Folie à deux

Folie à deux, French for madness of two, is a rare psychiatric syndrome in which the symptoms of a delusional belief are transmitted from one individual to another. It is also called shared psychosis or shared delusional disorder, and Charles Lasègue and Jules Falret first conceptualized it in nineteenth century French psychiatry, which is why it is also known as Lasègue-Falret syndrome. Later classifications renamed it shared psychotic disorder in DSM-IV and induced delusional disorder in ICD-10, although the research literature still mostly uses the original name. The same syndrome shared by more than two people may be called folie à trois or folie à quatre, and further terms include folie en famille and folie à plusieurs. The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders does not list it as a separate entity and tells clinicians to classify cases as delusional disorder or as another specified schizophrenia spectrum and psychotic disorder.