Tetrachoric correlations computed within the twin pairs (Table 1) were significant for MZ twins at both ages, whereas DZ twin correlations were generally lower and mostly non-significant, suggesting the presence of genetic influences. The results of model fitting are presented in Table 2. First, we fit a saturated model, which estimated a variance/covariance matrix for each group (10 parameters for each of 5 twin groups; 50 parameters total), thresholds for each twin for each variable (5 groups, 2 twins per group, 2 variables per twin; 20 parameters total), and regression coefficients for each definition variable (5 groups, 2 twins per group, 4 definition variables per twin, 2 variables per twin; 80 parameters total). More restrictive genetic models were then tested against the saturated model to determine whether they provided an adequate fit with a more parsimonious model. Our first genetic 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
