There are several limitations of this study. First, reports of the number of H.S. peers who smoked were retrospective. Thus, temporal proximity and mechanisms of peer influence on the transition to nicotine dependence could not be examined, but is a direction for future research in genetically informative longitudinal samples. Additionally, recall bias may have affected the magnitude of the main effect of peer smoking on risk for nicotine dependence. However a strong association has been well demonstrated in longitudinal studies [7, 32]. Moreover, there is little reason to suspect that the level of recall bias would vary by rs16969968 genotype. Second, we were unable to distinguish different types of peer relationships (e.g. romantic versus friendship) which may have differential impact on risk of nicotine dependence. However, we did conduct parallel analyses for same and opposite sex peers. These results were entirely consistent with the presented results and are available upon request. Third, the case–control design of this study does not allow conclusions regarding risks of nicotine dependence in the population as a whole but limits the interpretation of results to
