Folie a deux

# Overview
Folie a 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. The same syndrome shared by more than two people may be called folie a trois, folie a quatre, folie a famille or even folie a plusieurs (madness of many). Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-IV) (297.3) and induced delusional disorder (folie a deux) (F.24) in the ICD-10, although the research literature largely uses the original name.

# Presentation
This case study is taken from Enoch and Ball's 'Uncommon Psychiatric Syndromes' (2001, p181):
This syndrome is most commonly diagnosed when the two or more individuals concerned live in proximity and may be socially or physically isolated and have little interaction with other people.
Various sub-classifications of folie a deux have been proposed to describe how the delusional belief comes to be held by more than one person.
- Folie imposee 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 simultanee 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 a 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 a... diagnostic category and becomes legitimate because of the number of people holding it. When 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 are labelled instead as mass hysteria.
Being defined as a rare pathological manifestation, folie a deux is rarely found in general psychology or social psychology text books, and is relatively unknown outside abnormal psychology, psychiatry and psychopathology.

## Related phenomena
There have been reports that a similar phenomenon to folie a deux had been induced by the military incapacitating agent BZ in the late 60s, and most recently again by anthropologists in the South American rainforest consuming the hallucinogen ayahuasca (Metzner, 1999).

# In the media
- (1994) Heavenly Creatures is a film directed by Peter Jackson and starring Kate Winslet and Melanie Lynskey.  It was set in New Zealand and inspired by a true story where two teenage girls develop a relationship so strong and peculiar that they believe the only way to stay together was to kill one of the girls's mother. These girls were thought to have folie a deux.
- (1998) Folie a deux was the title of an episode from season 5 of The X-Files, aired in 1998, where Agent Mulder shares the belief with a telemarketer that employees of the telemarketing firm are monsters.
- (2006) 'Folie a Deux' is the title of a short film written and directed by Devin Anderson. The film was shot in 2006 and is currently in post-production.
- (2006) Folie a deux was referenced and defined in an episode from season 2 of Criminal Minds entitled "The Perfect Storm", which aired October 4, 2006 on CBS, in which a pair of serial killers kidnapped, tortured, and murdered several young women; in this episode, the primary, or dominant, perpetrator was a woman.
- (2007) The film Bug portrays a folie a deux involving a man and woman who believe they are infested with government-implanted, nano-technological insects.