500
Explain how a biological factor and a psychological factor come together to make it difficult for a person to stop having an obsessive thought.
As a psychological factor, people attempt to engage in thought suppression (i.e. to not think about things they find unpleasant). This interacts with biological processes in the brain, wherein one part of the brain tells another part to stop thinking about that thing, but this just reactivates the first part of the brain, which then re-sends the same message, which reactivates the first part again, and so on.
[What is this biological cycle called?]