Clearing up the confusion
Psychology Tomorrow Magazine asks "Do schizophrenics live in parallel universes?" There is some interesting territory to mine here, but let's clear up a big mistake in the question first.
Schizophrenia is a mental disorder which is characterized by paranoia, auditory and visual hallucinations. The afflicted may hear voices whispering "You smell bad, you should kill yourself," or believe that the CIA is bugging their phones. There is also a collection of other symptoms involved, like disorganized thinking and negative affect.
This article is talking about the popular (and mistaken) belief that schizophrenia means "split personality." In fact, when most people say "schizophrenia" what they mean is Dissociative Identity Disorder (DID), which is extremely rare.
DID is also a controversial topic in psychological circles. There is endless debate about what causes it, or if it is even a real thing. The generally accepted current theory is that the separate personalities split off in response to extreme trauma, typically endured when the afflicted person was a young child. If you have ever zoned out in a really boring staff meeting, it's a little like that, but times a million. And while you're zoned out, some other splinter personality may take over for a while.
My first thought about the theory that DID patients are suffering from some kind of Many Worlds glitch is depressing, because it implies that the same person experienced horrific sexual and physical abuse across several parallel universes. But this theory doesn't quite work, because the different personalities (called "alters") each have a different name, age, background, even gender. So for this theory to work, multiple people across multiple universes would have to be entangled somehow.
It's interesting to think that people in several different universes could be exchanging personalities. Dave in Universe A recedes, and Bob from Universe B takes over. Meanwhile, Carl in Universe C takes over Bob, and maybe Dave from Universe A takes over Carl. It gets pretty complicated.
Like I said, there are some interesting ideas to explore here, but DID or split personality is kind of a non-starter. What if we posit that some people with mental illness are just slightly out of phase with our own universe, and able to see a ghost of another universe? What if, for every person suffering from clinical depression in our universe, there is an identical person suffering from mania in the other universe. Maybe there is a parallel universe where the CIA actually does spy on all its citizens all the time. OH WAIT THAT'S US.