subjects, 36.0% were monozygotic (MZ) twins (n=216), 63.5% were dizygotic (DZ) twins (n=382). The 602 genotyped individuals were selected from twin pairs extremely discordant and concordant (EDAC selection) for alcohol-related problems, using a 22- item version of the Rutgers Alcohol Problem Index (RAPI; White & Labouvie, 1989) administered at age 18.5. EDAC selection has the benefit of enhancing statistical power by focusing on the most informative sibpairs, and it has been suggested as an advantageous sampling method for genetic linkage analyses (Gu et al. 1996). A full description of this selection is described in Latvala et al., 2011 (Latvala et al., 2011).
