No one knows what drives people with obsessive-compulsive disorder to do what they do, even when they’re aware that they shouldn’t do it, and when it interferes with normal life. That lack of understanding means about half can’t find effective treatment. But a new analysis of brain scans from hundreds of people with OCD, and people without it, may help. Larger than previous studies, it pinpoints brain areas and processes linked to OCD’s repetitive behaviors.