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Folie à deux ('folly of two', or 'madness [shared] by two'), referred to as Lasègue–Falret syndrome, additionally known as shared psychosis or shared delusional disorder (SDD), is a collection of rare psychiatric syndromes in which symptoms of a delusional belief, and sometimes hallucinations, are transmitted from one individual to another. The same syndrome shared by more than two people may be called folie à trois ('three') or quatre ('four'); and further, folie en famille ('family madness') or even folie à plusieurs ('madness of several'). The disorder, first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jules Falret, and is also known as Lasègue–Falret syndrome. Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-4 – 297.3) and induced delusional disorder (ICD-10 – F24), although the research literature largely uses the original name. This disorder is not in the current, fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), which considers the criteria to be insufficient or inadequate. DSM-5 does not consider Shared Psychotic Disorder (Folie à Deux) as a separate entity; rather, the physician should classify it as "Delusional Disorder" or in the "Other Specified Schizophrenia Spectrum and Other Psychotic Disorder". From Wikipedia
