The catch here is that there’s likely no ethical way to test whether pot “causes” schizophrenia, or is merely preferred by people who already have a tendency to become schizophrenic. That would take a lot of pot smoking. Animal models might be illuminating, but crazy monkeys are dangerous.
There’s no ethical way to prove it? Rubbish. I’ve seen the brain scans. Read the Oxford literature. In any case, when it doubt go without: If one isn’t sure it can be tested then don’t dare experiment on brains.