The aim of these analyses was to apply, using structural equations, a model containing a single genetic common factor to putative externalizing and internalizing disorders, and traits in MM and FF twin pairs, to determine their relative loadings on the common factor. To improve our power, many of the diagnostic categories were treated as polychotomous rather than dichotomous, which allowed us to use information about subthreshold levels of symptoms. To ensure that this was a plausible model, we first tested the fit of an independent pathway model containing a single genetic and a single unique environmental common factor, in addition to genetic and unique environmental variable-specific loadings, against a model that contained only a single shared and a single unique environmental common factor (and shared and unique environmental variable-specific loadings). Analyses were performed in the Mx software package using full information maximum likelihood (Neale et al. 2003). The fit of the models was assessed by Akaike’s information criterion (AIC; Akaike, 1987), which performs well for models of intermediate complexity as used in this report (Williams & Holahan, 1994).
