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On posts about mental health disorders, at the bottom of the post I'll refer to where it is specifically listed or generically referred to in the DSM-IV-TR, the manual used to diagnose mental disorders. 

Folie à Deux or Shared Psychotic Disorder is a delusion that develops in an individual who is in a close relationship with another person that already has a Psychotic Disorder with substantial delusions. The second person will then share some or all of the delusional beliefs of the first person. Clinicians have to exclude other Psychotic Disorders (i.e. Schizophrenia), any Mood Disorders (i.e. Depression), substance abuse or any medical condition (i.e. head injury) that may cause the shared delusions before diagnosing someone with Folie à Deux. It is extremely rare. 

1) Folie à Deux is French for "madness of two." It is pronounced (fuh lee a doo).  

2) The first person is referred to as the "inducer" or the primary case that "transmits" the delusional belief to the second person. The most common diagnosis for the primary case is Schizophrenia.  

3) Over time the views have changed. While the second person was often expected to be female, the more recent view is that it affects males and females equally. The second person was often thought to be younger but the inducer has an equal chance of being younger.  

4) If the relationship with the primary case is interrupted, the second person's delusional beliefs can diminish or disappear although separation alone is not sufficient. Medication as well as separation is used for treatment purposes. It is very dependent on the relational connection. Thus, it is often chronic because it occurs in long term relationships that are resistant to change. The incidence of Folie à Deux is more common in sisters than brothers, assuming that they live together, and more common in common-law/married couples than in siblings.  

5) There are 4 subtypes of Folie à Deux. a) Folie Imposée - delusions of a psychotic person is transferred to a person with a sound mind. b) Folie Simultanée - simultaneous appearance of psychosis in two intimately involved individuals. c) Folie Communiqée - the second person develops psychosis even after being separated from the inducer. d) Folie Induite - new delusions develop in the second person who is still under the influence of the inducer.  

6) I read a case where a healthy individual facilitated the inducing of delusional thoughts into a second person while not sharing those delusional beliefs themselves. In this case, the mother supported the daughter's delusions that she shared with her brother.  

7) Folie à Deux is also the title of an album from the rock band Fall Out Boy. There is a winery in Napa called Folie à Deux with Folie à Deux/Ménage à Trois wine sections on their site. The next time I am in California, I'd like to check them out.  

DSM-IV-TR - 297.3 Shared Psychotic Disorder (Folie à Deux) - under Schizophrenia and Other Psychotic Disorders  

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This post is for informational/educational purposes only and should not be used to self-diagnose. No diagnosis of an individual is implied by any post.