Model-fitting results for the FF twin pairs are presented in Table 2. Model I was our full model (Fig. 1A) with genetic factors A1 through A7 and full Cholesky decompositions for C and E. Model II dropped (i.e., forced to zero) all C parameters with a large improvement in our fit index (AIC = −24.2), indicating that shared environment played no substantial role in twin resemblance for AD or our AC measures. Model III dropped all the A parameters which, by contrast, produced a deterioration in model fit (AIC = +3.0) suggesting that genetic factors were critical. Our further model fitting, therefore, included only A and E parameters.
