Descriptive statistics and preliminary analyses were obtained/conducted using SAS 9.2 and JMP 8 (SAS Institute Inc., USA). Twin modeling was conducted in OpenMx (Boker et al. 2011) using the raw continuous data option. All twins were included in these analyses, including those who were members of incomplete pairs. In twin modeling, liability to phenotypes such as depression or alcohol use can be attributed to several latent sources of variance: additive genetic factors (A), shared environment (C) and unique environment (E). The C variance component represents environmental exposures and experiences that are shared by both members of a twin pair and contribute to twins’ increased similarity, irrespective of zygosity, in a given phenotype. Environmental factors that are unique to one twin are accounted for by the E component; these factors reduce twin similarity for a given phenotype. The E component also includes measurement error. Estimates of each of these variance components are calculated by comparing the phenotypic correlation between MZ twins (who share all their genes) with DZ twins (who share half of their genes, on average, identical by descent).
