Reported rates of comorbid disorders with OCD have varied widely across published studies over the past 4 decades, even within similar socio-cultural backgrounds. For example, studies from the United States (USA) report varying lifetime prevalence rates of comorbid major depressive disorder (MDD) between 19% (15) and 66.8% (16) among adults with a primary diagnosis of OCD. Similar variations are observed in lifetime prevalence rates of other comorbid illnesses including anxiety disorders [22% (17)−56.3% (16)], tic disorders [8.7% (18)−31.6% (19)] and psychotic disorders [2.9% (20)−14.4% (21)]. Beyond variations related to chance and differences in study design, sampling strategy and measurement error, valid socio-demographic influences could plausibly underlie these observed variations. For example, differences in the reported comorbidity rate for MDD in OCD study populations of Klein et al. (15) and Mancebo et al. (16) may stem from differences in the mean age (34.4 vs. 40.1 years) and gender distribution (males 52.7 and 45%) within their samples. In comparing comorbid anxiety disorder rates, studies by Deacon et al. (17) and Mancebo et al. (16) differ with respect to age (35.8 vs. 40.1
