Adoption results for self-reported personality: evidence for nonadditive genetic effects?

Twin studies consistently indicate moderate genetic influence on individual differences in personality as assessed using self-report questionnaires, with heritability estimates typically about 40%. In this first analysis of self-report personality data from the longitudinal Colorado Adoption Project, little evidence is found for additive genetic influence in parent-offspring and sibling adoption analyses based on a foundation sample of 245 adoptive families and 245 nonadoptive families with adopted and nonadopted children assessed yearly from 9 to 16 years. Although several factors might contribute to the discrepancy between twin and adoption results, we suggest that nonadditive genetic influence, which can be detected by twin studies but not by adoption studies, is a likely culprit. These findings have important implications for attempts to identify specific genes responsible for genetic influence on personality.