There are some limitations to the present analyses. First, the study was conducted on a sample of women. While similar evidence has been seen for males, there is also support for variations in the diversity and patterns of substance involvement across sexes (53). Further, the effect of GxE may be more attenuated in males (54) with a prior investigation finding greater evidence for positive peer assortment in girls (38). Thus, whether the same factor structure or genetic findings would be obtained in males remains unknown. Second, PSI reports were extracted from the twin respondent‘s reports from a questionnaire that preceded the interview. While there is some evidence that respondent reports of peer behavior may augment estimates of peer similarity, there is also growing evidence that, within the context of risk influences on the respondent’s behavior, perceived measures may be more predictive of respondent behavior (55). The questionnaire for PSI preceded the interview for RSI - we consider this to be a strength of our study – twins reported objectively on PSI as adolescents and at a later time point, when
