The key features of OCD are the existence of both obsessions and compulsions. Most of us do experience obsessions and compulsions from time to time. We all know people who set themselves extremely high standards. These people often have the obsessive traits of high achievement, meticulousness, punctuality, extreme neatness or cleanliness. Indeed, traits such as striving for perfection and meticulous hygiene can be seen to have a survival advantage. This advantage may help to keep these traits in the gene pool. In OCD, however, these tendencies have become an end in themselves, with the person feeling compelled to perform compulsions even to the detriment of everything else in their life. For example, whereas general hygiene in the kitchen can be seen to be an excellent thing, in the case of a person with OCD this may have got so out of hand that every waking hour is spent cleaning the kitchen. A woman with OCD and contamination fears had financial difficulties owing to her inability to do any paid work and also her excessive spending on cleaning materials. Both she and her young son were undernourished as too much time was spent cleaning to the detriment of cooking any food.

In DSM-5 (American Psychiatric Association, 2013), OCD has been removed from the category of anxiety disorders and instead is in a separate category of obsessive–compulsive and related disorders. As well as OCD, this category includes body dysmorphic disorders (previously categorised as a somatoform disorder), hoarding disorder (new diagnosis), trichotillomania (new diagnosis), skin-picking (new diagnosis) as well as OCD symptoms arising from substance misuse and medication and other organic disorders.