Obsessive-compulsive disorder (OCD) is more common than was once thought. Most people who develop it show symptoms by age 30.

There are several theories about the cause of OCD, but none have been confirmed. Some reports have linked OCD to head injury and infections. Several studies have shown that there are brain abnormalities in patients with OCD, but more research is needed.

About 20% of people with OCD have tics, which suggests the condition may be related to Tourette syndrome. However, this link is not clear.