Although the recently-published genome-wide association studies of OCD (Mattheisen et al. 2015; Stewart et al. 2013; International Obsessive Compulsive Disorder Foundation Genetics Collaborative (IOCDF-GC) and OCD Collaborative Genetics Association Studies (OCGAS) 2017) found no genome-wide significant associations, these studies demonstrated that common variants account for a significant proportion of OCD heritability (24–32%) (Davis et al. 2013; International Obsessive Compulsive Disorder Foundation Genetics Collaborative (IOCDF-GC) and OCD Collaborative Genetics Association Studies (OCGAS) 2017). They also indicate that the strongest associated variants in OCD genome-wide association study (GWAS) are enriched for expression quantitative trait loci (eQTLs) and methylation QTLs derived from frontal lobe, cerebellum, and parietal lobe tissue (Stewart et al. 2013), demonstrating that biologically meaningful associations exist within the top ranked SNPs and that increasing sample sizes will likely identify significant common variant associations for OCD risk. In addition to increasing sample size, another approach to improve power for GWAS is to reduce genetic heterogeneity. For example, if sex significantly modifies the effect of genetic variation on the risk of OCD, then combining males and females with OCD may weaken or
