model included additive genetic, non-additive genetic, and unique environmental influences (ADE) because in most cases the MZ correlations were more than twice the DZ correlations, suggesting the contribution of non-additive genetic factors. The initial ADE model (Model 1) allowed for the differences in parameter estimates between males and females, equated thresholds for members of same-sex twin pairs (males and females estimated separately), and equated the impact of SES on same-sex twin pairs (males and females estimated separately). Thresholds and regression parameters for members of DZO pairs were estimated separately from each other and from the same-sex pairs. Under this model, broad-sense heritability (including both additive and non-additive genetic influences) was significant at both ages (age 12: 37 and 38% for males and females, respectively; age 14: 45 and 80% for males and females, respectively). However, equating male and female parameters did not lead to a significant deterioration of fit (Model 2). Because there was some evidence of shared environmental influences (C) at age 12, we also tested ACE models. As with the ADE models, ACE models incorporating shared environmental influences (C) indicated that male and female parameters could be equated (Model 4 vs. Model 3). Because the full ADE model
