Journal of Personality and Social Psychology1998, Vol. 74, No. 3, 818-827Copyright 1998 by the American Psychological Association, Inc.0022-3514/98/S3.00Genetic and Environmental Influences on MMPI Factor Scales:Joint Model Fitting to Twin and Adoption DataJeremy M. Beer, Richard D. Arnold, and John C. LoehlinUniversity of Texas at AustinIn general, the shared family environment appears to play a negligible role in determining individualdifferences in personality and interests. Nevertheless, scattered reports of significant shared environ-mental influence on such variables appear in the literature. Using data from die Texas AdoptionProject (TAP), the current study attempted to replicate twin study findings of significant sharedenvironmental variance on four of nine Minnesota Multiphasic Personality Inventory (MMPI) factorscales (Rose, 1988). Conventional behavioral genetic analyses of the adoption data agreed in af-firming a significant shared environmental influence on individual differences in Religious Orthodoxyonly. Subsequent simultaneous modeling of Rose's twin data and TAP adoption data resulted in threescales (Extraversion, Inadequacy, and Religious Ortfiodoxy) showing significant shared environmen-tal influence. Again, effects were most substantial for Religious Orthodoxy, where the shared environ-ment accounted for nearly 50% of the variance. It is argued that assortative mating cannot explainthis finding.Numerous behavioral genetic studies have concluded thatshared familial environmental variation is an unimportantsource of individual differences in personality and interests(Betsworth et al., 1994; Loehlin & Nichols, 1976; Loehlin,Willerman, & Horn, 1987; Pederson, Plomin, McClearn, & Frib-erg, 1988; Plomin & Daniels, 1987; Rowe & Plomin, 1981;Scarr, 1992). Although originally counterintuitive (Bouchard,Lykken, McGue, Segal, & Tellegen, 1990; Costa & McCrae,1987; Tellegen et al., 1988), this claim is rarely greeted withsurprise today, at least among behavioral geneticists. In fact,recent reports have indicated that shared environmental variancemay be negligible in determining individual differences in so-cial, political, and religious attitudes and beliefs as well (Waller,Kojetin, Bouchard, Lykken, & Tellegen, 1990), especially whenassortative mating for these traits is taken into account (Eaves,Eysenck, & Martin, 1989; Martin et al., 1986).However, scattered throughout the literature there remain asmall number of studies drawing the opposite conclusion regard-ing a variety of personality, temperament, and attitude measures.Tellegen et al. (1988) reported significant shared environmentalinfluence on measures of two extraversion-related traits, PositiveEmotionality and Social Closeness, in a twin study using theirMultidimensional Personality Questionnaire. Bergeman et al.Jeremy M. Beer, Richard D. Arnold, and John C. Loehlin, Departmentof Psychology, University of Texas at Austin.This material is based on work supported under a National ScienceFoundation graduate fellowship. The research on which this article isbased was supported by Grant MH-24280 from the National Instituteof Mental Health and Grants BNS-7902918 and BNS-8209882 fromthe National Science Foundation. We thank Joe Horn and the late LeeWillerman for their helpful comments in reviewing the manuscript andPaul Costa for providing the factor scale item key.Correspondence concerning this article should be addressed to JeremyM. Beer, Department of Psychology, University of Texas at Austin, MezesHall 330, Austin, Texas 78712. Electronic mail may be sent tobeer @ mail.utexas.edu.(1993) found significant shared environmental effects on a mea-sure of agreeableness. Loehlin and Gough (1990) came to thesame conclusion regarding Norm-Favoring, a scale derived fromthe California Psychological Inventory. Complementing thesepersonality studies is one showing that, once contrast effects inparental ratings are taken into account, the shared environmentemerges as a significant source of variance in temperament aswell (Saudino, McGuire, Reiss, Hetherington, & Plomin, 1995).A twin study by Rose (1988) is particularly noteworthy be-cause it included measures of beliefs and interests as well aspersonality. Rose used nine MMPI scales derived via principal-components analysis by Costa, Zonderman, McCrae, and Wil-liams (1985). Shared environmental influence proved signifi-cant on four: Masculinity-Rsmininity (64%), Extraversion(24%), Religious Orthodoxy (61%), and Intellectual Interests(40%). Rose's sample was comparatively large (228 monozy-gotic [MZ1 and 182 dizygotic [DZ] pairs); at the time it wasthe largest twin study ever conducted using data from the MMPI.Of course, among the large number of behavioral-geneticstudies that exist it may be expected that findings like thosenoted earlier will sometimes result from chance alone; particularresults must be replicated before they are taken seriously, prefer-ably via multiple methods. With respect to Rose's findings, datafrom the Texas Adoption Project (TAP; Horn, Loehlin, & Willer-man, 1979) provide such an opportunity. The adoption designyields direct measures of both shared environmental and geneticvariation, rather than relying on differences between correla-tions, as does the twin study. In addition, it is less vulnerableto certain twin-study criticisms, such as the assumptions ofequal environments for identical and fraternal twins and randommating with respect to the trait in question. TAP data allowfor the assessment of similarity among genetically unrelatedindividuals reared together and genetically related personsreared apart (Loehlin, Horn, & Willerman, 1981). For thesereasons and others (twin and adoption methods are differentlyaffected by age differences, and by the presence of nonadditive818GENETIC AND ENVIRONMENTAL VARIANCE IN THE MMPI819genetic variance), the adoption design can produce differentresults than the twin design for the same traits (Tellegen et al.,1988), and thus provides a means of testing the validity andgeneralizability of Rose's conclusions.The adoption paradigm also allows for a direct test of thehypothesis that assortative mating can account for variation thatmight otherwise be ascribed to the family environment. Thishypothesis has recently been offered as an alternative explana-tion for what appear to be substantial effects of the sharedenvironment on measures of social and religious attitudes(Eaves, Eysenck, & Martin, 1989). Phenotypic assortment