We utilized an extension of the classical twin modelling, assuming a liability threshold model with three sources of liability: additive genetic (A), shared environment (C), and nonshared environment (E). In addition to twins we included full and half siblings. We assume that MZ twins share all their genes while DZ twins and full siblings share on average half of their genes, and half siblings share only a quarter of their genes identical by descent. The shared environment, reflecting family and community experiences, is the same within each pair although we examine the presence of a unique twin environment (T). The unique twin environment includes random developmental effects, environmental experiences not shared by siblings, and random error.
