In 1932, French psychiatrist Joseph Capgras and his intern Jean Reboul-Lachaux described Madame M., who insisted that her husband was actually an impostor who looked exactly like him. She did not see just one impostor husband, but at least 80 different ones over the course of ten years. In fact, doppelgangers replaced many of the people in Madame M.'s life, including her children, who she believed had been abducted and substituted with identical babies.

Who were these faux humans and where were they coming from? It turns out they were actually the individuals themselves — her husband, her children — but they didn't feel familiar to Madame M., even though she could recognize that they looked the same.

The Capgras Delusion

Madame M. had the Capgras Delusion, which is the belief that people, often loved ones, are not who they appear to be. Instead, people who experience the Capgras Delusion believe that these people have been substituted by doppelgangers or even robots and aliens who have crept into the flesh of unwitting humans. The delusion can also extend to animals and objects. For example, someone with Capgras Delusion might believe that their favorite hammer has been replaced by an exact duplicate.

These beliefs can be incredibly unsettling. Madame M. believed that her true husband had been murdered, and filed divorce from her "replacement" husband. Alan Davies lost all affection for his wife, calling her "Christine Two" to differentiate her from his "real" wife, "Christine One." But not all responses to the Capgras Delusion are negative. Another unnamed individual, though bewildered by the appearance of who he felt were a fake wife and children, never appeared agitated or angry toward them.

Causes of the Capgras Delusion

The Capgras Delusion can arise in many settings. For example, in someone with schizophrenia, Alzheimer's, or another cognitive disorder, the Capgras Delusion may be one of several symptoms. It can also develop in someone who sustains brain damage, like from a stroke or carbon monoxide poisoning. The delusion itself can be temporary or permanent.

Based on studies involving individuals with very specific brain lesions, the main brain areas thought to be involved in Capgras Delusion are the inferotemporal cortex, which aids in facial recognition, and the limbic system, which is responsible for emotions and memory.

There are several explanations for what might happen on a cognitive level.

One theory says that to identify your mom as your mom, your brain must not only (1) recognize your mom, but (2) have an unconscious, emotional response, like a feeling of familiarity, when you see her. This unconscious response confirms to your brain that, yes, this is your mom and not just someone who looks like her. The Capgras syndrome occurs when these two functions both still work but can no longer "link up," so that when you see your mom, you don't get that extra confirmation of her feeling familiar. And without that feeling of familiarity, you end up thinking she's an impostor even though you may still recognize other things in your life.

One issue with this hypothesis: people with the Capgras Delusion usually believe that only certain people in their lives are doppelgängers, not everyone else. It's unclear why the Capgras Delusion would select some people, but not others.

Another theory suggests that the Capgras Delusion is a "memory management" issue. Researchers cite this example: Think of the brain as a computer, and your memories as files. When you meet a new person, you create a new file. Any interaction you have had with that person from that point forward will be stored in that file, so that when you meet someone you already know, you access that file and recognize them. Someone with Capgras Delusion, on the other hand, may create new files instead of accessing the old ones, so that, depending on the person, Christine becomes Christine One and Christine Two, or your one husband becomes husband 80.

Treating the Capgras Delusion

Since scientists aren't quite sure what causes Capgras Delusion, there isn't a prescribed treatment. If the Capgras Delusion is one of multiple symptoms resulting from a particular disorder like schizophrenia or Alzheimer's, common treatments for those disorders, like antipsychotics for schizophrenia or medications that help boost memory for Alzheimer's, may help. In the case of brain lesions, the brain could eventually reestablish the connections between emotion and recognition.

One of the most effective treatments, however, is a positive, welcoming environment where you enter into the world of the individual with Capgras Delusion. Ask yourself what it must be like to be suddenly thrown into a world where your loved ones are impostors, and reinforce, not correct, what they already know. As with many plotlines for science fiction movies, the world becomes a much scarier place when you don't know if someone is actually who they appear to be, and you need to stick together to stay safe.