We supplemented our autosomal analyses with association analyses of SNPs on the X chromosome. We first conducted separate association analyses of males (N = 152,608) and females (N = 176,750) in the UK Biobank. We found a male-female genetic correlation close to unity. We also found nearly identical SNP heritability estimates for men and women, which is consistent with partial dosage compensation (i.e., on average the per-allele effect sizes are smaller in women) and implies that any contribution of common variants on the X chromosome to sex differences in the normal-range variance of cognitive phenotypes18 is quantitatively negligible.
