We examined romantic relationship status as the environmental moderator for these analyses in view of evidence that (1) involvement in a romantic relationship in young adulthood is associated with lower alcohol use (Fleming et al., 2010) and (2) that romantic relationship status changes the degree to which genetic influences are important for alcohol outcomes (Heath et al., 1989, Prescott and Kendler, 2001). Of particular relevance for this exploratory G×E hypothesis, recent analyses of the FinnTwin12 sample indicated that genetic variance for intoxication frequency was attenuated for those in a romantic relationship compared to those not in a romantic relationship (Barr et al., in press). This implies that genetic influences on intoxication frequency are less important for those who are in a relationship, and more important for those who are single. The results from these twin studies suggested that romantic relationship status would be a particularly good “candidate environment” when testing our hypothesis that SNPs in regulatory regions would be enriched for G×E effects. Twin studies of G×E effects using inferred genotypes typically show a fan-shaped pattern of effects, whereby additive
