Dissociative Identity Disorder (DID) or Multiple Personalities is thought to be caused by a genetic predisposition to dissociation coupled with severe childhood trauma.
The Showtime series, the United States of Tara, is a fairly accurate (though exaggerated) representation of dissociative identity disorder/multiple personality disorder.
When you realize that someone isn't who he (or she) says he is, you can be devastated. Here are some ways to recognize an alter ego and then move on with your life.