Although no quantitative age differences were observed, without longitudinal data it is impossible to discount the possibility of qualitative age differences (specific aetiological influences coming online at different ages). Finally, we took a conservative approach to control for twins sharing the same peers. However, this greater sharing of friends may be due to genetic influences on friendship formation and therefore represent a valid genetic influence on peer alcohol use. If this were the case, our estimates of genetic influences on peer alcohol use would be overly conservative.
