example, it has been proposed that early onset cannabis use may increase risks for the development of MDD among women (Patton et al., 2002), Alternatively, it has been suggested that drug use may develop as a result of attempts to self-medicate adverse emotional states (Khantzian, 1997). Finally, it has also been proposed that there may be no patterns of reciprocal causation between drug use and mental health but that their apparent comorbidity arises from the influence of shared and correlated risk factors that operate to increase risks of both drug use and of mental health problems. For example, our own research, based on an older cohort of Aus-tralian twins, suggests that the association between cannabis use and depression may be largely noncausal and arise from risk factors, particularly shared genetic predispositions that increase risks of both cannabis use and of developing major depression (Lynskey et al., 2004). While we have not examined mechanisms underlying the observed associations between cannabis use and other drug use/mental health in our current analyses, we hope to more fully exploit the advantages of twin data to more comprehensively address this question using the current data set.
