I know everyone's really excited about the paleo diet, and the wheat-schizophrenia connection, and all of it (trust me, so am I!), but not even dear Emily Deans herself is 100% sure that eating wheat causes schizophrenia. http://evolutionarypsychiatry.blogspot.com/2011/10/slam-dunked-and-wheat-belly.html
This isn't to say diet isn't hugely helpful, it most certainly is. But another big reason that schizophrenics often fare better in the developing world has to do with community and community psychotherapy. In a lot of older cultures, people who suffered these kinds of reality-altering mental illnesses would be guided by community elders/shamans and have a special place in the community, etc. That's essentially the loving support of a free therapist and widepsread community acceptance.
Also, a lot of research has been done into cultural disconnects and schizophrenia. For example, immigrants are vastly more likely to suffer from the disease than either the home population or the immigrated-to population. Also, the rate between cities and countryside is dramatic. So, some of that may also help to explain why people suffering from schizophrenia suffer worse in the Western world, where cultural alienation is par for the course for everyone, and most people live in cities.
Disclaimer: my mother is an immigrant who suffers from schizophrenia, and refused therapy/medication of any kind, and is basically withering away (it's degenerative if you can't get it under control). But, that being said, she certainly does much better in China, were she grew up, than in the US. And, of course, I always wonder whether eating a better diet, free of gluten and inflammation, wouldn't have gotten her just better enough that she could have taken the next step into psychotherapy and then remained as smart and together as the poster here. Makes me tear up a little to think about.