One of the mechanisms by which sex differences in OCD could arise is through genetic risk deriving from the sex chromosomes. We observed no significant associations on the X chromosome in either the combined or sex-stratified analyses. A QQ-plot indicated that there was no significant SDEs on the X chromosome (Supplementary Figure 5). Using REML, we estimated the X chromosome (1.6% of total SNPs) heritability as h2X=0.010 (se=0.005, p=0.006), which comprised 3.8% of total OCD heritability, and was consistent with expectation (Supplementary Figure 7). When analyzed in each sex separately, X chromosome heritability was not statistically different between females (h2FX=0.014, se=0.008, p=0.027) and males (h2MX=0.028, se=0.013, p=0.010) at 2.5% OCD prevalence. Results were again robust to estimates derived using a range of OCD prevalence (Supplementary Table 3).
