This book reports on the Harvard Preschool Project, a study of how child-rearing practices affect the development of social and intellectual competence in children from birth to age six. The study is based on the experiences of 31 subjects in their own homes in Massachusetts. Part I gives the introduction, background and strategy for determining (1) what is human competence in six-year-old children, and (2) how we learn the details of the interactions between early experience and the development of such competence. Part II describes in detail the methods and findings of the longitudinal, natural experiment begun in 1969. This information is divided into research on the child's development of competence and his stream of experience, and research on the child's human and physical environment. Tables, graphs, artwork, and figures are used to present the data. Part III discusses the tentative interpretations of the findings. A full presentation of assessment techniques, manuals, reliability test results, and detailed data is found in the appendix section. (ST)