Genetic epidemiological studies have established that most psychiatric disorders, as well as many nonpathological human behavioral traits, have a substantial genetic component. Investigations in genetic epidemiology therefore provide the scientific foundation for molecular genetic and genomic studies of human behavior and behavioral disorders (Kendler 1993, 2001; Plomin and Kosslyn 2001). Genetic epidemiology uses family, twin, and adoption studies to assess the contribution of familial, environmental, and genetic factors to a trait of interest. Family studies can establish that a given disorder "runs in families" but cannot easily distinguish whether such familiality is due to genetic or environmental factors. An everyday example of the distinction between genetic and familial (but environmental) traits is the difference between the ability to acquire language (a genetic trait that distinguishes humans from other species) and the native language spoken by a given person, which is familial, but entirely environmentally determined.