In the current study we examined age at first drink and AD in a genetically-informative framework, toward the end of estimating the magnitude of heritable and environmental influences on the two alcohol-related outcomes and quantifying the overlap in these influences. Results provide additional support for the strong association between early onset of alcohol use and AD and, by producing evidence that common genetic risk accounts in large part for this association, address the much-debated issue of whether the relationship is causal in nature. Consistent with prior studies1-3,5, initiation of alcohol use at a younger than average age was associated with elevated rates of subsequent AD. By operationalizing timing of first use as a three-level variable (as opposed to using a dichotomous indicator of early onset) we were able to detect the higher prevalence in the average vs. late onset group as well, which suggests a linear effect of age at first drink on AD. Also consistent with the existing literature, rates of AD were higher among men than women in our sample31-34, but, importantly, the relationship between early initiation and AD did not vary by sex.
