(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 fit the data slightly better than the full ACE model (according to both the RMSEA and the AIC), the ADE model was selected for further analyses. Additional analyses indicated that non-additive genetic influences (D) could be dropped without a significant deterioration of fit (Model 5) and that additive genetic influences specific to age 14 could be dropped from the AE model without a significant decrement in fit (Model 7), but that dropping additive genetic influences common to both ages led to a highly significant deterioration of fit (Model 6). It was also possible to drop the path linking unique environmental influences across the two ages (Model 8), indicating that the unique environmental influences (which include actual individual-specific effects as well as error variance) were not overlapping across the two assessments. Thus, the best-fitting model (Model 9) included additive genetic influences that were common to ages 12 and 14 and age-specific non-shared (individual) environmental influences. Standardized parameter estimates for this model
