Folie à deux (‘madness for two’), also known as shared psychosis[2] or shared delusional disorder (SDD), is a psychiatric syndrome in which symptoms of a delusional belief, and sometimes hallucinations,[3][4] are transmitted from one individual to another.[5] 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 was first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jean-Pierre Falret and is also known as Lasègue-Falret syndrome.[3][6]
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 DSM (DSM-5).
