Capgras syndrome is classified as a delusional misidentification syndrome, known as DMS or DMI. These delusions are monothematic, which means that they center around one idea and make the world a very strange place for the people who have them, as they see everyone in disguise or perhaps don't even recognize their own bodies.
Frégoli syndrome or Frégoli delusion
Named after a quick-change artist, Leopoldo Frégoli, the Frégoli delusion or Frégoli syndrome leads people to believe that the people around them are actually other people in disguise. For example, you see your doctor and think that it's actually your ex-girlfriend disguised as your doctor. One patient was convinced that two famous actresses put on the guise of her nurses and acquaintances in order to pester her [source: Edelstyn]. People with Frégoli delusion think that the people around them are capable of changing their appearance, dress and gender in a matter of moments, with only nearly imperceptible clues to their real identity.
Cotard's syndrome or Cotard's delusion
Cotard's syndrome or Cotard's delusion describes the belief that one is missing organs, body parts or has become emotionally dead. Some people with Cotard's think that they no longer exist because they don't feel anything. Cotard's usually occurs in tandem with psychoses like schizophrenia and bipolar disorder. Patients' complaints run from "My heart doesn't beat" to "I don't have muscles" [source: Baeza]. Someone with Cotard's might think that his brain is rotting, that he's missing some of his insides or that he's "melted away" [source: Christensen]. They think they're the walking dead.
People with intermetamorphosis think that familiar people in their lives have switched identities, both physically and psychologically. While Fregoli syndrome involves just a physical transformation, intermetamorphosis includes the self. You might, for instance, decide that your father is really your brother, and every time you see your father, you see him as having the physical and psychological attributes of your brother.
Déjà vu isn't a delusional misidentification syndrome, but it's just as disconcerting. Déjà vu happens to a lot of us. You see something and immediately feel like it's something you've seen before. Or someone says something and you look around and the comment, the environment, everything -- you could swear it's already happened. But some people have chronic déjà vu. They remember things that never happened, like already watching the news, going to a friend's funeral or meeting someone for the first time. For them, nothing is new. One man said he wouldn't go to a doctor to find out what was wrong because he'd already been. Only, he hadn't [source: Ottawa Citizen]. (For a more in-depth look at déjà vu, check out How Déjà Vu Works).
For more information on Capgras syndrome, the brain and related topics, check out the links on the next page.