This is one of a series of EXCERPTS from older articles put online by John Ray as a public service. The articles concerned are in general otherwise available only by special request to a University or other major library.


Journal of Personality and Social Psychology 1988. Vol. 54, no. 6. 1031-1039

Personality Similarity in Twins Reared Apart and Together

Auke Tellegen, David T Lykken, Thomas J. Bouchard, Jr., Kimerly J. Wilcox, Nancy L. Segal, Stephen Rich

We administered the Multidimensional Personality Questionnaire (MPQ) to 217 monozygotic and 114 dizygotic reared-together adult twin pairs and 44 monozygotic and 27 dizygotic reared-apart adult twin pairs. A four-parameter biometric model (incorporating genetic, additive versus nonadditive, shared family-environment, and unshared environment components) and five reduced models were fitted through maximum-likelihood techniques to data obtained with the 11 primary MPQ scales and its 3 higher order scales. Solely environmental models did not fit any of the scales. Although 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 model ranged from .39 to .58. Consistent with prewous reports, but contrary to widely held beliefs, the overall contribution of a common family environment component was small and negligible for all but 2 of the 14 personality measures. Evidence of significant nonadditive genetic effects, possibly emergenic (epistatic) in nature was obtained for 3 of the measures.

Until recently, almost all knowledge regarding environmental and genetic causal influences on stable personality traits has come from studies of twins reared together. The findings have been both remarkable and puzzling. On the genetic side, regardless 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 genetic model, the Falconer (1960) formula for heritability, [h2 = 2(Rmz - Rdz)], to those results yields a heritability of about 50. This leaves 50% of the variance to systematic environmental influences, measurement error, and temporal instability.

Particularly puzzling, and contrary to what many psychologists would predict, is the finding that almost none of the environmental variance is due to sharing a common family environment. Loehlin and Nichols (1976) reached this conclusion on the basis of personality data collected on a large National Merit Scholarship twin sample. Eaves and Young (1981) carried out an informative additional analysis of Extraversion-Introversion and Neuroticism scores from the same twin sample, using a biometric model similar to the one to be reported in this article. They fitted three models to the data. The first model allowed for unshared environment effects and additive gene effects. The second allowed for two environmental effects: shared family environment effects and unshared environment effects, but no genetic effects. The third specified the joint effect of all three. The results were straightforward: The second model, which only specified environmental parameters, did not fit the data for either sex or for the two sexes considered jointly. The first model, which assumes no common family-environment effect, fit all three cases and was as capable of fitting both sexes simultaneously (with identical parameter estimates) as it was of fitting the results of each sex separately. Adding the common familyenvironment effect did not improve the fit of the third model over the first. These results essentially confirmed Loehlin and Nichols's (1976) earlier analysis.

Loehlin and Nichols (1976) carried out many additional analyses of their data. In particular, they attempted to identify the effects of systematic within-family variations. Because identical twins share all of their genes, any differences between them must be environmental in origin. By relating within-pair zygosity-group differences in experiences to personality differences, they could answer questions such as, "Do identical twins who were dressed alike turn out to be more similar in personality than identical twins who were not?" The results were essentially negative.

Correspondence concerning this article should be addressed to Auke Tellegen, Department of Psychology, Elliott Hall, University of Minnesota, 75 East River Road, Minneapolis, Minnesota 55455.

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