To ascertain what motivates patients with obsessive-compulsive disorder (OCD) to perform compulsions.

Fifty-nine OCD patients underwent a comprehensive assessment, which included a structured interview designed to elicit one or more functions of their compulsions. Each patient was interviewed about the maximum of 3 compulsions.

The functions of 138 compulsions were identified, and compulsions were classified in accordance with OCD symptom subtypes as washing/cleaning (n=35), checking (n=33), mental/covert (n=23), ordering/symmetry (n=17), hoarding (n=10), and miscellaneous (n=20). Compulsions were most frequently performed to decrease distress or anxiety (n=87, 63%) or automatically, without patients thinking why they were doing it (n=87, 63%). However, the reasons for performing compulsions varied significantly, depending on the OCD subtype. In comparison with patients from other subtypes, those with checking compulsions were more likely to perform them because they believed something bad would happen if they failed to do so, patients with washing/cleaning compulsions were more likely to perform them to alleviate the feeling of disgust, and patients with mental and ordering/symmetry compulsions were more likely to perform them to achieve a “just right” feeling.

Identifying functions of compulsions improves understanding of the psychopathology of OCD and has important treatment implications. Cognitive-behaviour treatment approaches differ in accordance with the reasons for performing compulsions: exposure and response prevention tends to achieve better results when compulsions are driven by a need to decrease distress, anxiety or the feeling of disgust, whereas cognitive techniques may be more useful when compulsions are performed for other reasons.