Interestingly, DRD4 polymorphism moderated the effect of heavy drinking friends on alcohol use, but not on frequency of heavy drinking. This discrepancy suggests that overall alcohol use may be a more sensitive outcome measure of G × E effects than the frequency of heavy drinking which required participants to meet the cutoff of 6 or more drinks in a row. Together, the results indicate that heavy drinking friends may contribute to higher overall alcohol intake among those with a long DRD4 allele, but not necessarily to more frequent binge drinking. Incorporating multiple alcohol use phenotypes in future studies will be important to provide a more nuanced understanding of environmental and genetic effects on specific aspects of substance use.
