There are a few theories as to how Capgras syndrome works, some of them likelier than others. The psychoanalytical view is that perhaps the syndrome is a result of an Oedipus or Electra complex -- sexual desire for one parent, sexual jealousy for the other. People with the syndrome must be trying to resolve their guilt about these circumstances by identifying their parent as a parental look-alike. Some psychodynamic theories propose that Capgras syndrome has to do with repressed feelings. The psychodynamic approach, however, has been mostly discredited.
Many researchers think that Capgras syndrome is actually the result of something wrong with the brain, an organic cause. They look for lesions on the brain, signs of atrophy and other cerebral dysfunction. Although Capgras syndrome is usually seen in people who have psychotic disorders, more than a third of Capgras patients have signs of head trauma [source: Hirstein and Ramachandran]. Many Capgras patients also have other organic conditions, like epilepsy or Alzheimer's.
Still more doctors and researchers combine the idea of both a physical and a cognitive cause. There's something wrong with the brain, but why and how is Capgras syndrome occurring because of it? Maybe the organic cause leads to feelings of disconnectedness that lead to Capgras syndrome. Maybe it's too tough for people with a brain lesion to update memories when they see a person and the person looks slightly different. Your body is having a strange experience and your brain scrambles for a way to explain it.
Somewhere, the brain isn't communicating when it should be. This breakdown of communication might be happening between the part of the brain that processes the visual information for faces and the part that controls the limbic system's emotional response.
The argument seems to come down to whether Capgras syndrome is a problem of perception or of some other process, like memory. Hirstein and Ramachandran proposed that Capgras syndrome is a problem of "memory management." They give this example: Think of a computer. You make a new file and save it. When you want that information, you open the old file, add to it, save and close it again. Perhaps people with Capgras keep creating new files instead of accessing the old one, so that when you leave the room and re-enter, you are a new person, a person who looks like the one who left, but slightly different -- maybe your ears look bigger, or your hair a different hue. There's still a lot that science doesn't know about the human memory.
Some studies have also shown blind people with Capgras syndrome -- their delusion extends to the voice of a person, thinking that the voice is a voice of an impostor, instead of the face, so perhaps it isn't a face-processing problem at all. Other studies have shown people who were convinced by looking at a person that that person was an impostor, but they recognized the person's voice on the phone.