There are several limitations of the present study. The same rater reported their own and their close friends' alcohol use, which may have led to shared method variance inflating covariance artificially between the two assessments. Because adolescent reports of peer drinking may include bias [19], as their accounts may measure imperfectly peer's actual use and reflect their own use [49], it is possible that the genetic influences on perceived peer use may index genetic influences on the adolescent's reporting, and this needs to be taken into account when interpreting our findings. Although very few studies have presented correlations between perceived and actual peer use, one study in adolescents has reported that the correlations were not perfect, but substantial (0.63–0.68) [24]. Furthermore, our results are in agreement with an earlier univariate twin study that was based on peers' own reports of a combined measure of alcohol and cigarette use [19] and also found evidence of genetic influences.
