While this study advances the burgeoning literature on G × E interactions in alcohol use, the results are limited by several factors. Participants' heavy drinking was defined as 6 drinks in a row for all participants, which is a more stringent cutoff than the commonly used 4 drinks for females and 5 for males, and it does not account for sex differences in alcohol use. Thus, the results for heavy drinking may have been attenuated by these differences and may not replicate across studies using lower and sex-specific thresholds. Friends' alcohol use was measured with items that did not provide specific definitions of ‘drinking’ or ‘heavy drinking,’ so the answers may have been affected by subjective interpretations of these terms. Friends' alcohol use was also measured differently in adolescence and adulthood, with a focus on any alcohol use among friends in adolescence but heavy use among friends in adulthood. This difference could serve as an alternative explanation for the developmental differences in findings regarding the moderating role of DRD4 genotype. It will be important to replicate the present results with
