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 quantitative genetic research is essential to the understanding of developmental change and continuity. The largest and longes longitudinal adoption study is the Colorado Adoption Project, which has generated much of the rich data on the progress from infancy to early childhood on which the authors draw throughout the book.