Genetic and Environmental In uences on Personality in Adult Russian Twins

Transcription

1 INTERNATIONAL JOURNAL OF BEHAVIORAL DEVELOPMENT, 1999, 23 (2), Genetic and Environmental In uences on Personality in Adult Russian Twins Kimberly J. Saudino and Jeffrey R. Gagne Boston University, USA Julie Grant Pennsylvania State University, USA Anna Ibatoulina Kent State University, Ohio, USA Tatinana Marytuina and Inna Ravich-Scherbo Moscow University, Russia Keith Whit eld Pennsylvania State University, USA The present study eplored genetic and environmental contributions to personality in a sample of twins participating in the Adult Russian Twin Study (ARTS). Subjects included 79 monozygotic (MZ) and 51 dizygotic (DZ) twin-pairs residing in the metropolitan Moscow area, Russia (mean age 42.2 years). Twins completed self-report questionnaires assessing the personality dimensions of neuroticism, etraversion, monotony avoidance, and impulsivity. For all four dimensions, model- tting analyses yielded estimates of heritability consistent with previous behavioural genetic ndings (h 2 ranging from.49 to.59). Also consistent with previous research is the nding that shared environmental variance is negligible for each dimension. Requests for reprints should be sent to Kimberly J. Saudino, Psychology Department, Boston University, 64 Cummington St, Boston, MA, 02215, USA. Julie Grant is now at the Department of Psychiatry, Washington University School of Medicine, St Louis, MO This research was supported by the Center for Developmental and Health Genetics at The Pennsylvania State University. c 1999 The International Society for the Study of Behavioural Development

2 376 SAUDINO ET AL. These results suggest that the factors that in uence individual differences in personality in the Russian culture do not substantially differ from those that in uence personality in more Western cultures. Probably one of the most robust ndings in the behavioural genetic literature is the nding that personality is genetically in uenced. Twin and adoption studies of personality in infancy, early childhood, adolescence, adulthood, and old age consistently yield moderate heritabilities for most major personality dimensions (see Loehlin, 1992; Plomin, Chipuer, & Loehlin, 1990 for reviews). However, heritability is a statistic that describes genetic contributions to the phenotypic variance of a trait within a particular population of individuals. For any given phenotype, differences in the population under study may result in differences in estimates of heritability (and environmentality). Therefore, the heritability of personality may not necessarily be the same for all cultures. Thus far, there is little empirical evidence of cross-cultural differences in genetic and environmental contributions to individual differences in personality. American, Australian, British, Finnish, and Swedish twin studies of etraversion and neuroticism, often considered to be the two super factors of personality, have been remarkably similar in suggesting heritability estimates in the range of.40 to.50 for both dimensions (e.g. Eaves, Eysenck, & Martin, 1989; Floderus-Myrhed, Pedersen, & Rasmuson, 1980; Loehlin & Nichols, 1976; Pedersen, Plomin, McClearn, & Friberg, 1988; Rose, Koskenvuo, Kaprio, Sarna, & Langinvainio, 1988). This means that between 40% and 50% of the variability in etraversion and neuroticism is due to genetic differences among the individuals studied. The remaining variance is attributed to environmental factors. However, the studies cited consistently nd that shared family environment accounts for a negligible portion of variance in etraversion and neuroticism. The environmental in uences that are important to these personality dimensions are those factors that are not shared by members of the same family (i.e. environmental in uences that are unique to family members). The present study adds to this body of research by eploring genetic and environmental contributions to individual differences in neuroticism, etraversion, and the related traits of monotony avoidance and impulsivity, in a sample of adult Russian twins. Previous behavioural genetic studies of personality have involved adults in democratic societies. In contrast, the twins in our Russian sample have eperienced most of their lives under a communist regime. As compared to individualist democratic societies, which involve government by popular representation; free enterprise; freedom of religion; private ownership; and variable employment, the

3 GENETIC AND ENVIRONMENTAL INFLUENCES 377 collectivist communist society of the Soviet Union prior to perestroika was highly restrictive. Life was regimented and state censorship and propaganda was used to shield citizens from outside in uences (e.g. capitalism). There was one large social class (i.e. very few rich or poor), one political party, one religion (i.e. atheism), no private ownership, and little opportunity to choose one s vocation (Law, 1975). Conformity was encouraged and individual differences, a factor highly valued in Western cultures, were disregarded (Korochkin, Gindilis, & Bulayeva, 1993). Thus, it can be argued that our Russian sample represents a truly unique population. Consequently, it is possible that the heritability of personality might differ for this population. Although we had no speci c predictions, there are two possible ways in which the uniqueness of the Russian eperience may affect outcomes from behavioural genetic studies of personality. First, the restricted circumstances eperienced by Russian citizens might reduce environmental variance and hence, increase heritability. Conceptually, heritability is the ratio of genetic variance to total phenotypic variance (i.e. genetic variance + environmental variance). Therefore, if environmental variance is reduced in a population, the relative in uence of genetic variance will increase. On the other hand, the types of environments eperienced by individuals within a communist culture may differ from those eperienced in democratic societies. That is, Russians may eperience unique environments that are not commonly eperienced by those in Western cultures. For eample, for most of their lives, our sample eperienced a highly regimented life lled with censorship, propaganda, a general shortage of consumer items, and long queues to buy products, along with the more positive aspects of guaranteed housing, and free or inepensive public services (Law, 1975; Smith, 1990). It may be that individual differences in these and other uniquely Russian environments contribute to variance in personality. Hence, it is possible that novel environments might result in shared environmental in uences in personality. Sample METHOD The Adult Russian Twin Study (ARTS) sample was recruited from the Moscow Twin Registry, Institute for Psychology, the Moscow University, Russia (Whit eld, Grant, Ravich-Scherbo, Marutina, & Ibatoulina, 1997). All participants resided in the metropolitan Moscow area. Participants included 79 MZ twin pairs (26 male, 53 female) and 51 DZ same-se twin pairs (20 male, 31 female). The mean age at time of testing was (SD = 8.93) years. Twin zygosity was established using physical similarity

4 378 SAUDINO ET AL. criteria from the Nichols and Bilbro (1966) self-report zygosity questionnaire. This method of zygosity classi cation has been shown to yield accuracy of over 90% when compared to tests of single-gene markers in blood (Nichols & Bilbro, 1966). Procedure and Measures Participants were tested in groups of 4 10 twin pairs at the Institute for Psychology, The Moscow University. During the 3-hour test session, twins were assessed on a variety of physical, cognitive, personality, and demographic measures. At the end of the test session, subjects received 12,000 rubles (approimately US$6.00) for their participation. The personality dimensions of neuroticism, etraversion, monotony avoidance, and impulsivity were assessed using previously established selfreport measures of personality (see later). For all measures, items were translated into Russian by a native Russian speaker and were then backtranslated into English by a second native Russian speaker. This procedure ensured that the meaning of the items had not been distorted as a result of the translation process. Neuroticism and etraversion were assessed using a short form of the Eysenck Personality Inventory (EPI; Floderus-Myrhed et al., 1980). Each scale consists of nine items, scored either 1 = Yes or 0 = No. Internal consistency as measured by Cronbach s alpha was.69 for neuroticism and.64 for etraversion. Although these reliabilities are somewhat lower than those reported for the full scales in the Eysenck Personality Questionnaire (EPQ; Eysenck & Eysenck, 1975), they are comparable to those reported for the EPI short-form (e.g. Pedersen et al., 1988). Similarly, as is the case with other studies using the EPI short-form (Floderus-Myrhed et al. 1980; Pedersen et al., 1988), neuroticism and etraversion were negatively correlated (r = ±.30). Measures of monotony avoidance and impulsivity, using the Karolinksa Scales of Personality (Shalling, Edman, & Asberg, 1983), were included in the present study as complements to the shortened version of the etraversion measure. Monotony avoidance items assess Zuckerman s sensation-seeking dimensions of susceptibility to boredom and disinhibition (Zuckerman, 1971) (e.g. I prefer people who are eciting and unpredictable ). Items on the impulsivity scale re ect quick decision making and the tendency to act on the spur of the moment (e.g. I often rush into new things ). For both dimensions, participants were asked to rate on a 5-point Likert scale how well each statement ts their personality (e.g. 1 = ts eactly to 5 = does not t at all ). Scale scores were calculated based on the sum of responses to ten 5-point Likert items. In the present sample, monotony avoidance demonstrated adequate internal

5 GENETIC AND ENVIRONMENTAL INFLUENCES 379 consistency (a =.73); however, the internal consistency of the impulsivity scale was low (a =.56) and therefore, the results for this scale should be viewed cautiously. Because twin resemblance can be in ated by variance due to age and gender, scores were residualised for age, gender, and age-gender interaction effects (McGue & Bouchard, 1984). All twin analyses were then conducted on the residualised scores for each measure. Design and Analyses The twin method involves comparing genetically identical (MZ) twins with fraternal (DZ) twins who share approimately 50% of their segregating genes. Genetic in uences are implied when co-twin similarity covaries with the degree of genetic relatedness. Thus, if heredity affects a trait, the twofold greater genetic similarity of MZ twins is epected to make them more similar than DZ twins. Intraclass correlations typically serve as indices of co-twin similarity. An MZ correlation that is signi cantly greater than the DZ correlation suggests genetic in uence. An estimate of heritability (h 2 ), the genetic effect size, can be derived by doubling the difference between the MZ and DZ correlations (Plomin, DeFries, McClearn, & Rutter, 1997). Heritability is the proportion of observed variance for a trait that can be attributed to genetic in uence. The remaining variance is attributed to environmental factors which includes all nonheritable in uences. The environmental variance component can be decomposed into shared and nonshared environmental in uences. Shared environmental variance (c 2 ) is twin resemblance that is not eplained by genetic variance. Thus, c 2 includes those environmental in uences common to both members of a twin pair that enhance co-twin similarity. DZ correlations that eceed onehalf the MZ correlation suggest the presence of shared environmental in uences. Doubling the ecess DZ co-twin similarity not accounted for by h 2 provides an estimate of c 2. Nonshared environmental variance (e 2 ) is a residual variance that includes measurement error and environmental in uences that are unique to each individual. Differences within pairs of MZ twins are due to nonshared environmental in uences, thus e 2 can be estimated by the etent to which the MZ correlation is less than unity. In addition to simple correlational analyses, model- tting analyses were used to assess the genetic and environmental in uences on each personality dimension. In the present study, maimum-likelihood model- tting analyses were performed on twin variance/covariance matrices using LISREL 8 (Jöreskog & Sörbom, 1993). The full univariate model, depicted as a path diagram in Fig. 1, uses twin covariances to decompose the phenotypic variance of a measure into genetic and environmental

6 380 SAUDINO ET AL. FIG. 1. Full univariate (additive) model. components. The rectangles represent the phenotypic variances of each twin. The circles represent latent genetic and environmental factors. The curved double-headed arrows indicate correlations between the variables they connect, whereas the single-headed arrows represent partial regressions of the measured variable on the latent variable. The Ga factors refer to additive genetic in uences the sum of the average effect of all genes that in uence a trait. Based on the degree of genetic relatedness, the Ga factors correlate 1.0 and.5 for MZ and DZ twins respectively. Shared environment (Es) refers to the in uence of shared rearing environments on twin resemblance. Because all twins were reared in the same family, both MZ and DZ correlate 1.0. Finally, the En factors re ect nonshared environmental variance and measurement error. Nonshared environmental in uences are those environmental factors that are unique to each member of a twin pair and that make twins different from each other (e.g. illnesses or accidents, and measurement error) and are therefore depicted in the path diagram as residual arrows for each twin representing the remaining variance not eplained by genetics or shared environment. Model- tting procedures provide a more elegant analysis of genetic and environmental in uences because they estimate multiple parameters simultaneously, test the model, make assumptions eplicit, provide parameter estimates, and permit tests of alternative models (Loehlin, 1987; Neale & Cardon, 1992). For eample, reduced models without the genetic and/or shared environmental parameters can be tested against the full model. Because these reduced models are nested within the full model, the change in chi-square from the full model to the reduced model estimates the signi cance of the parameter(s) not included in the reduced model. Degrees of freedom (df ) for the change in chi-square is equal to the difference in df between the two models.

7 Descriptive Statistics GENETIC AND ENVIRONMENTAL INFLUENCES 381 RESULTS Means and standard deviations for the four personality measures by gender and zygosity are listed in Table 1. To evaluate mean differences, two-way (gender zygosity) analyses of variance (ANOVAs) were conducted for each measure. There were signi cant main effects for gender and zygosity, but no signi cant gender zygosity interactions. Consistent with previous literature (e.g. Eysenck & Eysenck, 1969; Hanin, Eysenck, Eysenck, & Barrett, 1991; Lynn & Martin, 1997), females scored signi cantly higher on neuroticism [F(1,256) = 12.64, P,.001], and lower on etraversion [F(1,256) = 6.63, P,.05]. Females also scored signi cantly higher than males on impulsivity [F(1,256) = 7.91, P,.01]. With regard to mean differences between zygosity groups, MZ twins scored signi cantly higher on etraversion [F(1,256) = 8.63, P,.05], and monotony avoidance [F(1,256) = 3.88, P,.05]. More critical to our current focus on individual differences, MZ and DZ twin groups did not signi cantly differ in total variances for each measure, thus ful lling a basic assumption of the twin method. Intraclass Correlations Table 2 presents twin intraclass correlations for all dimensions studied. A pattern of signi cant MZ correlations and nonsigni cant DZ correlations emerged for all dimensions. For etraversion, monotony avoidance, and impulsivity, the MZ correlations were signi cantly greater than the corresponding DZ correlations (P,.05). The difference between MZ and DZ correlations for neuroticism approached signi cance (P,.06). Thus, genetic in uences appear to be contributing to individual differences in these personality traits. Etraversion, monotony avoidance, and impulsivity displayed a pattern of DZ correlations that are less than one-half the MZ intraclass TABLE 1 Means and (Standard Deviations) of the Personality Variables by Se and Zygosity MZ DZ Females Males Females Males M (SD) M (SD) M (SD) M (SD) Neuroticism 6.09 (2.24) 4.92 (2.16) 6.24 (2.41) 5.30 (2.09) Etraversion 4.29 (1.91) 4.96 (1.61) 3.63 (1.77) 4.20 (1.98) Monotony avoidance (6.55) (7.47) (7.04) (7.11) Impulsivity (4.78) (5.58) (5.30) (5.40)

8 382 SAUDINO ET AL. TABLE 2 Twin Intraclass Correlations MZ DZ Neuroticism.48*.23 b Etraversion.61*.13 a Monotony avoidance.54*.14 a Impulsivity.48* ±.06 a a Difference between MZ and DZ correlations are signi cant at P,.05. b Difference between MZ and DZ correlations approached signi cance at P,.06. * P,.05. correlations, suggesting that nonadditive genetic variance (i.e. dominance among alleles, or epistasis, the interaction of alleles across loci), may be contributing to individual differences in these behaviours. However, in a simple twin design, the power to detect nonadditive genetic variance is negligible. Therefore, in our model- tting analyses, we t a nonadditive model only when a simple additive model did not yield an adequate t to the data. Model-fitting Analyses Table 3 presents the t statistics for tests of alternative models. For neuroticism, etraversion, and monotony avoidance, we t the full model (Ga, Es, En) described in Fig. 1 and three reduced models: (1) a model that included additive genetic effects and nonshared environmental effects (Ga, En); (2) a model that included shared and nonshared environmental effects (Es, En); and (3) a model that included only nonshared environmental effects (En). The Ga, En model provides a test of the signi cance of the shared environment parameter. A signi cant difference in chi-square ( 2 D ) between the chi-squares for the full model and the Ga, En model would indicate that shared environmental variance cannot be eliminated from the model without a decrement in t (i.e. that shared environmental variance is signi cant). Similarly, the 2 D between the full model and the Es, En model allows a test of the signi cance of the genetic parameter. As indicated by the nonsigni cant chi-squares listed in Table 3, the full model (Ga, Es, En) provided a satisfactory t to the data for neuroticism, etraversion, and monotony avoidance. However, for each of these personality dimensions, the shared environment parameter could be dropped from the full model (i.e. Ga, En model) without a change in chi-square between the full model and the reduced model. Thus, shared environmental in uences are not signi cant. In contrast, dropping the

9 GENETIC AND ENVIRONMENTAL INFLUENCES 383 TABLE 3 Chi-squares ( 2 ) and Change in Chi-square ( 2 D ) from Tests of Alternate Models Model Full Ga, Es, En Ga, En Es, En En Neuroticism df = df = df = * df = 5 2 D 0.00 df = ** df = * df = 2 Etraversion df = df = * df = * df = 5 2 D 0.00 df = * df = * df = 2 Monotony avoidance df = df = df = * df = 5 2 D 0.00 df = * df = * df = 2 Impulsivity Additive Model: * df = * df = * df = * df = 5 2 D 0.00 df = * df = * df = 2 Nonadditive Models: Full Gd, Ga, En Gd, En df = df = 4 2 D 0.00 df = 1 Ga, additive genetic parameter; Gd, nonadditive genetic parameter; Es, shared environmental parameter; En, nonshared environmental parameter; 2 D, change in chi-square when parameters are dropped from the full model. Data in bold denote the best tting model. *P,.05; **P,.07. genetic parameter from the full model (i.e. Es, En model) resulted in a poorer t. The chi-square difference between the full model and the Es, En model was signi cant for etraversion and monotony avoidance, and approached signi cance (P,.07) for neuroticism. Similarly, a model that included only nonshared environmental variance did not t the data for any dimension. This is not surprising because nonshared environmental in uences result in differences between twin siblings, and the En model would imply that there is no resemblance between co-twins, which as can be seen from the intraclass correlations, is not the case for our sample. Thus, for neuroticism, etraversion, and monotony avoidance, the best- tting model (i.e. the model that best describes the data), is a model that includes additive genetic effects and nonshared environmental in uences (i.e. the Ga, En model). For impulsivity, we t both additive and nonadditive models to the data. The signi cant chi-square for the additive full model (i.e. Ga, Es, En) indicated that this model did not adequately describe the data. Similarly, a

10 384 SAUDINO ET AL. model that included Ga and En also provided a poor t. However, as indicated by the signi cant change in chi-squares between the full additive model and reduced models, the t of purely environmental models (i.e. Es, En, and En only) were signi cantly worse than the models including the additive genetic parameter. Thus, genetic variance is signi cant for impulsivity. Because the additive models failed to t the data, and yet there was evidence of signi cant genetic in uences, we then t nonadditive genetic models to the data. Nonadditive genetic variance refers to the effects of genes that are not linear and additive (e.g. dominance among alleles, or alternate forms of a gene; epistasis, the interaction of alleles across loci). With the twin design, shared environmental effects and nonadditive genetic effects are confounded and cannot be estimated within the same model because both are based on the etent to which DZ covariance deviates from one-half the MZ covariance. Therefore, the nonadditive model substitutes genetic dominance (Gd) for shared environmental effects (Eaves et al., 1989). MZ twins share all nonadditive genetic effects, whereas DZ twins share only a quarter of genetic variance due to dominance (Plomin et al., 1997). Thus, MZ co-twins are correlated 1.0 and DZ co-twins are correlated.25 for nonadditive genetic effects. As indicated by the nonsigni cant chi-square in Table 3, the full nonadditive model (i.e. Gd, Ga, En) provided a satisfactory t to the data for impulsivity. However, the additive genetic parameter could be dropped from the model without a change in chi-square. Thus, the best tting model for impulsivity is the reduced nonadditive model (Gd, En). Estimates of the percentage of total variance eplained by genetic and environmental factors derived from the best tting models are presented in Table 4. Genetic factors account for 49 59% of the phenotypic variance in personality. The remaining variance can be attributed to nonshared environmental factors. DISCUSSION Cross-cultural research eamining individual differences in personality at the phenotypic level has shown that the structure of personality is remarkably robust across cultures. Speci cally, research eploring similarities and differences between personality in Russia and other cultures has found that although there may be differences in mean levels for personality dimensions, the factor structure of Russian personality is similar to that of Western cultures (Bodunov, 1993; Brief & Comrey, 1993; Digman & Shmelyov, 1996; Hanin et al., 1991; Paunonen et al., 1996). Thus, the same basic traits can be used to describe individual differences in personality in Russian samples as in other cultures. The present study addressed the

11 GENETIC AND ENVIRONMENTAL INFLUENCES 385 TABLE 4 Parameter Estimates, Standard Errors, Percentages of Variance, and Chi-squares for the Best Fitting Model Parameter Estimates Standard Errors Dimension Ga/Gd Es En 2 (df = 4) P Neuroticism % Variance Etraversion % Variance Monotony avoidance % Variance Impulsivity % Variance Ga, additive genetic parameter; Gd, nonadditive genetic parameter; Es, shared environmental parameter; En, nonshared environmental parameter. Dashes ( ) indicate parameters that were not estimated in the best tting models. question of individual differences in Russian personality at the level of genetic and environmental aetiology. That is, does the relative importance of genetic and environmental in uences on individual differences in the core dimensions of neuroticism, etraversion and related traits differ for our Russian sample? The answer would appear to be no. Our ndings that the personality dimensions assessed in the Adult Russian Twin Study (ARTS) are moderately in uenced by genetic factors and nonshared environments are generally consistent with results from American, Australian, British, Finnish, and Swedish twin studies (e.g. Eaves et al., 1989; Floderus-Myrhed et al., 1980; Loehlin & Nichols, 1976; Pedersen et al., 1988; Rose, Koskenvuo, Kaprio, Sarna, & Langinvainio, 1988). In previous research eploring genetic and environmental contributions to personality, heritability estimates based on simple additive genetic models have been found to range from.41 to.60 for neuroticism, and.50 to.74 for etraversion (Henderson, 1982). Similarly, model- tting analyses combining the ndings from twin and adoption studies of neuroticism and etraversion across American, Australian, British, Finnish, and Swedish cultures have produced heritability estimates of.41 for neuroticism and.49 for etraversion (Loehlin & Rowe, 1992). Thus, there is little difference between our ndings of heritabilities of.49 and.59 for neuroticism and etraversion, respectively and those reported in the literature. The heritability of monotony avoidance in the present study was.53. This estimate is considerably higher than the heritability of.23 reported for the same measure in the Swedish Adoption/Twin Study of Aging (Pedersen et al., 1988); however, it is consistent with estimates reported

12 386 SAUDINO ET AL. for related measures of sensation seeking. For eample, Fulker, Eysenck, and Zuckerman (1980) found that 58% of the phenotype variability in a measure of general sensation seeking was due to genetic factors. Perhaps more speci c to the present study, Koopmans, Boomsma, Heath, and van Doornen (1995), report heritabilities ranging from.48 to.62 for the sensation-seeking dimensions of susceptibility to boredom and disinhibition the two scales that out monotony avoidance measure was designed to tap. The present results for impulsivity were consistent with previous research both in terms of the magnitude of the genetic effect (i.e. h 2 =.49) and sources of genetic variance. Impulsivity was the only dimension for which a simple additive model did not t the data. A model that included nonadditive genetic variation was required to provide a reasonable account of co-twin resemblance. Although previous research has also found evidence of nonadditive genetic in uences for impulsivity (e.g. Loehlin, 1992; Pedersen et al., 1988; Tellegen et al., 1988), in general, twin studies have only modest power to discriminate the relative importance of additive and nonadditive genetic variance. Given our small sample and the moderate internal consistency for the impulsivity scale, the safest conclusion is that heritability of this personality dimension is signi cant and substantial. Larger sample sizes and additional kinship groups (e.g. twins reared apart) would enable us to evaluate better the relative importance of nonadditive genetic variance for impulsivity and the other personality dimensions eamined in ARTS. In theory, heritability estimates should be higher in more egalitarian societies because such societies allow fewer arbitrary differences in eperiences between individuals (Scarr, 1993). Communism, with its goal of equality of social conditions for all members of the collective, and its consequent environmental restrictions, might therefore be epected to result in higher estimates of genetic in uence. However, as previously discussed, in the present study, the heritability estimates for all dimensions were similar to those found in behavioural genetic studies of personality in Western cultures. This suggests that the more restrictive collective environments that our adult Russian twins may have eperienced did not substantially in uence estimates of genetic variance. Thus, although there are a number of environments or eperiences in which, for much of their lives, our Russian sample encountered less variability than the Western world (i.e. freedom of religion, private ownership), it would appear that these environments are not the environments that are important to individual differences in personality. Or, at least, these restrictive environments do not have an enduring effect on individual differences in personality. Because our sample was assessed after the fall of communism, it is possible that our results re ect the recent democratisation of Russia

13 GENETIC AND ENVIRONMENTAL INFLUENCES 387 and its increased opportunities for its citizens. That is, heritabilities for personality might have been higher prior to perestroika. Without being able to turn back the clock, we will never know the answer to this question. Our ndings are also consistent with previous research in that there was no evidence for shared environmental in uences on personality. We had speculated that perhaps our Russian sample eperienced different types of environments than those eperienced in Western cultures and that these uniquely Russian environments might be important to personality. Such culture-speci c environments might result in signi cant shared environmental variance. This was not the case. As with other samples of twins, it is not those environmental in uences that are common to family members (e.g. parental rearing style, schools, neighbourhoods) that are important to individual differences in personality, rather it is those environments that are unique to individuals within a family. In conclusion, although it can be argued that there are considerable cultural differences between adults raised in Russian and Western cultures, the results of the present study suggest that these cultural differences do not substantially in uence the aetiology of individual differences in personality. That is, the factors that in uence individual differences in personality in the Russian culture do not differ from those that in uence personality in more Western cultures. In this way, Russian and Western cultures can be viewed as providing functionally equivalent opportunities for species-typical development in personality (Scarr, 1993). Manuscript received June 1998 Revised manuscript received September 1998 REFERENCES Bodunov, M.V. (1993). Factor structure of the Pavlovian Temperament Survey in a Russian population: Comparison and preliminary ndings. Personality and Individual Differences, 14, Brief, D.E., & Comrey, A.L. (1993). A pro le of personality for a Russian sample: As indicated by the Comrey Personality Scales. Journal of Personality Assessment, 60, Digman, J.M., & Shmelyov, A.G. (1996). The structure of temperament and personality in Russian children. Journal of Personality and Social Psychology, 71, Eaves, L.J., Eysenck, H.J., & Martin, N.G. (1989). Genes, culture and personality. London: Academic Press. Eysenck, H.J., & Eysenck, S.B. (1975). Manual of the Eysenck personality inventory. San Diego, CA: Educational and Industrial Testing Service. Eysenck, S.B., & Eysenck, H.J. (1969). Scores on three personality variables as a function of age, se and social class. British Journal of Social and Clinical Psychology, 8, Floderus-Myrhed, B., Pedersen, N., & Rasmuson, I. (1980). Assessment of heritability for personality based on a short form of the Eysenck Personality Inventory: A study of 12,898 twin pairs. Behavioral Genetics, 10,

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