We utilized a classical twin model assuming three sources of liability: additive genetic (A), shared environment (C), and unique environment (E). The model assumes that MZ twins share all their genes while DZ twins share on average half of their genes identical by descent, and that the shared environment, reflecting family and community experiences, is the same within each twin pair. Unique environment includes random developmental effects, environmental experiences not shared by siblings, and random error. We examine the relative fits of two trivariate models applied to the three externalizing syndromes of DA, CB and AUDs: an independent pathway and a common pathway model. In both of these models, each of the three variance components (A, C and E) are decomposed into two parts: one that is common to all three phenotypes (denoted Ac, Cc, and Ec) and one that is specific to each one of them (As, Cs, and Es). In the independent pathway model, the paths from Ac, Cc, and Ec go directly to the observed phenotypes. In the common pathway model, the paths from Ac, Cc, and
