Split Personality Disorder

Medically known as dissociative identity disorder, this is "a psychiatric diagnosis that describes a condition in which a single person displays multiple distinct alter egos, each with its own pattern of perceiving and interacting with the environment". Even though the topic/validity of DID is commonly disputed, one cannot deny the scariness of it. I'm reminded of Edward Norton in PRIMAL FEAR; the tagline to that movie goes: "Sooner or later, a man who wears two faces forgets which one is real." (*Shudder*)