Correlation versus causation. Cannabis use and psychotoic disorders like schizophrenia are definitely correlated, but last I heard there is no evidence of causation (Hall et al., 2000). If my information is outdated, please let me know.
Personally, I think it's far more likely that schizophrenics, or pre-schizophrenics, are drawn to cannabis as a form of self-medication.
BMJ 2002;325:1199 ( 23 November )
http://www.bmj.com/cgi/content/full/325/7374/1199
here is the conclusion paragraph:
Conclusions: Cannabis use is associated with an increased risk of developing schizophrenia, consistent with a causal relation. This association is not explained by use of other psychoactive drugs or personality traits relating to social integration.
Personally, I think it's far more likely that schizophrenics, or pre-schizophrenics, are drawn to cannabis as a form of self-medication.