A theory of developmental change in quantitative phenotypes applied to cognitive development.

A model is presentedfor the changes infamilial resemblance as afunction of age. The model allows for separate developmental components of genetic and environmental effects andfor the influence ofearlier phenotypic values on current measurements. Genetic and environmental effects may be specific to occasions or constant over time. Expected covariances are derived within individuals and between relatives measured at different ages. Parameter substitution shows that models with different assumptions about the mechanism ofdevelopment yield different predictions for temporal changes in family resemblance. The application of the model is illustrated by the analysis of published longitudinal data on cognitive development. The data suggest that the continuity of cognitive performance over time and the increase in heritability with age reflect the cumulative long-term effects of a single set of genes expressed throughout development. The quality of the shared environment changes from family to family over time but appears to exercise a long-term effect on cognitive development.