Having a substance dependent parent was generally predictive of earlier onset alcohol use, drinking to intoxication, smoking, regular smoking, and cannabis use. However, across most substance use variables, offspring of unaffected parents with a substance dependent identical cotwin exhibited greater risk of early involvement than offspring of substance dependent parents. Risk to offspring of unaffected parents with a substance dependent fraternal cotwin was on average small and nonsignificant. While suggestive of genetic versus environmental transmission from parental substance dependence, a somewhat different pattern was observed for cigarette use. Risk to offspring of unaffected parents with a substance dependent identical (but not fraternal) cotwin was elevated in models of early smoking; however, risk was much greater for offspring of cannabis dependent parents, especially through age 8, than for offspring of alcoholic parents or unaffected parents. Thus, for very early use of cigarettes, results suggest an environmental mode of transmission from parental cannabis versus alcohol dependence, perhaps involving modeling of smoking behavior. Relatively permissive attitudes regarding underage substance use, particularly smoking, might also play a role.
