In dissociative identity disorder, formerly called multiple personality disorder (the phenomenon behind Robert Louis Stevenson’s “Dr. Jekyll and Mr. Hyde”), at least two seemingly separate people occupy the same body at different times, each with no knowledge of the other. This can be seen as a more severe form of borderline personality disorder. In borderline personality disorder, there is one dramatically changeable personality with an intact memory, as opposed to several distinct personalities, each with an incomplete memory. People with dissociative identity disorder have two or more (on average, eight to ﬁfteen) personalities or personality fragments that control their behavior at different times. Often there is a passive, depressed primary identity who cannot remember personal history as fully as can the other more hostile, protective, or controlling identities.