Because allele frequencies in the candidate genes typically investigated in G×E studies often differ across ethnicities, an ethnicity-by-environment interaction is a particularly plausible alternative explanation for G×E findings from ethnically heterogeneous samples. Of the 47 studies, 26 used an ethnically homogeneous sample, 10 used an ethnically heterogeneous sample, and 11 did not provide information about ethnicity. Most but not all of those studies that failed to provide information about the ethnic compositions of their samples were conducted in Europe and presumably used ethnically homogeneous samples. Thus, stratification is a possible alternative explanation for around one-fifth of these G×E results.
