The magnitude of C and E contributions to both cannabis use initiation and problematic use indicates that environmental factors (which are often modifiable) also play a substantial role. In a longitudinal twin study, Korhonen et al. (65) identified some of the environmental factors that predict cannabis as well as other illicit drug use. They found that paternal drinking behaviour was a significant familial predictor (although this could also be a manifestation of their shared genetic vulnerability). Other predictors they mention are early smoking onset, drinking to intoxication, having peers who smoke cigarettes or have acquaintances with drug experience, and aggressive behaviours among males (65). Scherrer et al. (66) also found that perceptions of substance use among siblings, friends and school peers are strongly associated with cannabis abuse/dependence in young adults.
