Shared psychotic disorder (folie à deux) is a rare disorder characterized by sharing a delusion among two or more people in a close relationship. The inducer (primary) who has a psychotic disorder with delusions influences another nonpsychotic individual or more (induced, secondary) based on a delusional belief. It is commonly seen among two individuals, but in rare cases, can include larger groups. For example, it can occur in a family and is called folie à famille.
Jules Baillarger was the first to report this condition in 1860. During the 19th century, psychiatrists in Europe suggested different names. In France, it has been called “folie communiquee“(communicated psychosis) by Baillarger. In German psychiatry, it was named “Induziertes Irresein” by Lehman and Sharfetter. In 1877 Lasegue and Falret coined the term “folie à deux.” The French word “folie à deux” means madness shared by two. In the early 1940s, Gralnick, in his review of 103 cases of folie à deux, described four types of this disorder. He defined it as a psychiatric entity characterized by the transfer of delusions from one person to one or several others who have a close association with the primarily affected person. The four types are as follows: