We followed a stringent quality control (QC) protocol that was designed to minimize occurrence of false positive signals, which included checking the relatedness of samples (i.e. verifying the relationship reported from the participating clinical centers) and reported sex (based on deviations from expected heterozygosity rates based on x-chromosomal markers in the analysis; PLINK standard parameters were used). In addition, evidence for genotyping errors at the sample / marker level was evaluated by searching for an excess of “Mendelian inconsistencies” in the data (as indicated by substantial deviation from the empirical distributions in both measurements). Multidimensional scaling (MDS) analyses were performed on singleton OCD cases and unselected controls, as implemented into Plink 26. Samples were removed when they significantly deviated in the first two MDS dimensions (> 4 standard deviation from the mean). Although inherently robust against population stratification, substantial heterogeneity in the population-based study cohorts would have lead to decreased power for our analytical approach (see below). More details on the QC process are provided in the supplementary materials and methods section. Use of the described filters resulted in a
