Heritable influences explained a substantial amount of variance in initiation status for alcohol (44%), cigarettes (62%), and cannabis (77%). Shared environmental factors had a modest influence on alcohol use initiation (10%) but did not contribute significantly to the onset of cigarette or cannabis use. There was evidence for a considerable degree of overlap in genetic factors influencing initiation status for alcohol and cigarettes (rA1–A2 = 0.70; CI: 0.61–0.75) and alcohol and cannabis (rA1–A3 = 0.49; CI: 0.36–0.83), but the genetic correlation between cigarettes and cannabis was more modest (rA2–A3 = 0.25; CI: 0.13–0.26). A large degree of overlap was observed in individual-specific environmental influences on initiation of cigarette smoking and onset of cannabis use (rE2–E3 = 0.95; CI: 0.80–0.97). Unique environmental contributions to the onset of alcohol use were also correlated (to a lesser degree) with those contributing to cannabis use initiation (rE1–E3 = 0.27; CI: 0.19–0.51), but we were able to drop the E pathway from alcohol to cigarettes without a deterioration in model fit (Δχ2(1) =−2.00).
