To provide a control for the possibility that the abstainer/nonabstainer and user/control differences observed at some of the clustered, reproducibly-positive SNPs were due to occult ethnic/racial differences in the frequencies of alleles at these same SNPs between abstainers and non-abstainers or between abusers and controls, we compared the present results with those that we have previously obtained from comparisons of allele frequency data in self-reported African-American vs European-American control individuals, focusing on SNPs that display ethnicity difference scores that lie in the outlying +/- 2.5% of all differences (Table 1).
