Journal of Personality and Social Psychology1988, Vol. 54, No. 6,103 1-103 9Copyright 1988 by Ihe Amn Psychological Association, Inc.0022-3514/88/$00.75Personality Similarity in Twins Reared Apart and TogetherAlike TellegenUniversity of MinnesotaThomas J. Bouchard, Jr., Kimerly J. Wilcox, andNancy L. SegalUniversity of MinnesotaDavid T. LykkenDepartments of Psychology and PsychiatryUniversity of MinnesotaStephen RichDepartment of Pathology and Laboratory Medicine andInstitute of Human GeneticsUniversity of MinnesotaWe administered the Multidimensional Personality Questionnaire (MPQ) to 217 monozygotic and114 dizygotic reared-together adult twin pairs and 44 monozygotic and 27 dizygotic reared-apartadult twin pairs. A four-parameter b i o m e t r i c model (incorporating genetic, additive versus nonaddi-tive, shared family-environment, and unshared environment components) and five reduce d modelswere fitted through maximum-likelihood t e c h n i q u e s to data obtained with the 11 primary MPQscales and its 3 higher order scales. Solely environmental models did not fit any of the scales. Al-though the other reduced models, including the simple additive model, did fit many of the scales,only the full model provided a satisfactory fit for all scales. Heritabilities estimated by the full modelranged from .39 to .58. Consistent with previous reports, but contrary to widely held beliefs, theoverall contribution of a common family-environment component was small and negligible for allbut 2 of the 14 personality measures. Evidence of significant nonadditive genetic effects, possiblyemergenic (epistatic) in nature, was obtained for 3 of the measures.Until recently, almost all knowledge regarding e nvi ro nme n -tal and genetic causal influences on stable personality traits hascome from studies of twins reared together. The findings havebeen both remarkable a nd puzzling. On the genetic s id e, regard-less of the trait studied, the intraclass correlation for fraternal,or dizygotic (DZ), twins has approached .25; that for identical,or monozygotic (MZ), twins has approached .50 (Goldsmith,1983; Nichols, 1978). Application of the simplest geneticmodel, the Falconer (1960) formula for heritability, [h2 =2(RMz - RDZ)], t o thos e results y i e l d s a h e r i ta b i li t y of a bo u t . 5 0 .This leaves 50% of the variance to systematic environmentalinfluences, measurement error, and temporal instability.Particularly puzzling, and contrary to what many psycholo-gists would predict, is the finding that almost none of the envi-This research has been supported by grants from the University ofMinnesota Graduate School, the Koch Charitable Foundation, theSpencer Foundation, the National Science Foundation (BNS-7926654),the Pioneer Fund, and the Harcourt Brace Jovanovich Publishing Com-pany.We thank the following people for the time and effort they have g iv ento testing the twins: Margaret Keyes, Jeff McHenry, Elizabeth Rengel,Susan Resnick, Joy Fisher, Jan Englander, and Ann Riggs. We are in-debted to our colleagues and collaborators on the Minnesota Study ofTwins Reared Apart project, Elke Eckert and Leonard Heston for theirhelp and advice. We owe special thanks to our colleagues, Greg Careyand Matt McGue, for their valuable biometric advice and assistance.We also thank Jack Darley for his resolute and dedicated support andthe Minneapolis W a r Memorial Blood Bank, Herbert Polesky, D i r e c t o r ,for the blood testing.Correspondence concerning this article should be addressed to AukeTellegen, Department of Psychology, Elliott Hall, University of Minne-sota, 75 East River Road, Minneapolis, Minnesota 55455.ronmental variance i s d u e to sharing a common family environ-ment. Loehlin and Nichols (1976) reached this conclusion onthe basis of personality data collected on a large National MeritScholarship twin sample. Eaves and Young (1981) carried outan informative additional analysis of Extroversion-Introversionand Neuroticism scores from the same twin sample, using abiometric model similar t o the one to be reported in this article.They fitted three models to the data. The first model allowedfor unshared environment effects and additive gene effects. Thesecond allowed for two environmental effects: shared family-environment effects and unshared environment ef fe ct s, but nogenetic effects. The third specified the joint effect of all three.The results were straightforward: The second model, whichonly specified environmental parameters, did not fit the datafor either sex or for the two sexes considered jointly. The firstmodel, which assume s no common family-environment effect,fit all three cases and was as capable of fitting both sexes simul-taneously (with ide nt ica l parameter estimates) as it was o f fittingthe results of each sex separately. Adding the common family-environment effect did not improve the fit of the third modelover the first. These results essentially confirmed Loehlin andNichols's (1976) earlier analysis.Loehlin and Nichols (1976) carried out many additionalanalyses of their data. In particular, they attempted to identifythe effects of systematic within-family variations. Because iden-tical twins s h a r e all of their genes, an y differences between themmust be environmental i n origin. By re la ti ng w it hin -pa i r z y g o s -ity-group differences in experiences to personality differences,they could answer questions such as, "Do identical twins whowere dressed alike turn out to be more similar in personalitythan identical twins who w er e not? " The results were e s s e n t i a l l ynegative. Greater similarity in the twins' experiences could ac-10311032TELLEGEN, LYKKEN, BOUCHARD, WILCOX, SEGAL, RICHcount for only a small fraction of the MZ twins' similarity inpersonality. Loehlin and Nichols (1976) concluded the fol-lowing:Thus, a consistent—though perplexing—pattern is emerging fromthe data (and it is not purely idiosyncratic to our study). Environ-ment carries substantial weight in the determination of personal-ity—it appears to account for at least half the variance'—but thatenvironment is one for which twin pairs are correlated close tozero. Attempts to treat twins alike do not lead to greater similaritybetween them. In short, in the personality domain we seem to seeenvironmental effects that operate almost randomly with respectto the sorts of variables that psychologists (and other people) havetraditionally d ee me d important in personality development,