Results from the current study suggest a number of possible directions for future work in this area. Extension of this line of research to African-American males and members of other under-represented ethnic groups will address critical questions about the generalizability of these findings and their implications for tailoring prevention efforts to the needs of various risk groups. Examination of genetic and environmental contributions to additional transitions in the course of substance use (e.g., symptom onset) will address the extent to which these influences fluctuate over time, potentially introducing the need for different intervention approaches at different stages of use. Investigation of possible mediators of the correspondence of age at onset for these three substances of abuse, such as externalizing psychopathology (attributable in part to the same genetic factors that influence substance use initiation [McGue et al., 2001]), will further our understanding of the pathway of risk that can lead to substance use disorders. Finally, our estimates of the relative contribution of heritable and environmental influences on the timing of first alcohol, cigarette, and cannabis use provide a starting point for
