The present study had strengths and limitations. We used OCD cases from the EGOS and NORDiC cohorts, the two largest OCD studies in Sweden to examine the role of genetic and environmental factors. The EGOS cohort utilized the NPR for its sampling frame, thus it is an epidemiological cohort minimizing selection biases, while the NORDiC recruited through specialty OCD clinics across Sweden, a sampling frame more typical of case-control studies. This difference in sampling frames could introduce heterogeneity into our study. Nonetheless, when we evaluated this possibility by estimating the heritability induced by contrasting OCD cases from EGOS to OCD cases from NORDiC, and doing the same for controls, both estimated heritabilities were not significantly different from zero. Hence, while there could be subtle heterogeneity between the cohorts, it must be small. Furthermore, for both cohorts, reliance on inclusion as a result of individuals seeking care at mental health hospitals/clinics can inadvertently exclude those with milder forms of the disorder who may seek treatment from primary care providers and/or those who do not present to clinical services at all. If
