The results from the model fitting procedure are summarized in Table 2. The baseline ACE model (Model 1) included the unique twin environment (T) and age regression parameters. Dropping the T parameters (Model 2) resulted in no deterioration in model fit compared to Model 1, ΔAIC = −3, χ2(6) = 8.61, p = 0.19. Constraining the male and female A, C, and E paths to be equal (Model 3) resulted in significant deterioration in fit compared to Model 2, ΔAIC = 239, χ2(7) = 253.39, p = 7 · 10−51. Dropping the age regression parameters (Models 4 and 5) resulted in significant deterioration in fit compared to Models 1 and 2, ΔAIC = 490, χ2(12) = 513.63, p = 3 · 10−102 and ΔAIC = 490, χ2(12) = 514.26, p = 2 · 10−102, respectively. In sum, the best fitting model was an ACE model with age regression where the parameters were allowed to vary across sex (Model 2). The parameter estimates and 95% CIs for this best fitting model are presented in Figure 1.
