Folie à deux
Folie à deux (‘madness for two’), also known as shared psychosis or shared delusional disorder (SDD), is a psychiatric syndrome 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’).
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).