# Shared Psychotic Disorder (Folie à Deux) – History and Classification Shared delusional disorder, historically called *folie à deux* (French “madness of two”), was first described by Lasègue and Falret in 1877. In the late 19th and early 20th centuries clinicians described subtypes (folie **imposée**, **simultanée**, **communiquée**, **induite**) reflecting whether one person imposes a belief on another or two develop delusions together. In ICD‑10 it was listed as **“Induced Delusional Disorder”** (F24), and DSM‑IV used the term **Shared Psychotic Disorder**. Modern classifications no longer give it a standalone code: **DSM‑5‑TR** subsumes it under *Other Specified Schizophrenia Spectrum and Other Psychotic Disorder* (e.g. “delusional symptoms in partner of individual with delusional disorder”), and **ICD‑11** treats it as a rare form of delusional disorder (6A24). The ICD‑11 text explicitly notes that “rarely, delusional disorder may occur at the same time (or closely associated in time) in two people… referred to as ‘shared or induced delusional disorder’ or ‘folie-à-deux’,” with remission of the second person’s delusions upon separation. In other words, the historical concept lives on in a descriptive form, not as an independent diagnosis. # Current Diagnostic Criteria and Dynamics In practice, a **shared psychosis** involves a “primary” person with a psychotic disorder (usually a delusional disorder or schizophrenia) who has fixed false beliefs, and a “secondary” person who gradually adopts similar beliefs. The secondary typically has little or no mental illness of their own at onset. In DSM‑5‑TR, identifying the *delusional symptoms in the partner* is an example of “other specified” psychotic disorder. Merck’s manual notes that shared psychosis *“usually occurs in a person or group… who are related to a person with a significant delusional disorder or schizophrenia.”* Importantly, it points out that the primary is usually the socially **dominant** partner, who “imposes the delusion” on the submissive secondary. Clinicians must carefully determine chronology: typically the primary developed the delusion first, and the secondary later acquiesced. If the dyad is separated and the secondary’s delusions abate (as often happens), this confirms the induced nature. However, clinicians also recognize that roles can sometimes blur or even reverse, especially in long-standing or complex relationships. # Risk Factors and Relationship Contexts Certain relational and psychosocial factors predispose to shared delusions. Almost all reported cases involve **close, longstanding relationships** – most commonly spouses or intimate partners, siblings (especially twins), parent-child pairs, or other family members. Proximity and isolation are key: couples who live in **social isolation** (cut off from outside information or alternative viewpoints) are at greatest risk. In such situations, the primary’s interpretation of events goes unchallenged, and any ambiguous stimuli are explained to fit the delusion. Chronic stress or shared trauma (e.g. bereavement, illness) can also create a vulnerable psychological climate. Personality and power dynamics matter: typical secondaries often have a **dependent, passive, or anxious personality**, while the inducer is more dominant or forceful. The smaller person may initially agree just to keep the peace, and over time actually comes to *believe* the delusion. In families, a single dominant individual can even draw multiple relatives into a *folie à famille*. Shared delusions are not limited to dyads. There are reports of “folie à trois” or more, including in modern settings where the link is digital rather than physical. For example, a recent case series described three young men who interacted daily in an online gaming guild for years and developed a common persecutory belief that they were under “digital surveillance” by enemies. In that scenario the indi​vidual who first harbored the paranoia gradually influenced the others through constant online contact. This shows that **“virtual cohabitation”** (intense online association) can suffice to transmit delusional ideas when people are isolated from counterinformation. Regardless of setting, the relationship usually involves dependency: the secondary often becomes emotionally or psychologically reliant on the primary, and the pair functions as a closed system. # Distinguishing from Shared Beliefs or Conspiracy Thinking Not all shared beliefs are psychopathological. **Cultural or political beliefs** held by many are not folie à deux. What marks a shared psychosis is idiosyncrasy and irrationality of the belief. For example, a couple sharing a mainstream religious or political conviction (even a fringe one) is *not* automatically pathologic. By contrast, folie à deux involves highly **implausible, personally-held delusions** that collapse under logical scrutiny. Forensic experts emphasize this distinction: if two people share an unusual belief that is **not widely endorsed** and causes significant distress or dysfunction, it raises concern. In practice, psychiatrists often ask: “Would these beliefs be common among others in their culture or community, or are they isolated to this duo?”. This issue has been highlighted in cases of “targeted individuals” and “gang stalking” beliefs. In some internet communities, people claim *corporate or government surveillance, harassment or mind-control*. As one review notes, members of such groups allege covert multi-entity conspiracies (e.g. spy agencies using directed-energy weapons, implanted devices, AI hacking). These narratives are shared and amplified online. **However, courts and clinicians typically regard these shared conspiracies as delusional** when presented in isolation – because they are evidence-free and fantastical. In one analysis, for example, targeted-individuals’ claims were uniformly seen as “irrational and likely delusional” by judges. By contrast, folie à deux is a **clinical diagnosis** for two people with such a fixed delusion *only within their closed relationship*, not part of a broader movement. In short, shared psychosis is suspected when the belief is singular to the pair and resistant to all reality testing, unlike conspiracy beliefs which often have wider social channels and less absolute rigidity. # Clinical Assessment Assessment of a suspected shared psychosis requires a careful, multi-angle approach. Ideally, the clinician **interviews each person separately**, so that neither influences the other’s account. One must establish who developed the belief first and whether the secondary ever expressed doubt or had alternative explanations. Collateral information is crucial: doctors seek family members, friends, or any records to confirm the timeline. StatPearls advises obtaining “collateral history about both partners from a third person,” since often only the pair are available during evaluation. The clinician then assesses each person’s full mental state. If the secondary has truly become delusional, they will endorse the same false belief with equal conviction; otherwise they may be more ambivalent when asked alone. Psychometric tools and objective records (e.g. diaries, communications) can help reconstruct events. Throughout, one must rule out other causes: for example, ensure the primary does not have a schizotypal personality or the secondary a neurocognitive disorder that might explain the shared belief. A key diagnostic clue is **response to separation**: if the induced person’s delusion fades when apart from the primary, this strongly supports a shared delusional diagnosis. If delusions persist in both despite separation and treatment, a revision of diagnosis is warranted. # Ethical and Practical Considerations Managing shared psychosis raises ethical challenges. One common intervention is **separation** of the dyad, which often precipitates remission in the secondary. However, forcibly isolating two adults can infringe on their autonomy and bonding. Clinicians must balance beneficence (breaking the cycle of delusion) against respect for the individuals’ relationship. In practice, separation is pursued when the secondary is willing or when there is urgent risk (e.g. threats or self-harm). Hospitalization may be needed if either person is a danger to self or others, but involuntary commitment is reserved for those meeting legal criteria. Guardianship or conservatorship is **rarely used** solely for shared delusions; instead, mental health treatment and social support are preferred. Family therapy or psychoeducation can sometimes be offered to the wider group to rebuild communication and reality-testing. Other ethical issues include medication consent: the primary (and secondary, if ill) often **lack insight** and refuse treatment. Thus clinicians must work gently to build trust. When the dyad does enter treatment, the focus is on symptom reduction. Boards and ethics bodies generally support antipsychotic treatment for the primary patient, even involuntarily if necessary, and supportive care for the secondary, since non-treatment risks harm. Throughout, clinicians should clarify that shared delusions are **not evidence of truth**, but manifestations of illness, without affirming any unverified claim of real surveillance or poisoning. # Treatment Strategies Treatment of a shared psychotic disorder involves two tracks. First, the *primary patient* is treated as having a standard psychotic disorder: antipsychotic medication (often second-generation agents) is indicated. Psychosocial interventions (therapy, support groups) are also offered to address insight and coping. If the primary is hospitalized or isolated, this removes the reinforcing partner’s influence. Merck’s manual notes that **psychotherapy** (individual or joint) can help in shared cases, but that the primary will usually need pharmacotherapy. Second, the *secondary* (induced) person is supported. If their delusion resolves with separation, they may need only brief intervention: reassurance, counseling, and education about psychosis. If any residual belief remains, short-term antipsychotic treatment or reality-oriented therapy may be used. Family therapy can address underlying relational issues (dependency, communication patterns). Follow-up evaluation is important: in some cases, a secondary who becomes fully independent and departs from the delusional system remains well, but in others they may unmask a separate psychotic or mood disorder requiring longer treatment. Overall, prognosis is better when both partners receive appropriate care: studies suggest that prompt treatment and, when possible, **voluntary separation** lead to rapid improvement in the secondary. # Case Vignette (Fictional) Maria (age 45) and her husband Alejandro (age 48) have lived rurally and rarely socialized outside their marriage. Over six months, Alejandro became convinced that a foreign intelligence agency was secretly monitoring their home: he believed microphones were hidden in their walls and that their mobile phones were being tapped. He reported hearing faint ticking sounds as “signals” and insisted he was being poisoned by chemicals in the water. At first Maria laughed this off, but as Alejandro’s fear grew, she began to monitor the water and check the house daily for bugs. Their relationship became tense; Alejandro grew fearful of leaving the house and even accused Maria of conspiring with the agents. Maria, who has a passive personality, found herself agreeing with Alejandro’s suspicions to calm him, even though deep down she doubted them. Concerned neighbors convinced Alejandro to see a psychiatrist. In separate interviews, the doctor learned that Alejandro’s delusions began after he lost his job five years ago and he had become withdrawn. Maria, by contrast, admitted she had been skeptical at first but gradually “got used” to the idea that someone was watching them. Neither had any history of psychosis individually. The psychiatrist also interviewed their adult daughter (who reported that Alejandro repeatedly talked about “the spies” and that Maria seemed anxious and always tried to reassure him). Lab tests and brain scans were normal. The doctor diagnosed Alejandro with Delusional Disorder (persecutory type) and noted Maria’s matching delusions likely arose from his influence. The couple was advised to separate temporarily: Alejandro was admitted to a hospital unit (voluntarily) where he continued antipsychotic medication. Maria went to stay with her sister for two weeks. Within days, Maria’s fears rapidly subsided – she realized the only “proof” had been Alejandro’s coaxing. Alejandro’s delusions remained fixed, but he was calmer on medication. Over the next month, Alejandro learned to question his paranoid thoughts in therapy. Maria also attended counseling to process her anxiety and improve critical thinking. Eventually they reunited and attended couples therapy, setting ground rules for checking any future fears against reality (for example, having neutral third parties verify). Neither received a sensational “cure” from conspiracy theories; rather, Alejandro’s long-standing delusions were managed medically, and Maria’s induced beliefs receded under support. Both were helped by separating delusional symptomatology from their love and daily life, and by regaining trust through structured care. **Sources:** Authoritative psychiatric and forensic psychiatry texts and case reports on shared psychosis have been used throughout, including DSM‑5‑TR and ICD‑11 descriptions, StatPearls and Merck clinical reviews, and relevant literature on induced delusional disorder. These sources emphasize the rarity of folie à deux, the importance of distinguishing it from broader conspiracy beliefs, and the central role of isolation, dominance dynamics, and thorough assessment in diagnosis and treatment.