Table 1 depicts the multivariate twin models we evaluated. The initial baseline model (1) included single genetic, shared environmental, and nonshared environmental common factors along with criterion-specific genetic, shared, and nonshared environmental influences. Next, we compared the fit of alternative models that omitted, respectively, the criterion-specific shared environmental factor (model 2), the criterion-specific genetic factor (model 3), and then both together (model 4). The BIC values for models 2 and 3 were both clearly superior to BIC for model 1, with fit being slightly better for model 2 than model 3. The BIC for model 4, which omitted both the criterion-specific shared environmental factor and the criterion-specific genetic factor, was in turn much worse. Therefore, working from model 2, we then dropped (in models 5, 6, and 7, respectively) the single genetic, shared environmental, and nonshared environmental common factors. The BIC values for these 3 models were all substantially poorer than for model 2. Since simplification of model 2 did not improve the fit, we then sought to improve fit by including additional common factors. Specifically, in models 8, 9,
