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Traditionally, scientists have not completely understood the cause of obsessive-compulsive disorder (OCD). There has been disagreement about whether the disorder is primarily biological in nature, or whether it is a result of learned behavior. Others have argued that OCD is a blend of these two factors.

However, growing evidence suggests that chemical imbalances may play a major role in the development of OCD. Recent studies have indicated that low levels of the neurotransmitter serotonin (a chemical that helps nerve cells communicate in the brain) may contribute to OCD. Many patients with OCD who take medications that boost serotonin levels show improvement in symptoms. In addition, people who have brain injuries sometimes develop OCD, which further suggests that a physical problem in the brain can cause the disorder.

OCD also appears to increase metabolism in the basal ganglia (region of specialized nerve cells) and frontal lobes (the upper brain area) of the brain. This may cause the repetitive movements, rigid thinking and lack of spontaneity in OCD. Finally, people with OCD have been shown to have higher than normal levels of the hormone vasopressin, which raises blood pressure and reduces excretion of urine.

Heredity is also believed to play a role in the development of the disorder. Children with close family members (e.g., parents) with a history of OCD also have a higher risk for developing the condition themselves.

Onset of OCD appears earlier in boys than in girls, and incidence levels first peak in puberty (between ages 10 and 12 years) and then again in a person’s 20s and 30s. Although scientists have not been able to trace any particular genes to OCD, people with close family members with a history of OCD have a slightly higher risk for developing the condition.