Familial resemblance in the second-to-fourth digit ratio (2D:4D), a proxy for prenatal androgen action, was studied in 1,260 individuals from 235 Austrian families. In agreement with findings from twin studies of 2D:4D, heritability estimates based on parent-child and full-sib dyad similarity indicated substantial genetic contributions to trait expression (57% for right hand, 48% for left hand 2D:4D). Because twin studies have found nonadditive genetic as well as shared environmental effects on 2D:4D to be negligible or nil, these family-based estimates in all likelihood reflect the narrow-sense (additive genetic) heritability of the trait. Directional (right-minus-left) asymmetry in 2D:4D was only weakly heritable (6%). The pattern of same-sex and different-sex parent-child and full-sib correlations yielded no evidence for X-linked inheritance. This is surprising, considering evidence for associations of male 2D:4D with sensitivity to testosterone (functional variants of the X-linked androgen receptor gene). 2D:4D was particularly strongly heritable through male lines (father-son and brother-brother correlations), thus raising the possibility that Y-linked genes (such as the sex-determining region SRY) might influence 2D:4D expression.