Folie à deux is a psychiatric syndrome in which symptoms of a delusional belief are projected from one individual to another. The same syndrome shared by more than two people may be called folie à troisfolie à quatrefolie en famille or even folie à plusieurs (“madness of many”). Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-IV) (297.3) and induced delusional disorder (F.24) in the ICD-10, although the research literature largely uses the original name. The disorder was first conceptualized in 19th century French psychiatry.

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 emotional contagion, which could be made independently in either case.

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 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 his or her 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 either the situation where two people considered to suffer independently from psychosis influence the content of each other’s delusions so they become identical or strikingly similar, or one in which two people “morbidly predisposed” to delusional psychosis mutually trigger symptoms in each other.
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