Genetic variance in adaptive personality traits

Personality test data from the California Psychological Inventory on a sample of 147 pairs of normal adolescent twins, seventy-nine identical and sixty-eight fraternal same-sex pairs, were evaluated within the framework of evolutionary biology. All eighteen of the trait intraclass correlation coefficients for the identical twins were significant at less than the 0.01 level compared to nine for the fraternals. When heritability of the traits was estimated from the intrapair variances, seven had one-third or more of the within-family variance significantly associated with genetic factors. A factor termed person orientation or extraversion-introversion hadthe greatest genetic variation. Introversion was entertained as a derivative of the social attachment observed in infants by Schaffer and Emerson and considered by them to be a pervasive innate response tendency.