Speaking of correlation that is precisely what epidemiological studies have consisted failed to show and there is no causation without correlation. Specifically, should there be a causal link between marijuana and schizophrenia, there should be a positive correlation between marijuana consumption and schizophrenia, but such a correlation is conspicuous by its absence. Despite a massive increase in the number of Australians consuming the drug since the 1960s, Wayne Hall of the University of Queensland found no increase in the number of cases of schizophrenia in Australia. http://www.november.org/stayinfo/breaking3/MJScience.html Mitch Earleywine of the University of Southern California similarly found the same with regard to the US population http://www.november.org/stayinfo/breaking3/MJScience.html and Oxford’s Leslie Iversen found the same regard to the population in the UK. http://www.stats.org/stories/2007/will_one_joint_schizoid_july30_07.htm According to Dr. Alan Brown, a professor of psychiatry and epidemiology at Columbia University,
“A 40% increase in risk sounds scary, and this was the risk linked to trying marijuana once, not to heavy use. To epidemiologists a 40% increase is not especially noteworthy-- they usually don’t find risk factors worth worrying about until the number hits at least 200% and some major journals won’t publish studies unless the risk is 300 or even 400%. The marijuana paper did find that heavy use increased risk by 200-300%, but that’s hardly as sexy as try marijuana once, increase your risk of schizophrenia by nearly half!
By contrast, one study found that alcohol has been found to increase the risk of psychosis by 800% for men and 300% for women.
"If anything, the studies seem to show a possible decline in schizophrenia from the '40s and the '50s,"http://dir.salon.com/story/news/feature/2005/09/19/reefer_madness/index.html
As Szalavitz notes, this is marked contrast to what happened with cigarette consumption and lung cancer. “
When cigarette smoking barreled through the population, lung cancer rose in parallel; when smoking rates fell, lung cancer rates fell.”http://www.huffingtonpost.com/maia-szalavitz/reefer-inanity-never-tru_b_58353.html
Much of the evidence linking marijuana to schizophrenia suggests not that it causes schizophrenia per say but rather that it causes the earlier onset of symptoms in people who would sooner or later develop schizophrenia. Much to Gordan Brown’s dismay, this is opinion of Dr Iddon.
Dr Iddon, the chairman of the all-party parliamentary group on drugs misuse [Britain], said the study did not convince him it was time to return cannabis to class B. "I don't think the causal link has been proved. I think cannabis might - possibly for genetic reasons - trigger psychosis at an earlier age." The MP, who is also a member of the science and technology select committee, said there was a danger of criminalising "hundreds of thousands of young people" if the status of the drug was changed. "If Gordon Brown changes the class of the drug, it won't be evidence-based but for political reasons," he said.