This article deals with the contribution of genetic and environmental factors to individual differences in the three major dimensions of personality (Psychoticism, Extraversion, and Neuroticism). Twin studies indicate, and family studies confirm within limits, the strong genetic determination of these and many other personality factors, additive genetic variance accounting for roughly half the total phenotypic variance. On the environmental side, shared family environment plays little or no part, all environmental effects being within-family. Assortative mating, important in the formation of social attitudes, has little impact on personality. Dominance may be important for Extraversion. Epistasis (emergenesis) may account for the comparative low values of dizygotic (DZ) twins' correlations. Evidence for differential heritability of traits is present, but not very strong. It is concluded that behavioral genetics forms a vital part of the psychological understanding of the causes of individual differences in personality.