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 previous 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, [h 2 = 2(R M z -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 enviThis research has been supported by grants from the University of Minnesota Graduate School, the Koch Charitable Foundation, the Spencer Foundation, the National Science Foundation (BNS-7926654), the Pioneer Fund, and the Harcourt Brace Jovanovich Publishing Company.We thank the following people for the time and effort they have given to testing the twins: Margaret Keyes, Jeff McHenry, Elizabeth Rengel, Susan Resnick, Joy Fisher, Jan Englander, and Ann Riggs. We are indebted to our colleagues and collaborators on the Minnesota Study of Twins Reared Apart project, Elke Eckert and Leonard Heston for their help and advice. We owe special thanks to our colleagues, Greg Carey and Matt McGue, for their valuable biometric advice and assistance. We also thank Jack Darley for his resolute and dedicated support and the Minneapolis War Memorial Blood Bank, Herbert Polesky, Director, for the blood testing.Correspondence concerning this article should be addressed to Auke Tellegen, Department of Psychology, Elliott Hall, University of Minnesota, 75 East River Road...

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Psychological researchers typically distinguish five major domains of individual differences in human behavior: cognitive abilities, personality, social attitudes, psychological interests, and psychopathology (Lubinski, 2000). In this article we: discuss a number of methodological errors commonly found in research on human individual differences; introduce a broad framework for interpreting findings from contemporary behavioral genetic studies; briefly outline the basic quantitative methods used in human behavioral genetic research; review the major criticisms of behavior genetic designs, with particular emphasis on the twin and adoption methods; describe the major or dominant theoretical scheme in each domain; and review behavioral genetic findings in all five domains. We conclude that there is now strong evidence that virtually all individual psychological differences, when reliably measured, are moderately to substantially heritable.

Since 1979, a continuing study of monozygotic and dizygotic twins, separated in infancy and reared apart, has subjected more than 100 sets of reared-apart twins or triplets to a week of intensive psychological and physiological assessment. Like the prior, smaller studies of monozygotic twins reared apart, about 70% of the variance in IQ was found to be associated with genetic variation. On multiple measures of personality and temperament, occupational and leisure-time interests, and social attitudes, monozygotic twins reared apart are about as similar as are monozygotic twins reared together. These findings extend and support those from numerous other twin, family, and adoption studies. It is a plausible hypothesis that genetic differences affect psychological differences largely indirectly, by influencing the effective environment of the developing child. This evidence for the strong heritability of most psychological traits, sensibly construed, does not detract from the value or importance of parenting, education, and other propaedeutic interventions.

A summary of 111 studies identified in a survey of the world literature on familial resemblances in measured intelligence reveals a profile of average correlations consistent with a polygenic mode of inheritance. There is, however, a marked degree of heterogeneity of the correlations within familial groupings, which is not moderated by sex of familial pairing or by type of intelligence test used.