. Beta coefficients are significant at p <.05. ' Predicted variables are signed MZ within-pair differences. (

6 108 LAURA A. BAKER AND DENISE DANIELS SIBLING INTERACTION 8 I 8 JO UJ i s (0 MZ twins fs DZ twins B Biological S ibs E3 Adopted S ibs SC PARENTAL TREATMENT MZ twins DZ twins Biological Sibs Adopted Sibs PEER-GROUP CHARACTERISTICS I e Q "o m SI < UJ Q MZ Twins DZ Twins Biological Sibs Adopted Sibs PCO Figure 1, Average absolute scores for SIDE scales in three domains: (a) Sibling Interaction, including Sibling Antagonism (SA), Sibling Closeness (SC), Sibling Caretaking (SCT), and Sibling Jealousy (SJ); (b) Parental Treatment, including Paternal Affection (PA), Paternal Control (PC), Maternal Affection (MA), and Mater- pp

7 DIFFERENTIAL EXPERIENCE IN TWINS 109 As mentioned earlier, the correlations for MZ pairs in Tables 4 and 5 reflect true environment-behavior associations. Because the only source of differences between members of an MZ twin pair is the nonshared environment, neither the SIDE scales nor within-pair differences in personality or affect variables are influenced by genetic factors. In light of the facts that the MZ personality difference scores carry unreliable error variance and that the twins were only in moderate agreement on SIDE scales, it was encouraging to find that there were more SIDE/ personality-difference relations than expected by chance alone and that these relations were found to be consistent across both co-twins' SIDE reports. Discussion Unlike the sibling adoption design, this twin study showed significant genetic influence on sibling differential experience measured by the SIDE. Nonetheless, the SIDE was sensitive enough to be associated with MZ twin personality differences, which must be directly related to the nonshared environment. Both of these important findings are next discussed in greater detail. Genetic Mediation of Sibling and Twin Nonshared Experiences To explore further the differences in findings between results from the twin and adoption designs, we turn to Figure 1, which presents absolute scores on the 11 subscales for MZ and DZ twins in the present study as well as for adoptive and biological siblings (from Daniels & Plomin, 1985). In general, for all sibling types, the greatest within-pair differences in sibling experiences appear to have been reported for sibling interaction scales and the least differences for scales reflecting parental treatment. Further, with the exception of parental treatment scales, adoptive siblings consistently reported far more differences than MZ twins, with DZ and nontwin biological siblings' reports somewhere in between. It is also noteworthy that the differential experience scores for the latter two groups were remarkably similar for a majority of the scales (the few differences are discussed next). Of particular interest here, however, is the finding that for many subscales, scores demonstrated a general decrease in differential experience across increasing genetic relatedness: from adoptive to biological, and DZ to MZ pairs. This is especially apparent in the peer-group characteristic domain, where significant differences are found between DZ and MZ twins in the present study and between adoptive and biological siblings (Daniels & Plomin, 1985) for all four subscales. Implications of differences among siblings of varying degrees of relatedness should be considered. First, there may be more genetic influence in the SIDE than previously thought on the basis of the adoptive/biological sibling studies. Of particular importance here is the observation that the differences between MZ and DZ twins appear far more substantial than those between adoptive and biological siblings in Figure 1. Because the difference in average genetic relatedness is equivalent (one-half) for both comparisons, this may suggest the presence of genetic dominance, epistasis, genotype-environment correlation, or a special MZ twin environment with respect to experiences tapped by the SIDE. The SIDE may not, therefore be a pure measure of the environment but may instead partially reflect inherent personality differences between siblings. Furthermore, because DZ twins report somewhat more similar experiences than biological siblings on scales of Maternal Control, F(l, 199) = 4, p <.05; Paternal Control, F{1, 208) = 7.59, p <.01, and Peer Delinquency, F(l, 177) = 3.724, p <.06, there is some indication of a special twin environment for DZ twins as well. Of course, age varies within the biological and adoptive sibling pairs but not within the twin pairs. Thus, the contemporaneous nature of twin as compared with nontwin sibling relationships may contribute somewhat to reduced differential experience as measured by the SIDE. Second, to the extent that the SIDE does measure experiences relevant to psychological differences in personality and adjustment, these findings might alternatively indicate that the equalenvironments assumption is not valid when calculating heritability from twin designs. However, because associations of SIDE scales with MZ within-pair differences in the personality and affect measures used in this study were only moderate at best, there may be less concern about violation of this critical assumption in twin designs. It is certainly true that MZ twins reported far fewer differences than DZ twins in their experiences with parents, peers, and each other while growing up. However, those MZ pairs who reported fewer differences than other MZ pairs do not appear to be far more similar in adult behavior. In other words, any past experiences tapped by the SIDE do not appear strongly related to personality or affect differences, at least as measured in this study. Third, retrospective reporting of adult twins may diminish the amount of the nonshared environment relevant to adolescents. Adult reports of adolescent experience while growing up could reflect current adult perceptions, making the comparisons to Daniels and Plomin's (1985) younger sample less meaningful. It is possible that there is greater genetic variance in adult perceptions of earlier experiences than in adolescents' perceptions of more recent experiences. Clearly additional studies are needed of twins concurrently reporting on their experience before conclusions about genetic involvement in the SIDE may be fully understood. MZ Twin Nonshared Experiences Related to Their Personality Differences Despite the fact that the SIDE revealed some genetic effects, it did show some systematic associations with twin differences nal Control (MC); and (c) Peer-Group Characteristics, including Peer College-Orientation (PCO), Peer Delinquency (PD), and Peer Popularity (PP). (Scores are presented separately for monozygotic [MZ] and dizygotic [DZ] twins in the present study, as well as for adoptive siblings and nontwin biological siblings from Daniels and Plomin [1985].)

8 110 LAURA A. BAKER AND DENISE DANIELS in personality and affect. Because MZ twins are 100% genetically identical, these SIDE/personality relations must be direct functions of the nonshared environment. In particular, parental treatment differences were most strongly predictive of twin differences in depression and psychological well-being. Additionally, adult MZ twins' reports of their sibling and peer experiences when growing up showed associations with current personality differences in extraversion, masculinity, and affect. Methodological and conceptual considerations suggest these relations should be taken seriously. More associations were revealed than by chance alone, and the odds against finding relations in the first place were quite large because they are based on difference scores that carry increased error variance. The links between twin experience and twin personality differences were expected in terms of magnitude of correlations (e.g.,. 3-.5) and for conceptual reasons (e.g., sibling with a more popular peer group was more extraverted as an adult). Even though the adult twins retrospectively reported on their young adult experiences, the resulting links to their personality differences are comparable to those found in adolescent and young adult samples of nontwin siblings. Similar to the current twin study, differences in sibling jealousy and antagonism have been linked to differences in sibling emotionality (Daniels, 1986), differences in parental closeness are associated with differences in sibling behavioral adjustment (Daniels, et al., 1985), and differences in peer popularity are correlated with sibling sociability and emotionality differences (Daniels, 1986). In summary, adult MZ twins' reports of their experiences when they were growing up and living at home do link to current personality differences in extraversion, masculinity, and affect. Because of the demonstrated genetic influences in the SIDE, the use of the twin design has been critical for identifying pure environmental factors that are linked to personality characteristics. Although the conclusions are limited due to the correlational nature of the environment/behavior associations, this study represents another step toward a greater understanding of the nonshared environment. References Bern, S. L. (1974). The measurement of psychological androgyny. Journal of Consulting and Clinical Psychology, 42, Bradburn, N. M. (1969). The structure of psychological well-being. Chicago: Aldine. Cliff, N. (1987). Analyzing multivariate data. New York: Harcourt Brace Jovanovich. Daniels, D. (1986). 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