Think back to a time where you had a bit too much to drink. The next day, could you remember what your drunken self had done? Now, remove the alcohol. If you still drew a blank, you would have something in common with these famous cases of dissociative identity disorder.
Dissociative identity disorder (DID), formerly referred to as multiple personality disorder, is when a person manifests two or more distinct personalities that switch off controlling the body. The "blackout" periods are where the dissociative part comes in. Often the personalities have no memory of what went on when other personalties were in charge. In other words, the individual may have no clue that the other personalities even exist.
Usually, there is a dominant personality and one or more "alters." Which personality is dominant can actually change over the years. Sometimes the alternate personalities appear as hostile, foreign invaders to the dominant personality, and can be mistaken for cases of demonic possession. Real cases of DID are extremely rare, and are almost always the result of extreme trauma or abuse. The disorder serves as a psychological coping mechanism in which the psyche compartmentalizes the trauma. Ultimately, the disorder is a failure to integrate various memories, personality traits, identity, and consciousness into a single, multidimensional persona.
These are real people with dissociative identity disorder. All of their stories are true, and well documented. Beyond the sensationalism of tales like Dr. Jekyll and Mr. Hyde is a very real psychological concern that, while rare, can be extremely difficult to live with.