This has always puzzled me. How is it that the majority of layfolk came to think that the term “schizophrenia” really means “multiple personalities” when it does not? Were such disorders all lumped together at one time, perhaps?
I think its because they can hear voices but that isn’t at all the same thing as having distinct personalities take them over.
I think it might be originally because of a different understanding of the root words. Schizophrenia was coined from the Greek for ‘split mind’, and a multiple personality disorder looks very obviously like a way that someone’s mind is split up.
However, the word was coined for a different kind of ‘split’ in which the person has broken away from reality, or something like that, and it looks like this interpretation is struggling to be widely accepted.
According to Wikipedia, ‘The first known misuse of the term to mean “split personality” was in an article by the poet T. S. Eliot in 1933.’ It wouldn’t surprise me if the misunderstanding has come from popular culture. Plenty of people still think that autistic=card-counter.
Given the controversies that have surrounded both terms, it should be no surprise that a clear answer is difficult.
The matter is discussed in quite some detail by the eminent philosopher and historian of science Ian Hacking in his Rewriting the Soul: Multiple Personalities and the Sciences of Memory (Princeton, 1995, esp. chapter 9). One of the main points of Hacking’s overall argument is that the notion of multiple personalities, regardless of its validity, much predates 1954 and The Three Faces of Eve. In particular, he focusses on the debates in France about the notion during 1874-86.
Then Eugene Bleuler coins the term schizophrenia in 1908. This is the particularly awkward bit. As Hacking discusses, it became common in histories of the mental health for writers to claim that Bleuler subsumed earlier notions of multiple personalities under his broader diagnosis of schizophrenia. In those versions, schizophrenia was originally defined to include multiple personalities, though it would have been possible to be schizophrenic without displaying them. However, Hacking points out that Bleuler was more careful than that. Roughly, he drew a parallel between the two conditions, but with multiple personalities the patient was switching between personalities, while in schizophrenia they could manifest themselves simultaneously.
Hacking seems clearly correct that later historians have sloppily read Bleuler. On the other hand, Bleuler did associate the notions, at least roughly, and what Hacking doesn’t really address is whether that’s enough to explain the connection in popular culture.