The controversial diagnosis of having two or more distinct people, each with his or her own identity and personality, inside one person’s body. More commonly known by its older name, multiple personality disorder. This sort of disorder is often causes by some form of trauma that was part of their childhood, for example, physical abuse/sexual assault/emotional abuse (usually repetitive in nature).
The disorder is thought to be a way that people cope. Dissociation helps them leave the situation…we all do that when we daydream, but this disorder takes it to another level entirely where the dissociations become real and they begin to mold themselves into another identity entirely.
Example: When “Sybil”, Jane’s other personality, is not dominant, Jane cannot recall anything Sybil has said.
WebMD has a great little section on this disorder here: http://www.webmd.com/mental-health/dissociative-identity-disorder-multiple-personality-disorder