Regarding the cotwin-control analyses, it is possible that selective attrition biased our results, although our examination suggests that any selective attrition for smoking-related behaviors contributed to an underestimation of conditional odds ratios and thus reduced our likelihood of finding differences. Additionally, given the small numbers of pairs discordant for early cannabis use, cannabis dependence, and any illicit drug dependence, our conditional logistic regression effects have broad confidence intervals that do not allow us to exclude important effects.
