The direction of all effects indicated that a larger number of the minor allele was more likely in the earlier onset group. Regarding rs8040868, note that according to Hapmap LD data from the merged Phase I, II, and III CEU sample (Utah residents with ancestry from northern and western Europe) this SNP is highly correlated with rs16969968 (r2 = 0.802), which has been associated with severity of smoking and/or early onset of smoking and alcohol use in several studies (Saccone et al. 2009a, b; Weiss et al. 2008; Bierut et al. 2008; Sherva et al. 2009; Chen et al. 2009; Stevens et al. 2008). However, in our subsample from the Add Health data, the r2 value between these two SNPs is only 0.52, which may explain the lack of association with rs16969968.
