associations and of the mechanisms underlying them remains a topic of considerable debate. In addition to the advantages to studying this important question inherent in the twin design, the current study also contained several unique aspects of data collection that we hope will help elucidate these mechanisms. Preliminary analyses presented here, while not comprehensively addressing this, do suggest that more intensive analyses of the data collected here may offer valuable insights into the mechanisms underlying these associations. Specifi-cally, our data highlight the important associations between age of onset of cannabis use and increasing opportunities for the use of other illicit drugs. In future studies based on these data, we hope to explore the extent to which the well known associations between declining age of onset of cannabis use and increasing rates of other illicit drug use and drug abuse/dependence can be explained by a combination of factors including opportunity and access to other drugs as well as peer and related factors. Resolution of this issue has important implications for policy and prevention of substance-related problems.
