Folie à deux (FAD) was first described in 19th century France. Since then, the concept has been elaborated, and several subtypes of FAD have been successively reported in France. In contrast, studies in German-speaking psychiatry mainly focused on the conceptual boundary between reactive/endogenous psychosis and etiological hypothesis (ie, psychogenesis vs genetic predisposition). In North America, Gralnick wrote a seminal review and redefined four subtypes of FAD by adopting the European classical concepts. More recently, "shared psychotic disorder" in DSM or "induced delusional disorder" in ICD-10 was branched off from FAD. However, several classical subcategories of FAD were not included in these recent definitions, the nosological significance of which should not be underestimated. We examined demographic data of FAD case reports published from the 19th to the 21st century and found that some of the earlier hypotheses, such as females being more susceptible, older and more intelligent individuals being more likely to be inducers, and sister-sister pairs being the most common relationship, were not supported. The controversial issue of the etiology of FAD-association of subjects or genetically driven psychosis-was re-examined in light of recent studies.

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