Folie a Deux

Folie a deux, French for madness of two and also known as shared psychotic disorder or shared delusional disorder, is a rare psychiatric phenomenon in which the symptoms of a delusional belief appear to be transmitted from one person to another within a close relationship. Nineteenth-century French psychiatrists Charles Lasegue and Jules Falret first conceptualized the condition, which is sometimes called Lasegue-Falret syndrome after them. Clinicians have described four variants of the pattern: folie imposee, in which a dominant person imposes a delusion onto a second, more passive person; folie simultanee, in which two people develop the same delusion together through mutual influence; folie communiquee, resembling the imposed form but where the second person eventually develops additional delusional content of their own; and folie induite, in which one already psychotic person's existing delusions are intensified by a second person. Diagnostic manuals have treated the condition inconsistently over time: the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders classified it as shared psychotic disorder, the International Classification of Diseases classified a similar condition as induced delusional disorder, and the fifth edition of the Diagnostic and Statistical Manual removed it as a distinct diagnosis altogether, directing clinicians instead toward general delusional disorder or another specified psychotic disorder category. Documented cases associated with the pattern include the 1933 case of the Papin sisters in France and, in the psychological literature discussing shared delusion more broadly, subsequent cases examined for evidence of shared psychosis between closely bonded individuals.