A genetic and environmental analysis of the California psychological inventory using adult twins reared apart and together

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-fitting 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 influence (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 findings are highly consistent with the larger behavior genetic literature on genetic and environmental influences on personality. A mean spousal correlation of 0.20 for the 34 scales based 111 pairs suggests that there is only modest assortative mating for the CPI scales. (C) 1998 John Wiley & Sons, Ltd.