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...

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.

Monozygotic twins reared apart from an early age were used to test the hypothesis that there is a significant genetic component to job satisfaction. Thirty-four monozygotic twin pairs who had been reared apart completed the Minnesota Job Satisfaction Questionnaire as part of a comprehensive work-history assessment. Three subscales were formed from the job satisfaction items to reflect intrinsic, extrinsic, and general satisfaction with the current (or major) job. Intraclass correlations were computed to estimate the proportion of observed variability resulting from genetic factors for all job satisfaction items and for the three subscales. Resulting values indicated that approximately 30% of the observed variance in general job satisfaction was due to genetic factors. Additional analysis indicated that these results obtained even when job characteristics such as complexity, motor skill requirements, and the physical demands were held constant via partialing methods. Finally, the data indicated significant heritabilities for several of these job characteristics, which is consistent with the hypothesis of a genetic disposition to seek and remain in similar environments (jobs). Implications of these findings for theories of job satisfaction, selection, and job enrichment are discussed.

Gingivitis and periodontitis are among the most common diseases known to man. Although bacterial plaque is generally accepted as the primary etiologic agent, little information is available concerning the influence that host genetic factors have on these diseases. The purpose of the present study was to examine the relative contribution of environmental and host genetic factors to clinical measures of periodontal disease through the study of both reared-together twins and monozygous twins reared apart. Probing depth, clinical attachment loss, gingivitis, and plaque were assessed from the Ramfjord teeth in 110 pairs of adult twins (mean age 40.3 years), including 63 monozygous and 33 dizygous twin pairs reared together and 14 monozygous twin pairs reared apart. Bootstrap sampling was used to estimate and provide confidence limits of between-pair and within-pair variances, intraclass correlations and heritability. Based on ratios of within-pair variances or heritability estimates, a significant (P less than 0.05) genetic component was identified for gingivitis, probing depth, attachment loss and plaque. Heritability estimates indicated that between 38% to 82% of the population variance for these periodontal measures of disease may be attributed to genetic factors. While there is general agreement that bacteria are important in the pathogenesis of the periodontal diseases, future etiologic studies should consider the role of host genetic influences.