Heritability of personality traits in adult male twins.

Personality test data from the California Psychological Inventory were collected on 99 pairs of identical and 99 pairs of fraternal adult male twins. Heritabilities were comuted for all 18 scales and compared to the heritabilities for "pure" scales with overlapping items omitted. Two of the pure scales, Responsibility and Femininity, had zero heritabilities, whereas all of the full scales had moderate to high heritabilities. It was concluded that item overlap has contributed significantly to previous failures to find evidence for the differential heritability of personality traits as measured by the CPI. CPI items were classified into genetic or environmental categories and separate factor analyses of items in these categories revealed more differences than similarities in factor structure. The genetic personality factors included Conversational Poise, Compulsiveness, and Social Ease. Environmental factors included Confidence in Leadership, Impulse Control, Philosophical Attitudes, Intellectual Interest, and Exhibitionism. Compared to the genetic factors, each of the environmental factors accounted for only a very small percentage of the variance.