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.
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.
When a large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession, and are instead labelled as mass hysteria.
As with most psychological disorders, the extent and type of delusion varies, but the non-dominant person's delusional symptoms usually resemble those of the inducer.
[10] Prior to therapeutic interventions, the inducer typically does not realize that they are causing harm, but instead believe they are helping the second person to become aware of vital or otherwise notable information.
Psychology Today magazine defines delusions as "fixed beliefs that do not change, even when a person is presented with conflicting evidence.
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.
[15] While there is no exact cause of shared psychosis, there are several factors that are contributors depending on different cultures and communities and taking into consideration the individual's circumstances, including their environmental changes and relationships.
Shared delusional disorder is diagnosed using the DSM-5, and according to this, the patient must meet three criteria:[10] Reports have stated that a phenomenon similar to folie à deux was induced by the military incapacitating agent BZ in the late 1960s.
[19][20] 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.