A relative of reduplicative paramnesia, it is the delusional belief that your loved ones have been replaced by look alike impostors. The victims are otherwise completely coherent and lucid. The delusion was first described in 1923 by the French psychiatrist Joseph Capgras.
The illness is linked to damage to the part of the brain that connects vision to emotion. The victims can recognize the features of the person they know, and they feel an emotional link to that person, but the image doesn’t stir that emotion because of the damage. Ergo, they assume that this person must be an impostor.
Interestingly, because there is no damage to the part of the brain that links hearing to emotion, if they hear the voice of the loved one, they recognize it immediately.
In one of the most famous cases, a British man named Alan Davies suffered the delusion after a car accident and became convinced that his wife had been killed in the accident (the women living with him was an impostor). He successfully sued the other driver for the same damages as if his wife had been killed because the anguish for him was the same.
The video above is the first half of a TED talk by neurologist Vilayanur Ramachandran, during which he discusses the delusion, as well as phantom limb syndrome and synesthesia. It is certainly worth watching the whole thing.
Rivka Galchen, a very talented writer, wrote the novel Atmospheric Disturbances, which is a compelling psychological tale about a man who either suffers from Capgras or is truly being deceived. She tipped me off to the Alan Davies story.