As with the FF pairs, the fit improved when we dropped all C parameters (model II) and deteriorated when we dropped all A parameters (model III) (Table 4). Compared to the full model, the model fit was not improved by dropping A1 (model IV), A2 (model V), or A3 (model VI). It is particularly noteworthy that we could not drop A3, which reflects genetic influences unique to AD. In contrast to the female twins, the best-fit model for the males was model II, parameter estimates from which are seen in Fig. 3.
