The genotypes of 2 149 individuals were imputed at over 2 million SNPs using the CEU HapMap samples and the IMPUTE program [40]. We retained for analysis the 1 705 237 genotyped and imputed SNPs with, on average over individuals, probability ≥0.9 assigned to their most likely genotype. These were analysed using a standard linear model implemented in R, with SNP coded as the expected minor allele count. In addition to PC1, age and sex were included as covariates in each model, and the interactions of these terms with the SNP were also tested. The age term used for the linear model was the age at the time of psychological assessment, not that at recruitment.
