We searched PubMed and Google Scholar® for all twin and adoption studies of AUD. In addition, we searched the bibliographies of all of the relevant studies to further add samples to our analysis. To ensure completeness, we followed up by contacting leading researchers in the AUD genetics field. Studies were included in our analyses if they: (i) examined AUD, here defined as a syndrome including heavy alcohol use with significant adverse social, psychological and/or medical consequences, a definition broadly similar to the DSM-IV concepts of alcohol abuse or dependence (APA, 1994); and (ii) presented sufficient data to permit re-analysis. In practice, across the various studies, AUD was defined by clinical interview, hospital record diagnosis or temperance board registration. If multiple studies were published using the same data, we used the report with the largest sample size. Because these studies utilized a variety of different analytic approaches, it was necessary to reconstruct the data for each study. The features of the available twin and adoption studies are listed in Tables 1 and 2, respectively. Importantly, because we focused exclusively on AUD, twin studies examining only drinking frequency or quantity were omitted.
