Prevalence
Shared delusional disorder is most commonly found in women with slightly above-average IQs, who are isolated from their family, and who are in relationships with a dominant person who has delusions. The majority of secondary cases (people who develop the shared delusion) also meet the criteria for Dependent Personality Disorder, which is characterized by a pervasive fear that leads them to need constant reassurance, support, and guidance. Additionally, 55% of secondary cases had a relative with a psychological disorder that included delusions and, as a result, the secondary cases are usually susceptible to mental illness. Treatment
After a person has been diagnosed, the next step is to determine the proper course of treatment. The first step is to separate the formerly healthy person from the inducer, and see if the delusion goes away or lessens over time. If this is not enough to stop the delusions, there are two possible courses of action: medication or therapy. Therapy can be provided as both personal therapy and/or family therapy. With treatment, the delusions, and therefore the disease, will eventually lessen so much so, that it will practically disappear in most cases. However, if left untreated, it can become chronic and lead to anxiety, depression, aggressive behavior, and further social isolation. Unfortunately, there are not many statistics about the prognosis of shared delusional disorder, as it is a rare disease, and it is expected that the majority of cases go unreported; however, with treatment, the prognosis is very good.
Various sub-classifications of folie à deux have been proposed to describe how the delusional belief comes to be held by more than one person:
Folie imposée
Where a dominant person (known as the primary, inducer, or principal) initially forms a delusional belief during a psychotic episode and imposes it on another person or persons (the secondary, acceptor, or associate) with the assumption that the secondary person might not have become deluded if left to his or her own devices. If the parties are admitted to hospital separately, then the delusions in the person with the induced beliefs usually resolve without the need of medication. Folie simultanée
Either the situation where two people considered to independently experience psychosis influence the content of each others delusions so they become identical or strikingly similar, or one in which two people "morbidly predisposed" to delusional psychosis mutually trigger symptoms in each other.Folie à deux and its more populous derivatives are psychiatric curiosities. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the persons culture or subculture." It is not clear at what point a belief considered to be delusional escapes from the folie à... diagnostic category and becomes legitimate because of the number of people holding it.