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.