Folie à deux — French for "madness of two" — is a rare psychiatric syndrome in which a delusional belief is shared between two or more closely associated people. It is formally classified in ICD-11 as Shared Psychotic Disorder. In clinical literature and practice, the original French term remains in common use.
Folie à deux represents a striking clinical phenomenon: a psychiatric delusion — ordinarily a breakdown in individual reality-testing — spreading from one person to another through the vehicle of a close and often isolated relationship.
The classic presentation involves two individuals — most commonly intimate partners, parent and child, or siblings — in which one person (the primary case) holds a well-established delusional belief system, and the other (the secondary case) has come to share those beliefs through prolonged close contact.
The primary case typically has a primary psychotic disorder — most commonly delusional disorder, schizophrenia, or psychotic depression. The secondary case is usually a person without a primary psychotic disorder whose adoption of the belief is driven by the relational dynamics of the shared environment.
The shared delusional content is typically persecutory — beliefs about being spied on, poisoned, persecuted, or conspired against. Less commonly, religious, grandiose, or somatic themes are shared.
The defining feature that distinguishes folie à deux from coincident psychotic illness in two individuals is that separation typically leads to resolution of the shared belief in the secondary case.
Folie à deux most commonly occurs within close-knit, socially isolated dyads — couples who live without significant outside social contact, parent-child relationships characterised by extreme enmeshment, or siblings in isolated living situations.
Social isolation is a critical factor. The absence of external reality-testing — input from other people, normal social contact — creates the conditions in which an encapsulated shared belief system can develop and be reinforced.
Other associated factors include a significant power differential in the relationship; emotional dependency; shared language or cultural background; and in some cases intellectual disability or reduced critical thinking in the secondary case.
Beyond the classic dyad, larger presentations — folie à trois, folie à famille, and in extreme cases larger group delusional systems — have been documented in the clinical and forensic literature.
Folie à deux has significance beyond its interest as a psychiatric curiosity. In some cases, the shared delusional system drives both individuals toward isolating or harmful behaviour with significant consequences.
The forensic dimension — where the shared delusion has motivated harmful actions — is an area where expert psychiatric evaluation is sometimes requested by legal or child protection authorities.
Where children are involved as secondary cases — absorbing delusional beliefs from a parent or caregiver — the clinical and safeguarding implications require urgent attention. Children in this situation may be at risk of significant developmental harm from prolonged exposure to a delusional family environment.
Diagnosis requires identification of the primary case and their primary psychiatric disorder; assessment of the secondary case and the nature and origin of their shared beliefs; evaluation of the relational dynamics that facilitated belief transmission; and assessment of how the shared belief is affecting the behaviour and functioning of both individuals.
Assessment is typically prompted by a concerned third party — a family member, GP, social worker, or school — rather than by the affected individuals themselves, who are convinced of the reality of their shared belief.
At IsraClinic, the clinical team can provide comprehensive assessment of folie à deux presentations and, where required, contribute expert clinical documentation for legal, child protection, or other institutional processes.
The primary treatment intervention is separation of the two individuals. In the classic presentation, separation alone leads to significant improvement or full resolution of shared belief in the secondary case — as they regain access to external reality-testing.
The primary case requires treatment for their underlying psychiatric disorder — pharmacotherapy for psychosis, depression, or other primary conditions, following the standard treatment approach for the relevant diagnosis. Digital prescriptions are issued through the Yarpa system.
Following separation and stabilisation, psychotherapeutic work may address the relational dynamics, the degree of enmeshment or dependency, and — particularly in the secondary case — the processing of having held delusional beliefs.
Folie à deux is almost never self-presented. It is typically identified by a concerned outsider — a family member who notices two people sharing unusual fixed beliefs, a school observing a child adopting a parent's delusional framework, or a GP or social worker concerned about an isolated couple or family.
If you are concerned that two people in a close relationship are sharing beliefs that appear delusional, fixed, and resistant to outside input — specialist psychiatric assessment is appropriate.
IsraClinic accepts patients for in-person consultation in Tel Aviv and online, in English, Russian and Hebrew. No referral is required.
Clinical Reviewer: Dr. Mark Zevin, MD — Senior Psychiatrist | IsraClinic | Last reviewed: 2026
Folie à deux is rarely self-identified. If you are concerned that two people close to you are sharing beliefs that seem disconnected from reality — specialist assessment is available in English, Russian and Hebrew.