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Imagine, if you will, that one by one your friends and family⁠— the people closest to you⁠— are being removed and replaced with exact duplicates. Although they are identical in appearance and manner, you are certain that these people are not your loved ones. They are impostors. While most people would become deeply paranoid in such a scenario, there are some individuals who experience such things every day without fear… and just wonder, “why?” Such is the life of people stricken with Capgras’ Syndrome.

A person with Capgras’ Syndrome suffers from the delusion that one or more of their close friends or family members have been replaced with exact duplicates, and they cannot be shaken from this belief in spite of an otherwise clean bill of mental health. In some instances, the person believes that they themselves are, in whole or in part, a duplicate. Unlike the paranoia expected from such a condition, there is never a motive assigned for the appearance of the duplicates – the patients do not believe someone is “out to get them,” but they are at a loss for an explanation why anyone would want to replace their loved ones.

This odd misperception is named after the French psychiatrist Jean Marie Joseph Capgras, who described the case of a Madame M. in 1923. The woman insisted that identical-looking persons had taken the place of her family. Over time her delusion expanded to include neighbors, friends and acquaintances. But Madame M. never bothered to get to know these impostors because it was her belief that each one regularly left to make room for the next double. In all, she eventually claimed to have had more than eighty husbands.

People suffering from Capgras’ Syndrome can sometimes even doubt their own identity after seeing their reflection in a mirror. One man pinched himself on the arm after seeing his reflection at the doctor’s office, and wondered aloud whether he and the man in the reflection were the same person. There was also a woman who flew into a jealous rage every time she caught sight of her own reflection, believing this “other woman” was trying to lure her husband away from her. Her husband eventually covered every reflective surface in the house in an effort to keep her from hurting herself. Oddly enough, she had no problem recognizing herself in the mirror of her makeup compact, but anything larger resulted in an assault on the imaginary impostor. Her doctor tried a novel solution: he gathered a number of mirrors of varying sizes, and had the woman view herself in each one. He started with the smallest and gradually moved to the next larger as soon as she recognized herself. Ultimately she was able to see herself in a full-length mirror, and she was cured from then on.

In some instances, individuals with the Capgras delusion see duplicate objects rather than duplicate people. One doctor reported a patient who believed that his poodle had been replaced with an identical dog, and another reported a patient who believed that during the night his running shoes and many other personal possessions were being replaced.

Capgras’ delusion always centers around just one of the subject’s senses. The most common is the sense of sight; for example, one person readily recognized his wife on the phone when speaking to her, yet when she arrived in the flesh he thought the impostor was actually his sister-in-law. Blind people have also been diagnosed with the disorder, and they believe that the voices of certain loved ones are actually coming from duplicates.

While the causes of Capgras’ syndrome are not specifically known, there is no shortage of theories. It has been shown that many people with the syndrome have brain lesions in the right temporal lobe from traumatic injuries, epilepsy, and other causes, yet there are also significant numbers of patients with no such damage in evidence. Also, there is a somewhat higher incidence of schizophrenia among people with Capgras, and in New Zealand there is a markedly higher incidence of the disorder among the Maori people than in the general population.

Some earlier researchers attempted to draw connections to Prosopagnosia, a condition which prevents some people from being able to recognize faces. By measuring a person’s galvanic skin response⁠— the amount of electrical resistance in the skin⁠— scientists can detect when an individual is experiencing emotions. Patients with Prosopagnosia show an emotional response to familiar faces, though they exhibit no conscious recognition. With a Capgras patient there is no such reaction. Though no emotional connection is present when shown a picture of their father, the patient will remark on the striking resemblance. This test also rules out mental illness as a definitive cause, since the emotional center of the brain would subconsciously react even with impaired perceptions.

Another proposed cause involves some form of damage or impairment in two lobes of the brain: One site of damage affecting the emotional connections with respect to people’s faces, and the other affecting the brain’s consistency-checking abilities.

In at least one case, doctors have successfully cured Capgras’ Syndrome by suspending a prescription of diazepam, yet in other cases symptoms have disappeared after administering anti-psychotic medication. To date, no single treatment has been found to be consistently effective, and so far there is no single theory that can explain all the reported cases of Capgras’ Syndrome. We do know, however, that the human mind uses many interlocking cognitive tricks to fill in the gaps of our observations, essentially building a simulation which allows us to interact with our world and society. When just one or two of those links go awry, the true complexity of that simulation is revealed by the fascinating problems that arise.