Early mental development is analyzed from an evolutionary viewpoint and related to the dynamic interplay of genetic programming, maturational status, and environmental influence. Data are reported from a large sample of twins and siblings who have been tested longitudinally from 3 months to 6 years of age. Monozygotic twins became increasingly concordant with age and also paralleled each other for the spurts and lags in development. Dizygotic twins became less concordant with age and eventually matched their singleton siblings as closely as one another. The overall results suggested that the course of mental development is guided by the intrinsic scheduling of the genetic program acting in concert with maturational status and environmental influence.