The most notable (and widely cited) contribution to this literature comes from Prescott and Kendler10, who addressed the issue in a study conducted with 8,746 adult twins. Their investigation revealed that the strong association between age at first drink and AD was attributable in full to familial sources of influence – both shared environment and genetics – and thus did not reflect a causal relationship. This study built on an extensive literature documenting the substantial genetic contributions to AD and a growing body of research addressing the relative genetic and environmental influences on the initiation of alcohol use. Heritability of AD has been estimated at about 50-60%12-16, but, in contrast, genetic factors have been reported to account for between 0 and 39% of the variance in alcohol initiation17-19, which is explained to a greater degree by shared environmental factors20-23. The majority of studies addressing heritability of alcohol use have defined initiation in terms of “ever use” without reference to the timing of first drink, which, given the variability in prevalence of AD by age at first drink, is a key
