Background and Criteria
Shared psychotic disorder, or folie à deux, is a rare delusional disorder shared by 2 or, occasionally, more people with close emotional ties. An extensive review of the literature reveals cases of folie à trois, folie à quatre, folie à famille (all family members), and even a case involving a dog. 
In the most recent update to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), shared psychotic disorder was removed as a separate disease entity and was included in the section on other specified schizoprenic spectrum and other psychotic disorders. In the updated version the entity exists as “delusional symptoms in partner of individual with delusional disorder.”  According to the DSM-5, in the context of a relationship, the delusional material from the dominant partner provides content for delusional belief by the individual who may not otherwise entirely meet criteria for delusional disorder.
ICD-10 diagnostic criteria for Induced Delusional Disorder (folie à deux) is as follows:
- Two people share the same delusion or delusional system and support one another in this belief.
- They have an unusually close relationship.
- Temporal or contextual evidence exists that indicates the delusion was induced in the passive member by contact with the active partner.
Delalle et al presented a case of folie à deux in which paranoid delusions were shared by a mother and her 15-year-old son.  In this case, the mother was considered the dominant psychotic individual. The son was the passive recipient. The parent-child relationship, like spousal and sibling relationships, is very common in folie à deux. The son was treated with medication.
Soriano et al presented a case of folie à deux that occurred between 2 sisters.  As in the parent-child relationship, the occurrence of shared psychotic delusions among siblings is common due to the close ties. As the authors of this study discussed with regard to the sisters, the occurrence among siblings can be particularly attributed to shared past experience or expectations. The sisters shared delusions of persecution and prejudice toward the Chinese community of their hometown. The sisters were separated and given psychopharmacological treatment.
Peritogiannis et al investigated a case of folie à quatre, which not only presented the issues of close familial ties as a contributing factor, but also the complications of occurrence in rural areas.  The authors described a situation in which 4 adult cohabitating siblings were revealed to share delusions about their neighbors. None of the siblings were cooperative, and the authors suggested that mobile psychiatric units that worked with PCPs would be useful in diagnosing and treating the condition in rural areas.
Roth et al investigated an interesting case of folie à deux in a married couple, in which the delusions in the husband, the primary person, were caused by Huntington disease.  In an example of organic psychosis leading to shared psychotic disorder, the wife adopted her husband's delusions through folie impossée, a category of folie à deux that occurs when the secondary person is otherwise mentally sound. Her condition quickly improved upon spatial separation from her husband.
Many highly detailed explanations of folie à deux pathogenesis have been developed, yet none appear to explain all aspects of the syndrome. A survey of the literature shows that most of those afflicted with the disease are women with higher intelligence quotient scores who are usually younger than their significant other (eg, partner, parent, sibling, friend). The survey further suggests that the primary patients are susceptible to schizophrenia and often are diagnosed with episodes of paranoid delusions.
Quite often, factors arise because of unhealthy or interrupted ego development during the early stages of life. As Freud suggested with his theories on the Oedipus and Electra complexes, children develop attraction to the opposite-sex parent, developing a greater sense of self by comparing and resisting identification with their same-sex male or female parent, recognizing that each is similar to or different from themselves. If the relationship between parent and child is filled with jealousy, rejection, or anger, or if the relationship becomes more sexual than that of a healthy parent-child relationship, symptoms of folie à deux generally express themselves.
The adult-child identifies inappropriately with the opposite-sex parent and often perceives or has delusions of shared sexual intimacy with the parent. The person who has the disorder tends to form symbiotic relationships with a significant other who shares a common psychiatric disorder; often, they too are susceptible to unhealthy bonding, lowered self-esteem, and lack of personal responsibility that would otherwise foster healthy interdependence within intimate relations.
Studies of individual cases have shown that delusional ideas and psychotic symptoms are rarely transmitted to a healthy individual whose partner displays unhealthy behavior resulting from a psychotic disease; however, a passive person may have a genetic predisposition to psychosis and, as a result, may develop this disorder. 
Subtypes and Characteristics
Although Harvey described the first case of phantom pregnancy associated with induced psychosis in 2 sisters in 1651, the term folie à deux dates to a classic report by Lasègue and Falret in 1877. In 1942, Gralnick published a classification of 4 folie à deux subtypes.  These subtypes are as follows:
Subtype A is termed folie imposée. The delusions of a person with psychosis are transferred to a person who is mentally sound. Both persons are intimately associated, and the delusions of the recipient disappear after separation. The mental status exam of both affected individuals would be significant for delusional thinking, lack of judgment and insight, poor attention and concentration, and affect may or may not be affected. Both individuals would be perseverative and sometimes preoccupied with limited relatedness.
Subtype B is termed folie simultanée. The simultaneous appearance of an identical psychosis occurs in 2 individuals who are both intimately associated and morbidly predisposed. The mental status exam of the affected individuals would be consistent with paranoia, lack of insight, disorganized thought processes in extreme cases, and lack of relatedness.
Subtype C is termed folie communiquée. The recipient develops psychosis after a long period of resistance and maintains the symptoms even after separation. The mental status exam may be consistent with hypervigilance, obsessive thinking, brooding, rumination, anxiety, and lack of reasoning.
Subtype D is termed folie induite. New delusions are adopted by an individual with psychosis who is under the influence of another individual with psychosis. The mental status exam would be similar to one of a psychotic patient, namely, paranoia; lack of reasoning, judgment, and insight; and poor relatedness. Limited eye contact, bizarre mannerisms, and magical thinking may be apparent on assessment.
Since the times of Lasègue and Falret, shared psychotic disorder has been identified more frequently in women, reflecting the traditional submissive role of females in the family. Nevertheless, no confirmation of increased susceptibility of females exists today. Both female and male secondaries are equally affected by female primaries.
The involved individuals have an unusually close relationship and are isolated from others by language, culture, or geography. This explains the high number of reported twin cases (especially sister-sister pairs), in which biological and psychological factors are shared. Most of the reported relationships have been within the nuclear family.
Distribution of the relationships in Western countries differs from those indicated by Japanese data, in which mother-child and spousal combinations are the most common.  Also in Japan (contrary to Western countries), more cases of have occurred in which younger partners affect older partners rather than older partners affecting younger partners. Thus, the conclusion has been made that close association contributes more to the development of shared psychotic disorder than age.
Individuals with shared psychotic disorder lack insight and therefore do not seek treatment. Originally, induced delusions were thought to disappear in a person once he or she was separated from the person with genuine psychosis. New data gathered by analyzing published case reports show that separation from the primary is not sufficient. Most often, recovery of the secondary follows separation from the primary and the administration of antipsychotic medications. Interestingly, rare cases have been reported in which the secondary experienced hallucinations while the primary did not.
The dominating primary case is most commonly represented by persons with schizophrenia, delusions, or mood disorders. In Western countries, both the original delusions in the dominant person and the induced delusions in the submissive person are usually chronic and either persecutory or grandiose in nature. In Japan, acute psychotic reactions have been noted to be delusions of a religious nature.
In the literature, shared psychotic disorder has also been referred to as folie à deux, shared imposed psychosis, infectious insanity, shared paranoid disorder, and symbiotic psychosis. 
|Traditional Views||Recent Views|
|Sex of the submissive person||Female||Females and males are equally affected.|
|Age of the secondary||The secondary is younger than the primary.||The secondary has an equal chance of being either younger or older than the primary.|
(in descending order)
|Two sisters (most common), husband and wife, mother and child, 2 brothers, brother and sister, pairs of friends, whole families||Incidence in married or common-law couples is equal to that in siblings (incidence in sisters is more common than that in brothers, assuming cohabitation)|
|Hallucinations||The delusion is similar to that of the inducer.||
In 2 reported cases, the secondary experienced hallucinations while the primary did not.
Secondary hallucinatory experiences occur less frequently and are less intense than primary hallucinatory experiences.
|level of intelligence in the secondary||Lower than the primary||Not found|
|Treatment||Separation from the primary||Separation by itself is insufficient. Effective treatment of the secondary requires neuroleptics and separation from the primary.|
Treatment & Management
The new standard of treatment for shared psychotic disorders includes the use of 2 agents. The atypical newer neuroleptics are the accepted mode of treatment for the spectrum of these disorders. Newer-generation anticonvulsants are also highly effective. Aripiprazole (Abilify) and quetiapine (Seroquel) are extremely effective in these cases.
Initiation of aripiprazole at 5-10 mg PO qd with a titration upward by 5-10 mg PO every 3-5 days until a 25-60 mg PO qd dosage is achieved should thwart and alleviate the symptoms of the psychoses. Quetiapine is initiated at 25-50 mg PO bid and increased by 50 mg PO bid every 3 days until symptom resolution is achieved. Maintenance doses of 200-600 mg can be achieved easily.
The following case studies share many similarities and shed light on the seriousness of the disease; the complications associated with the disease; and the dire need for hospitalization, chemical therapies, and separation of the enmeshed pair as a primary form of treatment. These do follow the subtypes.
Case Report 1- Treating Subtype B
Potash and Brunell suggest multiple-conjoint psychotherapy for the treatment of folie à deux in identical twins.  In this case, twins were noted to have adapted roles resembling their abusive parents. Each twin took on the role of one parent, and, together, they reinvented their unhealthy past within their relationship. The twins developed a coping mechanism whereby they became inseparable until their hospitalization.
The mental status was consistent with alert and oriented individuals whose thought content was significant for paranoia and preoccupations. Reasoning, judgment and insight was lacking. Attention was diminished and concentration was limited. EC was limited. Passive suicidal ideation with no acute plan, as well as violent and homicidal thinking was undertoned. Mood was irritable and affect was labile.
The twins presented with visual paranoia and shared delusional experiences; general hostility; feelings of betrayal and being taken advantage of; signs of depression, suicidal ideation, inseparability, despair about the future, and work problems; refusal to discuss the past for fear of reinjury; obsession with sex and/or promiscuity; and supporting, believing, and assuming identification with each other's delusions.
Indications for hospitalization
Hospitalization was required in order to ensure that the twins would remain separate throughout their treatment, would remain in a safe and controlled environment, and would not follow through on their suicidal threats. The twins feared the delusions and the confusion, and the hospital provided them with the sense of security that they would not come into danger when they were in a "blacked-out" state. Previous efforts at separating the twins proved ineffective, until hospitalization. Once hospitalization was established, the twins were treated with a regimen of multiple-conjoint psychotherapy and the issue of the twins echoing each other's delusions, going into depression, and making suicidal threats was thwarted.
The treatment in the hospital consisted of multiple-conjoint therapy. Through this treatment process, the twins met with a male and female therapist and were given the opportunity to have transference of their angry and confused feelings onto the therapist who most closely represented the sex of the mother and father. Through the treatment process and the expression of otherwise dangerous emotions in a safe setting, each patient had the opportunity to discover that her anger cannot harm her dependent partner or others and that the hostility she feels no longer needs to be denied or deflected into a delusional system.
Through the separation experience, each twin learned that being an individual provides safety and that being alone does not have to make one feel lonely and separate. Through this process of breaking dependency, each learned that she had her own ideas, beliefs, and interests that could be shared without the other having to accept it as her own reality. Working with both a male and female therapist, each twin was able to resolve many of her intimacy issues related to the same- or other-sex parent. Each developed a sense of health, using this chance to resolve many of the childhood dependency issues through the validation of the therapists and by developing their own individual identities.
In some cases, the interaction with the therapist was the girls' first experience with being treated with sensitivity. At the end of each session, in order to encourage discussion of the twins' differing perspectives, the therapists would direct any delusional thoughts to be more specific and to be attached to the feelings they covered up. This supported positive feelings that were then exchanged between the therapists and the patients themselves.
As the therapist would move progressively from the delusions to reality and feelings, the need for the twins to echo each other's delusions gradually disappeared. This process enabled the twins to recognize some of their distortions and pathology as they replaced them with healthy behaviors. Usually, by the end of the meeting, the twins expressed their fear and anger more directly and formed more constructive avenues for addressing their real-life complexities. As the patients' confidence increased and as positive transference occurred, the twins' childhood and family issues emerged and were dealt with constructively. Consequently, each twin began dealing with her own problems and feelings. So far, the multiple-conjoint model is the most beneficial means of treatment for folie à deux.
Case Report 2- Treating Subtype D
Emde, Boyd, and Mayo resolve folie à deux by allowing the patient to be responsible for developing healthy activities and relations while hospitalized and receiving medication therapies.  They examined a case of folie à deux involving a mother and daughter who were both diagnosed as having schizophrenic thinking disorder and a shared delusion that the husband/father was trying to poison them.
The patients present alert and oriented. Relatedness and eye contact were fair. Speech was logical. Mood was irritable and affect was labile. TC was characterized by delusions, obsessive rumination, and hypervigilance. Psychomotor agitation is present. Attention and concentration are preserved. Reasoning and judgment are poor and insight is lacking. No evidence has presented for suicidal or homicidal ideation.
The mother and daughter both exhibited signs of distortions in their communication, emotional distancing from others, masking of feelings in conflict situations, deviant behavior, isolation, and shift of roles across generational and sexual lines, including incest and homosexuality. The daughter behaved in a sexually explicit manner, attracting the father and the attention of other men, while she maintained a delusion of him wanting to impregnate her. The mother was diagnosed with catatonic schizophrenia. Both mother and child slept, ate, and bathed together.
Indication for hospitalization
Removing the daughter from the home before overt incest could occur was imperative. The mother needed to be hospitalized so that she could stop worrying and trying to control her daughter's behavior. She was overprotective and fearful that her daughter would be harmed. The mother and daughter switched roles when the daughter reached puberty, and the father began to pursue her as if she were the adult in the relationship with him. The mother behaved like a child, and the daughter became her caretaker. The dominant personality could not be determined in the psychosis; the role of parent and child interchanged frequently between the two. The mother and daughter were inseparable, and hospitalization was necessary in order to approach each of their complex symptoms.
Through hospitalization, separation was gradually introduced for the mother and daughter in order to establish role separation within the family and promote the development of healthy appropriate boundaries. The therapy for each included electroshock therapy and insulin, which facilitated temporary gains. However, these therapies achieved substantial improvement when coupled with phenothiazine therapy.
After 3 weeks, the daughter's speech became coherent and she returned to a school setting within the hospital. However, the fact that the daughter was still protective of her mother indicated that the similarity of folie à deux still existed between them. By the second month of hospitalization, both mother and daughter showed improvement, and the role of reeducating them was the major feature of the therapeutic setting. The mother was increasingly engaged in social and recreational activities together with a work assignment, while the daughter was more involved with school and with friends her own age. After 3 months of hospitalization, symbiosis was no longer present; both had independent feelings and reactions.
The mother was discharged with an understanding of her illness and a desire to make a better marriage with her husband. The daughter remained in the hospital for further support in her schooling, in building peer relationships, and in developing a stronger sense of independence and a distinctly separate identity. In the sixth month, the daughter and her mother were reexamined with psychological testing. By that time, both of their mental ideations were healthy and their stories were bound in reality; both recognized their early symptomatology and took responsibility for not returning to the unhealthiness of their earlier experiences. The father/husband was given opportunity to spend time with the daughter, developing a healthy relationship. The mother's jealousy was addressed as she and her husband worked toward developing intimacy in their relationship, separate from the daughter.
Case Report 3- Treating Subtype A
Brenman suggests a therapy of reconstruction in early folie à deux.  In looking at early childhood experiences, one must consider the effects of the outer environment on the unconscious inner environment.
A 28-year-old woman sought help regarding a suicidal depression, feelings of helplessness, and total inability to work. She was aware of her irrational, hateful, and angry feelings and thought she had been provided misinformation in her primary relationships. For example, her career was not satisfying because she felt her parents had pressured her choice, although she changed her career several times. She was very bright but could not complete her education. She called off several engagements when the pressures of intimacy became intolerable. She exhibited anxiety and fear when working with the therapist, and she would not allow him to diagnose her, believing that her own assessment would be correct. She was in denial and became fixated on achieving a sense of justice through an overactive superego.
Indications for hospitalization
In this case, no indication for hospitalization was present. Rather, the therapist treated her with a regimen of analysis and he generally attempted to assist the patient in realizing that her perspective did not foster the healthy successful lifestyle she sought.
Over the course of a year, the therapist tried to help the patient give up her need to control his opinion of her. The patient did not allow the analyst to develop a real experience of her feelings. After a while, he felt compelled to abandon his views and admire hers, creating a relationship between them that resembled a dependent relationship often seen in folie à deux.
Through psychoanalysis and reconstruction, the therapist attempted to develop the relationship between them to be one in which honesty in expressing feelings could serve to relieve the pressure of her past grievances from negative relationships. The value of the reconstruction was intended to allow her to express the distortions she perceived, to begin to see the truth of the matter, and to develop a different relationship to the stimulus that caused her pain. This provided her with a foundation for constructing new experiences and relationships. The purpose of reconstructing the transference was to analyze what was wrong in the past in order to form new foundations. The patient recovered a year later and regained the experience of her own feelings.
Case Report 4- Treating Subtype C
Bankier recommends treating folie à deux with antipsychotic medication and psychotherapy.  He studied role reversal in folie à deux, in which a man with alcoholism was sexually engaged with prostitutes. When his wife discovered his activity, he developed delusions that "prostitutes were following him with cars," and successfully convinced his wife of his delusions.
Mental status is significant for alert and oriented mentation. Speech is logical and coherent. Attention and concentration are preserved. Mood is irritable to euthymic. Affect is labile. Reasoning and judgment are limited and insight is lacking. The patients are limited in relatedness, delusional, and obsessive. They present with psychomotor excitation. No evidence is present for suicidal or homicidal ideation.
The patients, a husband and wife team, were both socially active, employed in professional positions, and affiliated with a religious/church life. The husband was an active alcoholic, was engaged in sexual promiscuity with prostitutes, and was spending large amounts of money. He began having paranoid delusions quickly thereafter. He would see prostitutes following him in cars, watching him on the streets, making improper phone calls to his home, and stealing the little bit of money he had left.
He became anxious and convinced his wife that his delusions were true. His wife, having no previous mental illness, believed him, and together they watched for suspect prostitutes and reported their harassment to the local police. She became ill following her husband's developing paranoid psychosis, suggesting a folie à deux. When she became very ill, she developed the delusion that prostitutes were entering her house to embarrass and harm her. She planned for her husband to kidnap a police officer in exchange for ransom, and, later, he was arrested for attempting to blow up the building that housed the headquarters of their supposed persecutors.
Indication for hospitalization
The wife was not charged as a partner in crime, and she refused hospitalization. She attended outpatient therapy for several weeks and reluctantly took a small dose of neuroleptic medication. The husband made an excellent recovery from paranoid schizophrenic illness following a hospital stay.
Clearly, the case is one of folie à deux, with both the husband and wife having shared delusions. They were associated intimately, supported each other's pathology, and reversed roles during the illness. At first, the husband was the dominant partner. When the wife became more ill, she believed the delusions to be true and took on a more dominant role, while he became submissive and enabled her illness to progress. The problem reached severe proportions when the wife convinced her husband to blow up the headquarters of their perceived persecutors; they were reported to the police.
After her initial anger at her husband subsided during the course of his hospitalization, the separation between the two allowed the wife to cease her psychotic behavior and make significant progress in his absence. The husband recovered completely from a diagnosis of a typical paranoid schizophrenic illness with treatment consisting of neuroleptic medications, group therapy, and occupational therapy. However, the wife was suspected of not taking her medication and, when last seen, still had delusional beliefs.
Bankier, following the traditional literature, recommends physical separation, antipsychotic medication, and psychotherapy to treat folie à deux.  He is supportive of conjoint-psychotherapy as an alternative therapy, together with neuroleptic medications to handle the treatment. Each of these methods has been used to a certain degree of success. However, many psychiatrists today are of the opinion that multiple-conjoint therapy further helps patients to deal with feelings of rejection, inducer-induced dimensions, anger, dependency, hostility, and distorted communication and provides healthier contrast to distortions in perspective. In some cases, medications may have to be administered jointly with the therapy to accelerate the process of recovery and further ensure successful completion of the readily accepted protocol for recovery.
Patient and Family Education
Numerous online resources are available to patients and their families. Many websites provide information about the disorder as well as information on support programs and resources. Clinicians may wish to consider directing patients and their families to any of the following websites for additional information on shared psychotic disorder.
Cleveland Clinic, Shared Psychotic Disorder