The phrase folie à deux — French for "madness of two" — sounds like something out of a nineteenth-century novel. In fact, it describes a real and recognisable clinical syndrome, first formally written up by the French psychiatrists Lasègue and Falret in 1877. It is now usually called shared psychotic disorder in the older DSM-IV, or induced delusional disorder in ICD-10, and is grouped with "other specified schizophrenia spectrum and other psychotic disorder" in DSM-5-TR. Whatever the label, the clinical picture is striking: one person develops a delusion, and someone close to them — a spouse, sibling, parent, or child — comes to share the same belief.
Shared psychotic disorder occurs when a person without primary psychotic illness comes to share the delusional beliefs of someone they live closely with — often resolving when the two are separated.
How it presents
The classic pattern involves two people in close, often isolated, contact. The first ("primary" or inducer) has a primary psychotic disorder — often schizophrenia or delusional disorder. The second ("secondary" or induced) is usually less dominant in the relationship and gradually adopts the delusion, sometimes elaborating on it. Themes are most often persecutory but can be grandiose, religious, or somatic.
Variants described in the literature include:
- Folie imposée — the secondary adopts the primary's delusion; symptoms typically resolve when the two are separated.
- Folie simultanée — both develop the same psychosis at the same time.
- Folie communiquée — the secondary develops their own primary psychotic illness over time.
- Folie induite — a person with their own psychosis takes on additional delusions from another patient.
Who is affected?
Most reported cases involve people living together with little outside contact. Sisters, mothers and daughters, and married couples are the most common pairs in the case literature. Risk factors that recur across reviews (Arnone et al., Acta Psychiatrica Scandinavica, 2006) include:
- Long-standing close relationship
- Social isolation from outside influences
- Significant power imbalance — one partner dominant, the other submissive or dependent
- Shared stressful life events
- Pre-existing personality vulnerability in the secondary (often dependent or schizoid traits)
- Cognitive impairment in the secondary
It can also occur in larger groups (folie à trois, folie à plusieurs), sometimes within entire families or small communities.
How common is it?
True shared psychotic disorder is rare. Most published estimates rely on case series rather than large epidemiological studies, but it is generally considered uncommon enough that any clinician encountering it is likely to remember the case for years. It probably occurs more often than it is diagnosed — partly because the pair often presents together with the same beliefs, and the secondary may not be recognised as separately ill.
Treatment
Step one: separate, gently
The single most consistent finding in the literature is that physically separating the inducer from the induced often leads to substantial improvement in the secondary, sometimes within days to weeks. This is not always practical — it may mean a hospital admission or temporary alternative housing — and it must be handled carefully, as both individuals may resist.
Step two: treat the primary
The inducer needs the standard treatment for their underlying disorder — usually antipsychotic medication and psychosocial support. Read our guide to finding the right medication.
Step three: support the secondary
The induced person may not need long-term antipsychotic medication if the delusion resolves with separation. Many benefit from supportive therapy, social reconnection, and treatment for any underlying anxiety, depression, or personality vulnerability.
The shared delusions involve plans to harm self or others, refusal of essential medical care (especially for children), or extreme isolation from the outside world. These situations sometimes require involuntary care or, when children are at risk, child-protection involvement.
What to do if you recognise this in your family
If you suspect a relative is sharing a loved one's delusion, the most useful step is to involve a mental health professional who can see them separately. Confronting the beliefs head-on rarely works and usually entrenches them further. Maintaining contact, even over the phone, with the induced person can support gradual reality-testing. NAMI's Family-to-Family programme is a useful starting point for relatives.
Why the diagnosis still matters
Modern manuals downplay the standalone diagnosis, but recognising the dynamic still matters because it changes treatment. A person who looks psychotic on assessment may simply be the secondary in a folie à deux, and their recovery may depend more on changing their environment than on a long course of medication.
This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.