In this sample of young adult male and female twins, we can conceptualize the lifetime comorbidity among alcohol, tobacco, and cannabis as alternative manifestations of a heritable latent trait. Further, gender differences in the relative influence of genetic and environmental factors on each substance are likely to arise from substance-specific effects. Based on our overall findings, the development of effective prevention and intervention strategies for these three substances will be dependent on identifying and applying biological and environmental strategies that reduce the risk that is both common and specific to these substances.
