Twin studies of alcohol use initiation indicate that genetic factors play a significant role in the timing of first alcohol use. Estimates of the proportion of variance attributable to heritable influences cover a wide range, from 0–72% (Koopmans and Boomsma, 1996; Maes et al., 1999; Pagan et al., 2006), but the majority of studies in this area have reported a more substantial contribution of common environmental than genetic factors. Common – also known as shared – environmental influences, are non-genetic factors that make members of a twin pair similar on a given phenotype, such as familial, school, and neighborhood influences. Shared environment has been found to account for approximately half of the total variance in initiation of alcohol use (Fowler et al., 2007; Koopmans and Boomsma, 1996; Pagan et al., 2006; Rhee et al., 2003). Findings from twin studies of alcohol dependence (AD) indicate a very different model for AD. Consistently across studies of both males and females, 50–60% of variance in AD is attributed to additive genetic influences and there is no evidence for a significant contribution of shared
