Folie à deux

Folie à deux is a rare phenomenon in which two people share a delusion, often after one person’s belief influences the other. It is also called shared delusional disorder.

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Delusion: A fixed false belief that persists despite strong evidence against it. The shared belief at the center of folie à deux is a delusion.
Charles Lasègue: A nineteenth-century French physician known for work in psychiatry and neurology. He co-authored an early influential account of shared delusions.
Delusional disorder: A psychotic disorder characterized by one or more persistent delusions. Unlike folie à deux, it does not require another person to share the delusion.
Psychiatric diagnosis: The process of identifying a mental disorder using clinical information and diagnostic criteria. Assessment must consider each person’s symptoms and the relationship between them.
Nosology: The study and classification of diseases and disorders. Changing diagnostic categories leave questions about the best classification for shared delusions.
Social influence: The ways people’s thoughts, feelings, or behavior are shaped by other people. Influence can help explain how one person’s delusion becomes shared.
Jules Falret: A nineteenth-century French psychiatrist who wrote about shared delusions with Charles Lasègue. His collaboration helped establish the phenomenon as a subject of psychiatric study.
Psychosis: A state involving impaired contact with reality, often including delusions or hallucinations. Folie à deux is discussed within the broader clinical territory of psychosis.
Differential diagnosis: The process of distinguishing among conditions that could explain the same symptoms. Clinicians must distinguish shared delusions from other explanations for similar beliefs.
Causal inference: Methods for determining whether one factor causes changes in another. It is difficult to separate interpersonal influence from shared circumstances or prior vulnerability.