The substantial contribution of genetic factors to the timing of first use found for alcohol, cigarettes, and cannabis indicates that although the prevalence of substance use is relatively low for African-American girls and young women compared with rates in males and other ethnic groups, early use is highly heritable. The considerable overlap between heritable factors that contribute to the timing of first alcohol use and those that influence onset of cigarette and cannabis use suggests that familial risk for problem drinking is a global marker of risk for early initiation of licit and illicit drugs. The fact that nearly all of the individual-specific environmental influences on onset of cigarette smoking and initiation of cannabis use were traceable to a common factor suggests that environmental exposures (e.g., association with deviant peers) that increase risk for starting to smoke at a young age also impact the likelihood of early onset of cannabis use. Our findings differ from those in studies based on primarily Caucasian samples that have addressed substance use initiation as a dichotomous construct (ever use), thus suggesting that efforts aimed
