Also called induced psychosis, folie à deux is a delusional disorder shared by two or more people who are closely related emotionally. One has real psychosis while the symptoms of psychosis are induced in the other or others due to close attachment to the one with psychosis. Separation usually results in symptomatic improvement in the one who is not psychotic.

Folie communiquée, folie imposée, folie induite, and folie simultanée are the four subtypes of folie à deux.