In light of work by quantitative geneticists, the authors reconsider the interaction of heredity and environment in the development of individual differences during infancy and early childhood. Quantitative genetics offers a general theory of the development of individual differences that suggests novel concepts and research strategies: the idea that genetic influences operate in age-to-age change as well as in continuity, for example. Quantitative genetics also provides powerful methods to address questions of change and continuity which are helpfully introduced in this study. Longitudinal...