Following evidence that alcohol consumption is coheritable with volume of the right insula and middle and superior frontal gyri (see Results), we examined whether same-sex twin and nontwin sibling pairs discordant for alcohol consumption differed from each other on brain volume in the HCP sample. These analyses examined whether aAUDIT-C was associated with insular or middle and superior frontal volume after accounting for sibling-shared genetic background and experience. Same-sex siblings were considered “high alcohol consumers” if their aAUDIT-C score was >0.5 SD above the sample mean (aAUDIT-C > 4.67), or “low alcohol consumers” if their score was <0.5 SD below the sample mean (aAUDIT-C < 1.54), respectively. A concordant sibling pair was defined as a pair who were both in the same category of consumption (i.e., high or low) and additionally scored within 1 SD of each other (low alcohol concordant pairs: n = 117; aAUDIT-C mean = 0.84, SD = 0.77; high alcohol concordant pairs: n = 54; aAUDIT-C mean = 7.08, SD = 1.4). There were 72 discordant sibling pairs (“low discordant”; aAUDIT-C mean = 1.25, SD =
