Folie à deux (French for 'madness of two'), also called shared psychosis or shared delusional disorder (SDD), is a rare psychiatric syndrome in which symptoms of a delusional belief 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...