Folie à deux (folly of two, or madness [shared] by two), also known as shared psychosis or shared delusional disorder (SDD), is a collection of rare psychiatric syndromes in which symptoms of a delusional belief, and sometimes hallucinations, are transmitted from one individual to another. The same syndrome shared by more than two people may be called folie à... trois (three) or quatre (four); and further, folie en famille (family madness) or even folie à plusieurs (madness of several). The disorder, first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jules Falret, and 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. This disorder is not in the current, fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), which considers the criteria to be insufficient or inadequate. DSM-5 does not consider Shared Psychotic Disorder (Folie à Deux) as a separate entity; rather, the physician should classify it as "Delusional Disorder" or in the "Other Specified Schizophrenia Spectrum and Other Psychotic Disorder". Signs and symptoms
This syndrome is most commonly diagnosed when the two or more individuals of concern live in proximity, may be socially or physically isolated, and have little interaction with other people.
For example, unsubstantiated or unverifiable claims of being followed by the FBI in unmarked cars and watched via security cameras would be classified as a non-bizarre delusion; while it would be unlikely for the average person to experience such a predicament, it is possible, and therefore understood by those around them. Mood-congruent delusions
These correspond to a persons emotions within a given timeframe, especially during an episode of mania or depression. For example, someone with this type of delusion may believe with certainty that they will win $1 million at the casino on a specific night, despite lacking any way to see the future or influence the probability of such an event. Similarly, someone in a depressive state may feel certain that their mother will get hit by lightning the next day, again in spite of having no means of predicting or controlling future events. Mood-neutral delusions
These are unaffected by mood, and can be bizarre or non-bizarre; the formal definition provided by Mental Health Daily is "a false belief that isnt directly related to the persons emotional state." An example would be a person who is convinced that somebody has switched bodies with their neighbor, the belief persisting irrespective of changes in emotional status. Biopsychosocial effects
As with many psychiatric disorders, shared delusional disorder can negatively impact the psychological and social aspects of a persons wellbeing. Unresolved stress resulting from a delusional disorder will eventually contribute to or increase the risk of other negative health outcomes, such as cardiovascular disease, diabetes, obesity, immunological problems, and others.