In stage 1, we conducted a mega-analysis combining genome-wide assocation study (GWAS) data from 17 separate studies (with a total of 9,394 cases and 12,462 controls; Table 1 and Supplementary Tables 1,2). We imputed allelic dosages for 1,252,901 autosomal SNPs (Table 1, Supplementary Table 3 and Supplementary Note) using HapMap3 as the reference panel1. We tested for association using logistic regression of imputed dosages with sample identifiers and three principal components as covariates to minimize inflation in significance testing caused by population stratification. The quantile-quantile plot (Supplementary Fig. 1) deviated from the null distribution with a population stratification inflation factor of λ = 1.23. However, λ1000, a metric that standardizes the degree of inflation by sample size, was only 1.02, similar to that observed in other GWAS meta-analyses2,3. This deviation persisted despite comprehensive quality control and inclusion of up to 20 principal components (Supplementary Fig. 1). Thus, we interpret this deviation as indicative of a large number of weakly associated SNPs consistent with polygenic inheritance4. We also examined 298 ancestry-informative markers (AIMs) that reflect European-ancestry population substructure5. Unadjusted analyses showed
