For the GWAS, the imputed gene dosage data were analyzed in PLINK.27 Covariates included: cohort (a four-category variable based on year of birth that was included to control for potential cohort effects: 1896–1929; 1930–1949; 1950–1969; and after 1970), age at assessment, sex and the first four principal components derived from a principal component-based analysis performed in PLINK on the measured genotypic data to cluster the samples along with HapMap reference samples (CEU, YRI, CHB and JPT). Results were filtered to include only SNPs with minor allele frequency >0.01 that also met an information criterion of >0.80. In total, 7 287 851 SNPs met these criteria.
