Yeah, but this population prone to schizophrenia would have: a) to know they're predisposed to this condition; b) to know that weed would worsen it; c) to believe that this effect on schizophrenia would be bad enough to outweigh smoking a joint. Or have another factor linked to schizophrenia that makes them avoid weed. It's possible, in principle, but I think you're stretching it too far.

And it's assumed the distribution of smoking per capita didn't change by much. I think that's a good assumption; again, it's possible that what you suggested happened, but it's a bit sketchy.