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 and/or alcohol and control probands, Merikangas and colleagues (Merikangas, et al., 1998) reported an eightfold increased risk of drug disorders which was largely independent from the familial aggregation of both alcoholism and antisocial personality disorder. There was also evidence of specificity of familial aggregation of the predominant drug of abuse, suggesting that there may be risk factors that are specific to particular classes of drugs as well as risk factors that underlie substance disorders in general. Similarly, studies have reported both that alcoholism is familial (Kendler, Davis, & Kessler, 1997) and that having an alcoholic parent is associated with a fivefold increase in the risk of alcoholism (Midanik, 1983). Family studies conclude that both alcoholism and other substance use disorders cluster in families, presumably due to heritable factors. However, the family design cannot distinguish whether the causes of familial similarity are genetic or environmental in nature. Further, there appear to be familial influences that confer a non-specific risk for drug
