Many ancient reports of "demonic possession" may very likely have been misunderstood cases of mental illness, including DID. While DID is often thought of as a modern creation, there is a rich, documented history of the illness.
The symptoms of DID were first diagnosed in 1791. At that time, hypnosis (then known as animal magnetism) was quite popular, and a doctor who practiced hypnotism used it to treat a patient who was switching between two distinct personalities -- the normal personality of the German woman, and a personality of a French woman. Under hypnosis, the French personality could easily be drawn out, and at the conclusion of a session the German personality would emerge from the hypnotic state.
Until about 1880, a commonly held belief was that everyone had a background consciousness that was greater than the consciousness responsible for the primary personality. Mental illness occurred when this greater consciousness became ill. The greater consciousness could then be brought out and treated through hypnosis.
Around the same time, doctors began to make a connection between the symptoms of DID and early childhood trauma, and also to recognize that more than one distinctly different personality could develop as a result of the mind compartmentalizing the trauma -- its attempt to protect the host personality.
A 22-year-old French patient, Louis Vivé, and his six distinct personalities made an appearance in an account published by his doctors in 1888 -- "Variations de la personnalité." The personalities didn't have overlapping memories, but doctors viewed the alters as hypnotic variations of the host personality and not truly separate personalities.
Another doctor of the same time period, Pierre Janet, had a different way of thinking, however. He was working with patients described as hysterical, and he concluded that some of them did have different, distinct personalities, born of things they'd witnessed during traumatic episodes.
The first real cure to be documented was in 1905 by Morton Prince. He published his account of a pseudonymous patient, "Miss Beauchamp," who exhibited three distinct personalities. Prince attempted to and -- by his own account -- fully succeeded in re-integrating the personalities and forcing them back into the subconscious, resulting in a unified and permanent personality.
In the 1970s, a doctor named Cornelia Wilbur treated a patient named "Sybil" and subsequently was the subject of a bestselling book on the case by Flora Rheta Schreiber. The book was later made into a movie. As we'll see in the next section, this brought about changes in treatment, public perception and public scrutiny.