Our model divides the sources of individual differences in liability to three forms of DA into additive genetic (A), shared environment (C), and unique environment (E). The model assumes that MZ twins share all their genes while DZ twins and full siblings 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/sibling pair. Unique environment includes random developmental effects, environmental experiences not shared by siblings, and random error. The aim of the models was to investigate to what extent genetic and environmental factors are the same for the three phenotypes. In the first model, an independent pathway model, we assume that each of the three variance components (A, C and E) consists of 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 second model, a common pathway model, we assume that the common variance components (Ac, Cc, and Ec) are mediated by
