Capgras Syndrome, also known as Capgras Delusion, is the irrational belief that a familiar person or place has been replaced with an exact duplicate — an imposter.

It is a disorder in which a person holds a delusion that a friend, spouse, parent, or other close family member (or pet) has been replaced by an identical-looking imposter. The Capgras delusion is classified as a delusional misidentification syndrome, a class of delusional beliefs that involves the misidentification of people, places, or objects (usually not in conjunction). It can occur in acute, transient, or chronic forms. Cases in which patients hold the belief that time has been "warped" or "substituted" have also been reported.

The delusion most commonly occurs in patients diagnosed with paranoid schizophrenia, but has also been seen in patients suffering from brain injury such as from an accident or stroke or from dementia.

Capgras syndrome is named after Joseph Capgras (1873–1950), a French psychiatrist who first described the disorder in 1923 in his paper co-authored by Jean Reboul-Lachaux, on the case of a French woman, "Madame M.," who complained that corresponding "doubles" had taken the places of her husband and other people she knew. Capgras and Reboul-Lachaux first called the syndrome "l’illusion des sosies", which can be translated literally as "the illusion of look-alikes.

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