The heterogeneity of OCD should always be considered in the light of psychiatric comorbidity, an approach that is facilitated in samples such as EGOS and NORDiC that are linked to national health registries. For example, in EGOS, using an epidemiological frame, approximately 40% of individuals with OCD have more than one psychiatric comorbidity, with anxiety disorders and major depressive disorder being most common (43). In addition, the severity of OCD was significantly higher in individuals with at least one additional psychiatric comorbidity compared to individuals with no psychiatric comorbidity: higher symptoms of obsessing and ordering, measured using the OCI-R, were observed in individuals with OCD and at least one additional psychiatric comorbidity (43). In future studies, it will be important to investigate how the combination of rare and common genetic variants differ in their relationship with the comorbid conditions.
