Folie à deux (French for "folly of two")

also known as shared psychosis or shared delusional disorder (SDD), is a rare psychiatric syndrome in which symptoms of a delusional belief, and sometimes hallucinations, are "transmitted" from one individual to another.

The disorder, first conceptualized in 19th century French psychiatry by Charles Lasègue and Jules Falret, 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. 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').