Table 3 provides details on standardized A, C and E contributions to variance, as a whole and as a function of each latent factor. The total h2 of alcohol use was quite low (<0.01) for ages 12–14 years (epoch 1). The h2 increased to 0.07 for ages 15–17 years (epoch 2), then to 0.40 for ages 18–21 years to 30–33 years (epochs 3–6). Due to the structure of the model, only factor A1 contributes to heritability at epoch 1. During epoch 2, factors A1 and A2 contribute equally to the heritability. The relative contributions of A1 and A2 shift during epoch 3, when factor A1 accounts for 30% of the total heritability in alcohol consumption, and A2 for 70%. This relationship is inverted at epoch 4 (i.e. A1 then accounts for 70% of the heritability). During epochs 5 and 6, A1 accounts for over 90% of the heritability in alcohol consumption. Thus, the effects of A2 are largely limited to a period in mid-adolescence to early adulthood, after which genetic influences from A1 are most relevant (Fig. 2).
