In addition to supporting the notion that multiple personality disorder is extrememly rare, I would argue, besides, that the genuine cases tend to be far less dramatic and colorful than the ones in best-selling accounts such as The Three Faces of Eve and Sybil. In fact, they amount to being a different mental disorder.
William James made a landmark case study of man with the disorder. The case he studied, however, involved a man who “cracked”, changed his name, and ran away from a life he found too stressful. When found, he appeared to honestly not remember that he used to have another name, or that he had fled from his obligations. His “new” personality was not wholly unlike his old one.
James’ achievement was not in discovering that such a thing could happen, but in describing it in pathological terms. In the same way, people were observed to have schizophrenia, manic depression, and a host of other disorders before medical science evolved to the point that these conditions were isolated and described in scientific terms.
The kind of behavior James identified is considerably different from the kind of melodramatic cases which make for good reading and cinema, where a sufferer continually switches from one identity to another at the drop of a hat. I would suggest that possibly a real but rare phenomenon ran afoul of show business hype and both its frequency of occurence and its actual symptoms became exagerated.
People were known to develop amnesia and skip town long before the advent of psychiatry, yet the modern “style” of multiple personality disorder which grabs the headlines seems to have escaped everyone’s notice before the 20th Century. It is one thing for the medical profession to categorize and describe a behavioral disorder. For medical professionals to claim they have “discovered” behavior of a kind no one else has ever observed, though, surely invites skepticism; it seems to me that a real-life Sybil would be awfully conspicuous, and would arouse a lot of comment, in any era of history.