This sounds like some bogus "study" pulled out of the asses of ultra conservatives (on the issue of Marijuana legalization). I'd be interested to look at the overwhelming amount of documented stories that suggest what you're saying here is true. Links?
Hot off the presses you can read the latest evidence by clicking on -
Well, if Dr. Large is indeed correct in his statistical meta-analysis, then this may be a step toward a causal---rather than a purely correlative---relation between the etiology of schizophrenia and cannabis use. However, it'd be premature to say that cannabis causes schizophrenia.
As the press release says, "earlier onset of severe mental illness among substance users is a result of cannabis use," but this does not say much about schizophrenics in general. Granted, there is high comorbidity between substance abuse and schizophrenia, but we don't know the extent to which these data pools overlap (e.g., it could be the case that most schizophrenics do not smoke marijuana, but those that do, tend to develop it faster. But since so few smoke it, not many have hastened development).
Moreover, it could be that marijuana only hastens schizophrenia when other factors are present in the diathesis-stress model, and these other factors are far more important. Also, it is important to note that this study does not say cannabis hastens schizophrenia; it only says that those who did get schizophrenia, and did smoke marijuana, tended to develop schizophrenia at an earlier age. So it could be that marijuana did not cause the disorder, it just hastened it in those who would have gotten it anyway. Because only 1% of the population gets the disorder, marijuana, at the worst, would seem to speed up the disease in some subset of that 1%, and we are unsure how much marijuana contributes to that speed up, and what percentage of schizophrenics it would affect.