OCD is a global phenomenon, with a substantial similarity across cultures in symptom clusters, gender distribution, age of onset, and comorbidities . Using the restricted DSM-5 definition (i.e., not including OCD-related disorders), the lifetime prevalence of OCD in the general population was calculated at 2–3% in several studies [2, 3]. Those percentages have been confirmed across cultures  and are supported the biological basis of OCD. Although the cultural, economic, and social factors play a role in the presentation of OCD (that is, the content of obsessions and the shape of the compulsions), its prevalence is equally distributed across the globe.
Although several approaches have been used to evaluate the role of heredity in OCD, twin and family studies remain the most common, as they allow comparisons between concordant monozygotic twins and discordant monozygotic twins with OCD, as well as familial aggregation. Based on their literature review, Van Grootheest and colleagues estimated that OCD is inherited in 27% to 47%  while childhood-onset OCD has an even greater genetic component (45–65%) [6,7,8]. The role of genetics in OCD has also been investigated by establishing OCD prevalence in family members of OCD patients. Numerous family studies have demonstrated the familial genetic contribution of OCD incidence. While the frequency of OCD and subclinical OCD differed within families across studies, the overall conclusion was that OCD and subclinical OCD are familial [9,10,11,12,13,14].
In accordance, relatives of OCD patients are twice as likely to develop OCD than those of healthy subjects, while rates in relatives of children and adolescents with OCD showed a tenfold increase relative to controls . Furthermore, based on their review and meta-analysis of the genetic epidemiology of OCD, Hettema et al. reported an aggregate odds ratio of 4, supporting the familial aggregation of OCD . In other studies, OCD prevalence among relatives of affected individuals was significantly higher than either the estimated population prevalence or rate among controls [17,18,19,20]. Significantly higher percentage of OCD cases was also reported by Grabe and colleagues in relatives of affected individuals based on both clinical and a general population study .
Tourette syndrome (TS) is a childhood-onset, neuropsychiatric disorder defined by multiple waxing and waning motor and phonic tics . A related diagnosis, chronic tic disorder (CT), is characterized by persistent motor or phonic tics. Community surveys, conducted in various countries over the past twenty years provide estimates of prevalence for TS ranging from 0.5 and 38 cases per 1000 children . Tourette syndrome/CT and OCD overlap in their phenomenological features, often cluster in families, and co-occur in affected individuals. About 30% of OCD patients have comorbid lifetime tic Disorders , and about 20% of TS/CT patients will suffer from OCD as well .
In DSM-5, OCD was separated from the anxiety disorders and was categorized within a new diagnostic category (denoted as ‘Obsessive–compulsive and related disorders’ or OCRDs), which includes, in addition to OCD, hoarding disorder, body dysmorphic disorder, and body-focused repetitive behaviors such as trichotillomania (hair pulling), onychophagia (nail biting), and excoriation (skin picking) .
Considering this spectrum-oriented view of OCRDs , its cumulative prevalence is estimated to be higher than that of OCD and can reach up to 9.5% . The diversity of disorders subsumed under this new OCRDs category warrants an epidemiological study that takes these conceptual changes into account.