Behavioral genetics is a fast-growing, multidisciplinary field which attempts to explain the influence of genetic and environmental factors on behavior through the lifespan. The preferred investigative technique for teasing out the differences between genetics and the environment is the longitudinal twin study. This book is the first complete publication from the MacArthur Longitudinal Twin Study (MALTS) that is by far the most ambitious and comprehensive longitudinal twin study to date. The goal of such an in-depth study was not to merely provide thorough descriptions of developmental change between the ages of one and three years, but to offer an original theoretical framework that explains how change occurs in different domains and how genetics and the environment influence those changes. This rigorous study will set the agenda for developmental psychology and behavioral genetics for decades to come.