The full model (depicted in Fig. 1 for FT16 and Fig. 2 for the VATSPSUD) calculated variance components separately by sex. Thresholds for each variable were adjusted by age to account for the variability in age in the samples. Additional models were tested to evaluate goodness-of-fit in which estimates of the variance components were constrained to be equal across sex. Estimates were obtained from observed twin data using maximum likelihood estimation in the software program Mx (Neale et al., 1999). Model fit was evaluated by Akaike’s Information Criterion (AIC), and the probability (p) value associated with the chi-squared statistic. Lower AIC values indicate an optimal balance between explanatory power and parsimony. Additionally, nonsignificant chisquared values (p > 0.05) indicate a good fit. We compared nested alternative models by the change in chi-square between models, which is used to evaluate the significance of dropping parameters. A significant change in chi-squared (p < 0.05) for the difference in degrees of freedom of the models indicates that the model with fewer degrees of freedom should be adopted, because the gain in degrees of
