Folie à deux (literally "a madness shared by two") is a rare psychiatric syndrome in which a symptom of psychosis (particularly a paranoid or delusional belief) is transmitted from one individual to another. (Related to the issue of shared delusions, there have also been occasional claims of shared visual hallucinations that are near-to-exact duplicates.) The same syndrome shared by more than one person may be called folie à trois, folie à quatre, folie à famille or even folie à plusieurs (madness of many). Recent psychiatric classifications refer to the syndrome as induced delusional disorder (DSM-IV) or shared psychotic disorder (ICD-10).

This case study is taken from Enoch and Ball's 'Uncommon Psychiatric Syndromes' (2001, p181):
Margaret and her husband Michael, both aged 34 years, were discovered to be suffering from folie à deux when they were both found to be sharing similar persecutory delusions. They believed that certain persons were entering their house, spreading dust and fluff and "wearing down their shoes". Both had, in addition, other symptoms supporting a diagnosis of paranoid psychosis, which could be made independently in either case.

This syndrome is most commonly diagnosed when the two or more individuals concerned live in close proximity and may be socially or physically isolated and have little interaction with other people.

Various sub-classification of folie à deux have been proposed to describe how the delusional belief comes to be held by more than one person.
Folie imposée is where a dominant person (known as the 'primary', 'inducer' or 'principal') initially forms a delusional belief during a psychotic episode and imposes it on another person or persons (known as the 'secondary', 'acceptor' or 'associate') with the assumption that the secondary person might not have become deluded if left to their own devices. If the parties are admitted to hospital separately then the delusions in the person with the induced beliefs usually resolve without the need of medication.
Folie simultanée describes the situation where two people, considered to independently suffer from psychosis, influence the content of each other's delusions so they become identical or strikingly similar.

Folie à deux and its more populous cousins are in many ways a psychiatric curiosity. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the person's culture or subculture" (see entry for delusion). It is not clear at what point a belief considered to be delusional escapes from the folie à... diagnostic category and becomes exempt because of the number of people holding it. While a large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession and may be labelled as mass hysteria.

Being defined as a rare pathological manifestation, folie à deux is rarely found in general psychology or social psychology text books, and is relatively unknown outside abnormal psychology, psychiatry and psychopathology.