In order to examine the structure of the correlations among INATT, HYP/IMP, CDP, and AlcProb, a multivariate model (see Figure 1) was fit to the data. This multivariate model simply extends the basic principles of the twin model and provides estimates of univariate parameters (i.e., a2, c2, d2, and e2 for each of the traits) in addition to cross-trait correlations, which can also be partitioned into genetic and environmental components. As evidenced in Table 2, the phenotypic correlations between the ADHD subtype scores (INATT and HYP/IMP) and AlcProb in these data were nonsignificant; however, it has been shown that the phenotypic correlational structure can be quite different from the underlying genetic and environmental structure (Cloninger, 1987; Heath & Martin, 1990; Stallings et al., 1996). Thus, the full model, guided by existing literature and the pattern of correlations in our data, allowed an ADE structure for INATT and HYP/IMP and an ACE structure for CDP and AlcProb and hypothesized one latent genetic factor (G) that influenced all four externalizing outcomes. The latent genetic factor was influenced by additive (A) and non-additive
