twins are assumed to share none of their E factors and all of their C factors. The Mx statistical program, which uses a raw data maximum likelihood estimation approach, was used to fit the models to the data (Neale et al., 2006). Corresponding to the principle of parsimony, models with fewer parameters are preferable if they do not result in a significant deterioration of fit. A useful index of parsimony and fit is Akaike’s information criterion (i.e., Δχ2 − 2Δdf) (Akaike, 1987).
