Only rarely has alcohol, cigarette, and cannabis use been examined in combination in genetically informative samples. Among the few to do so was Han et al.’s (1999) investigation with adolescent twins, which produced evidence of a common vulnerability factor that was explained largely by environmental factors shared by twins (63%, compared with 23% accounted for by genetic factors). Hopfer and colleagues (2001) reported similar findings in their study of alcohol use and cigarette smoking in an older adult cohort of female twins. Shared environmental influences, which accounted for a substantial proportion of variance in lifetime (ever use) measures of both alcohol and cigarettes, were highly correlated across substances. Findings from Koopmans et al.’s study (1999) using twins ranging in age from 12 to 25 years also indicated that, among the 12 to 16 year-olds, initiation of alcohol use and cigarette smoking were influenced in large part by the same shared environmental factors. In contrast, the association between the onset of alcohol use and cigarette smoking among 17 to 25 year-olds was attributable to common genetic factors. The relatively limited research
