It has long been known that first-degree relatives of alcohol or drug dependent probands have an increased risk for developing alcoholism (Hill et al., 1977; Merikangas et al., 1998) and drug dependence (Hill et al., 1977; Croughan, 1985) over that seen in the general population. Genetic influence on substance use disorders are well established based on comparisons of rates of these disorders in monozygotic and dizygotic twins (Gurling et al., 1985; Tsuang et al., 1998; Kendler et al., 2000) and in adopted away offspring of biological parents with alcoholism (Goodwin et al., 1973) and drug dependence (Cadoret et al., 1996).
