Your father comes in the door and you go to greet him. Suddenly, you stop. Why, this man isn't your father at all. He looks like your father, but no, you decide, that definitely isn't him. He's been replaced by an impostor!
Capgras syndrome is what's known as a delusional misidentification. It's the opposite of déjà vu. People with Capgras syndrome think that their spouse, family members or even their pets have been replaced with doubles. Imagine how disconcerting it would be to have someone who looks like a loved one sit down with you and know intimate details about your life, even though you're sure that this person is a trickster.
Capgras syndrome used to be considered very rare, but medical professionals are beginning to think that perhaps it isn't so rare after all. The more doctors that know about it, the more people they find who have it. Capgras was first described by two French doctors, Joseph Capgras, for whom the syndrome is named, and Jean Reboul-Lachaux. Their patient, Madame M., was convinced that her family and neighbors had all been replaced by lookalikes. She said she'd had 80 husbands -- one imposter would simply leave to make room for a new one.
Capgras syndrome isn't the same thing as face blindness, or prosopagnosia. People with prosopagnosia can see a face for the hundredth time and still not know who it is. You can walk right by your best friend and not recognize her even when she says hello (For a more in-depth look at face blindness, check out "Can face blindness explain why that person at work never says hi to me?"). People with prosopagnosia, however, show changes in their skin conductance when shown a picture of someone they know. Part of their brain recognizes this person emotionally, even if consciously they don't know who it is.
People with Capgras syndrome can perceive faces, and recognize that they look familiar, but they don't connect that face with the actual feeling of familiarity. That woman looks like your wife, but you don't feel that she really is your wife. You don't have the feelings you should have when you look at this person with your wife's face. Their skin conductance stays the same as it would if they were looking at a total stranger. It's a problem of disconnection. So what's going on?