There is growing evidence that OCD represents abnormal functioning of brain circuitry, probably involving a part of the brain called the striatum. OCD is not caused by family problems or attitudes learned in childhood, such as an inordinate emphasis on cleanliness, or a belief that certain thoughts are dangerous or unacceptable. Brain imaging studies using a technique called positron emission tomography (PET) have compared people with and without OCD. Those with OCD have patterns of brain activity that differ from people with other mental illnesses or people with no mental illness at all. In addition, PET scans show that in patients with OCD, both behavioral therapy and medication produce changes in the striatum. This is graphic evidence that both psychotherapy and medication affect the brain.