The Louisville Twin Study includes nearly 500 pairs of twins and their siblings who have participated in a longitudinal study of mental development, beginning in infancy and extending to adolescence. The results show that individual differences beginning in infancy and extending to adolescence. The results show that individual differences in intelligence progressively stabilized by school age, and each child followed a distinctive pattern of spurts and lags in mental development. Monozygotic twins became increasingly concordant over ages and matched each other closely for developmental trends. By contrast, dizygotic twins regressed to an intermediate level of concordance, comparable with that found for siblings and parent-offspring sets. A comprehensive home assessment was performed to identify the home/family variables that contributed to mental development. The overall results pointed to a strong developmental thrust in the growth of intelligence, which was principally guided by an intrinsic genetic ground plan. Qualitative features of home and family did, however, add significantly to prediction of offspring IQ. The results are interpreted in the context of Waddington's developmental model, and some recent advances in neurobiology and genetics are surveyed for their implications for developmental behavior genetics.