The aim of the current study is to characterize the relative contributions of genetic and environmental influences on timing of first alcohol use and AD, adding to a limited but important literature for the development of etiological and prevention models. Heritability estimates for AD are consistent across sexes12,13,25,26, but there is some evidence that shared environmental factors play a larger role in the initiation of alcohol use in females than males18,27, so we test for potential distinctions by sex in sources of variance. Most importantly, we test the hypothesis that the link between early initiation and AD can be explained by common genetic risk. Under this model, we would expect covariance between the two phenotypes to be attributed to genetic sources, with no significant overlap in individual-specific environmental influences on the two phenotypes.
