In the present study, the relationship between adolescent onset of cannabis use and repeated voluntary unprotected sex (rVUS) could be partially attributed to common genetic influences, but also to factors not shared by members of identical twin pairs. Problem behavior theory has previously organized early substance use and adolescent sexual activity into a common and heritable framework of impulsive and disinhibited behaviors (29;30). We identified the role of shared genetic influences on adolescent cannabis use and rVUS as well, substantiating the importance of considering shared predisposition in studies relating substance use to risky sexual behaviors.
