Quantitative genetic models indicated that both ever use of cannabis and early-onset cannabis use are influenced by genetic and environmental factors. In the full sample (3104 pairs with known zygosity), genetic (a2=0.43 [0.26-0.60]) and shared environmental (c2=0.25 [0.10-0.40]) influences together explained approximately 70% of the variance in lifetime cannabis use, with the remaining variance attributable to individual-specific factors (e2=0.32 [0.27-0.37]). When examined at the population-level, early use showed a somewhat larger shared environment component (a2=0.26 [0.03-0.52], c2=0.49 [0.26-0.69], e2=0.24 [0.18-0.31]). However, when restricted to pairs concordant for cannabis initiation (n=926 pairs with known zygosity of the 945 pairs concordant for ever use), familial influences on early cannabis use were primarily genetic (a2=0.42 [0.09-0.78]), shared environmental influences were moderate but non-significant (c2=0.33 [0.00-0.61]), and individual-specific factors explained the remaining 25% of the variance (e2=0.25 [0.18-0.35]).
