A Genetic and Environmental Analysis of the California Psychological Inventory Using Adult Twins Reared Apart and Together THOMAS J. BOUCHARD, Jr. 1 * MATT MCGUE 1 , YOON-MI HUR 2 , and JOSEPH M. HORN 3 1 Department of Psychology and Institute of Human Genetics, University of Minnesota, Minneapolis, MN 55455, USA 2 Dongjak-Ku, Shindaebangl-Dong, Hyundai APT 102±1707, Seoul 156-010, Sejopng University, Seoul, Korea 3 Department of Psychology, University of Texas, Austin, TX 78712, USA Abstract The California Psychological Inventory (CPI) was administered to a sample of 71 pairs of monozygotic and 53 pairs of dizygotic twins reared apart (MZA, DZA, adult twins) and 99 pairs of monozygotic and 99 pairs of dizygotic twins reared together (MZT, DZT, adult male twins). The twin reared apart data was age and sex corrected. The twin reared together data represented one sex and a narrow age range and was not standardized. The CPI was scored using the 1996 scoring keys, for its 20 folk scales, three vector scales, and 11 special purpose scales. The correlations for the four groups were subjected to model-®tting and the following four parameters estimated: (i) additive genetic variance (V a ), (ii) dominance variance (V d ), (iii) shared environmental variance (V c ), and (iv) idiosyncratic environmental variance (V e ). This design has considerable power to detect V c . The average estimate of V c for all variables was essentially zero. The average estimate of genetic in¯uence (V a  V d ) for all variables was 0.46. Consistent with these results either the MZT or the MZA correlations alone would have provided quite good estimates of the heritability of the traits. Measures of contact between the twins reared apart were unrelated to twin similarity. These ®ndings are highly consistent with the larger behavior genetic literature on genetic and environmental in¯uences on personality. A mean spousal CCC 0890±2070/98/050307±14$17.50 Received 18 June 1998 # 1998 John Wiley & Sons, Ltd. Accepted 30 July 1998 European Journal of Personality Eur. J. Pers. 12, 307±320 (1998) *Correspondence to: Thomas J. Bouchard Jr., Department of Psychology & Institute of Human Genetics, University of Minnesota, Minneapolis, Minnesota, 55455, USA. Contract grant sponsors: Pioneer Fund, The Seaver Institute; The University of Minnesota Graduate School; The Koch Charitable Foundation; The Spencer Foundation. Contract grant sponsor: The National Science Foundation. Contract grant number: BNS-7926654. Contract grant sponsor: Harcourt Brace Jovanovich Publishing Co. Contract grant sponsor: NSF. Contract grant number: GU-1598.