The response to this question may depend on who is answering it. A therapist who specializes in Cognitive Behavioral Therapy (CBT) may say that OCD is a result of the belief that an intrusive thought, impulse, or image and the resulting anxiety that is generated need to be neutralized, or else the thought will somehow come true, or a dire consequence will occur. The relief that occurs after performing the compulsion feels very good to the person, but at the same time it is the beginning of a pattern of behavior that will increase over time, now that the person has learned a way to rid him or herself of the obsession, he or she will be more likely to repeat the behavior the next time the obsessive thought occurs.
A psychiatrist may say that a person with OCD is experiencing a chemical imbalance. This is based on previous findings that have shown that certain medications, such as selective serotonin reuptake inhibitors (SSRIs), lead to an increase of serotonin in the brain and a decrease in OCD behaviors for some patients. Therefore, with the right combination of medications, psychiatrists believe OCD will decrease over time as the chemicals become balanced.
A neuroimaging researcher may say that OCD is the result of abnormalities in the physical structure of the brain. Neuroimages of the brain (such as PET scans, MRIs, etc.) show what areas are working, or “firing,” while a person is focusing on an obsession or performing a compulsion. These images are compared with images of the brain functions of people who do not have OCD and who are thinking the same thoughts or performing the same behaviors. A neuroimager is looking for differences between these two groups in how the brain functions, and then they form hypotheses about how disorders develop based on the differences they find.
It is probable that OCD develops because of a combination of all of these factors. More research is needed to say for sure how each one of these areas interact with and influence the others.