Separate genetic analyses conducted for alcohol, cigarettes, and cannabis use indicated that a sizeable proportion of variance in initiation status was attributable to additive genetic sources (A) for all three substances: 41% for alcohol, 74% for cigarettes, and 52% for cannabis. The contribution of shared environmental factors (C) to age at first use was more modest for all three substances, with point estimates of 0 for cannabis and 0.12 for alcohol and cigarettes. However, as seen in Table 2, confidence intervals for all three C estimates as well as estimated A values for alcohol and cannabis included 0. Dropping the C pathway did not result in a significant difference in model fit for alcohol (Δχ2(1) = 0.15), cigarettes (Δχ2(1) = 0.21), or cannabis (Δχ2(1) = 0.00). Dropping the A pathway also did not result in a significant difference in model fit for alcohol (Δχ2(1) = 1.39) or cannabis (Δχ2(1) = 2.57), but a deterioration in model fit was observed for cigarettes when the A pathway was dropped (Δχ2(1) = 5.66). The A and C pathways were dropped simultaneously to determine
