Family-based analyses in the HCP (n = 804) revealed that alcohol consumption and GMV of the right insula and right middle/superior frontal gyrus are moderately to largely heritable (Figure 2A; Table S1 in Supplement 1). Moreover, decomposition analyses showed that phenotypic correlations between frontal and insular GMV and alcohol consumption are attributable to shared genetic, but not unique environmental, influences (Figure 2B, Table S1 in Supplement 1). Post hoc analyses confirmed that shared environmental factors did not significantly contribute to the correlation of alcohol consumption and GMV (Table S1 in Supplement 1). Analyses within twin and sibling pairs in the HCP sample who were concordant or discordant for alcohol use revealed that relative to siblings who were concordant for low alcohol use, siblings who were concordant for high use or discordant for use (i.e., 1 high use, 1 low use) had lower insular and frontal GMVs (Figure 2C, D; Table S2 in Supplement 1). Further, GMVs did not differ between low and high alcohol-using members of discordant pairs. As shared genetic and familial factors are matched within pairs, this pattern
