Table 1 presents the results of the model fits for the different gender-specific models. It indicates that the no sex effects (sex homogeneity) model provides the best fit for all variables, suggesting that there were no quantitative or qualitative gender difference in aetiology. This model also equates the thresholds for males and females, suggesting that there was no difference in prevalence for males and females. Estimates of a2, c2, e2 appear in Table 2 as part of the bivariate analysis. The results for friends' alcohol use indicate significant genetic (approximately 28%), common environment (approximately 36.5%) and unique environment (approximately 35.5%) influences. For quantity of alcohol use the majority of the variance was explained by genetic influences (61%), although both common environment (20%) and unique environment (19%) were also significant. Similarly, for problem alcohol use, genetic influences explained the largest proportion of variance (46%) although common environment (29%) and unique environment (25%) were also significant. The best-fitting models were not significantly different from the fully saturated models (see Table 1), suggesting that they represented a good fit of the data.
