1) The studies with the most dramatic findings(300%, 600%, etc) only screened for symptoms of mental illness at some point in the subject's life. Qualifying symptoms are things like "paranoia", "recklessness", "social withdrawal" and "depression". Nearly everyone has had such "symptoms" at some point in their lives. These studies did not specifically research DSM-IV officially diagnosed mentally ill individuals. Read the fine print carefully. From the 2005 NZ "Study": "After carefully controlling for self-medication and other confounding factors, the British researchers found that those who had smoked cannabis three times or more before the age of 15 were much more likely to suffer symptoms of schizophrenia." Also, who the heck are they kidding with "three times"?
2) EVEN IF they did show a large amount of data on cannabis smoking correlating to a true DSM-IV diagnosis, studies showing ONLY a correlation of any two separate diseases or activities proves NOTHING about causation. For example - "Only about 25 percent of Americans in the general population currently smoke [tobacco], according to current estimates. But 80 percent or more of people with schizophrenia smoke [tobacco]".
If you believe the identical cannabis studies, you can say this also "proves" tobacco smoking increases the risk of "developing" schizophrenia by 320%. The data on this is exactly as strong as what you presented for cannabis, but presented without false conclusions or anti-cannabis hysteria. The statistics of alcohol or prescrption drug abuse by schizophernics are nearly identical also. The reasonable accepted medical reason for such correlations is that mentally ill people are more likely to self medicate with alcohol or drugs after suffering with the disease. One of the main symptoms of these illnesses is to not trust outside help and medicate on your own.
To prove direct causation, a study would need to show either A) direct biological evidence of damage from cannabis in test animals or humans(never has been proven at ANY dosage) or B) twin behavioral studies where one twin is given cannabis and another is not until a diagnosis happens (not a single case of this has ever been tested or observed). In fact, one of the few twin studies on cannabis says "Twin study fails to prove 'gateway drug' hypothesis" (published by JAMA)
3) There are statistics that show the rates for schizophrenia diagnosis did not increase even as drug abuse of ALL kinds exploded after the 1960s - especially widespread abuse of strong psychedelics that are far more mind-altering than cannabis. True schizophrenia diagnosis has always been recorded at slightly below 1% of the population - even through the recent great changes in politics, standards of living and substance abuse throughout the entire world. Look with your own eyes:
Percentage of Americans Who Have Tried Cannabis:
Source: National Survey on Drug Use and Health, 2006
Source: National Survey on Drug Use and Health, 2009
Rates of Schizophrenia Diagnosis:
Source: Schizophrenia Bulletin, Vol 24(4), 1998, 529-535
