Several secondary analyses support the high genetic correlation, suggesting that there is little or no difference in the ADHD results for male and female individuals. First, no genome-wide significant heterogeneity is observed when meta-analyzing the sex-specific GWAS results (Supplemental Figure S3). Second, a GWAS of sex-by-genotype interactions for ADHD identifies no variants with differential effects by sex, nor does it show any deviation from the null distribution of test statistics genome-wide (Supplemental Figure S4). Similarly, GWAS results for ADHD in the combined sample with or without including sex as a covariate are nearly perfectly correlated, with a low standard error (rg = .97, SE = .007). Narrowing the focus to only ADHD cases also finds no genome-wide significant differences between male and female cases (Supplemental Figure S5). Although some genome-wide inflation is observed for this final analysis in the iPSYCH sample, it is not replicated in the PGC data and appears to be attributable to one locus driven by a single low-frequency genotyped SNP (minor allele frequency = 0.02). Investigation of this locus shows no support for differences between male and female cases in neighboring genotyped SNPs, suggesting that the signal is likely a technical artifact (Supplemental Figure S6).
