age. A social explanation was proposed for these findings in that early initiators were said to face a nonpermissive environment (e.g. availability, community norms, adult supervision, etc.) [Breslau et al., 1993]. In addition, we performed an ad hoc exploratory analysis to determine whether there were any trends with the five loci and the delta between AOI and AOS. Utilizing a subset of samples with both phenotypes available (Add Health, CADD, COGA, COGEND, NYS-FS, YFS), we found no evidence for association between the AOI–AOS time differential and any of the loci used for this study (data not shown). The similarities and differences in results across SNPs for the two phenotypes are interesting also to consider in the context of work finding an association between externalizing behaviors and rs8040868, which is a locus not well-tagged by the SNPs investigated here [Stephens et al., 2012]. It is possible that some of the differences between AOI and AOS results are related to more general underlying behaviors, such as novelty-seeking and conduct disorder. Future studies aimed at disentangling possible unique genetic contributions to these phenotypes are merited. An alternative explanation for these findings could be biological. Age at initiation is related to behavioral traits such
