This groundbreaking volume synthesizes the results of the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, which yielded longitudinal data on more than 9,000 individuals. The authors trace how risk for depression, anxiety, eating disorders, antisocial behavior, alcoholism, and substance abuse emerges from the interplay of a variety of genetic and environmental influences. Major questions addressed include whether risk is disorder-specific, how to distinguish between correlational and causal genetic and environmental factors, sex differences in risk, and how risk and protective factors interact over time.