In considering the relative merit of attempts to identify genes conferring susceptibility to addictive diseases, a necessary first step is to evaluate the evidence concerning the extent to which substance use disorders may be influenced by heritable factors at all. Such evidence can be derived from a range of family-based genetically informative research designs including family, adoption and twin studies (Kendler, et al., 2000; True, et al., 1999; Tsuang, et al., 1998; G.R. Uhl, Elmer, Labuda, & Pickens, 1995). Early family-based studies provided initial clues to potential heritable influences on addictive disorders by examining the risk of substance use disorders in the first-degree relatives of individuals either with or without a substance use disorder. For example, in a study of a large sample of individuals meeting criteria for alcohol dependence and their siblings, Bierut and colleagues (Bierut, et al., 1998) reported that, relative to control individuals, the siblings of alcohol-dependent probands had elevated rates of alcohol dependence (50% for men and 25% for women). Similarly, in a study of adult first-degree relatives of probands with dependence on opioids, cocaine, cannabis
