The latest edition of the Canadian Journal of Psychiatry has a comprehensive review of the evidence on whether cannabis contributes to causing psychotic mental illness – the best known being schizophrenia.
It has been known for a long time that there is a link between cannabis use and psychosis, but it was not known whether cannabis contributed to the development of psychosis, or whether people with psychosis were just more likely to smoke cannabis because it helps dispell some of the unpleasant emotions and feelings associated with the condition.
There is now good evidence that cannabis can contribute to the cause of psychosis, particularly during adolescence and early adulthood.
At a population level, this effect is detectable but small.
At the individual level, the effect seems to be quite variable. Recent research has suggested that the risk of developing psychosis when using cannabis is heavily influenced by what version of the COMT gene a person has.
The main conclusions of the Canadian Journal of Psychiatry review are summarised in an editorial, but for those wanting the in-depth lowdown, the full paper is also available online.
Link to August 2006 Canadian Journal of Psychiatry.
Mind Hacks 
I’d like to address the contribution of science for resolving the controversy. Science is useless here to my opinion, and I’ll tell you why.
In my view, cannabis Sativa (and Indica) had been made illegal because of historical reasons – which began at the first years of the 20th century. The western states of the USA started to fear the influx of Mexican-Americans.
Later in that decade, small farmers felt bad about the large farms that used cheaper Mexican labor. Fears were in place, because jobs and welfare resources became scarce.
Ofcourse it’s not legit to arrest mexicans just because their mexicans, so one of the “differences” that the farmers found that time was that Mexicans were marijuana smokers. The first state marijuana prohibition was established in 1915, not in the name of science, but in the name of the people wanting to keep the Mexicans out.
Other states quickly created prohibition laws: Wyoming (1915), Texas (1919), Iowa (1923), Nevada (1923), Oregon (1923), Washington (1923), Arkansas (1923), and Nebraska (1927). These laws were targeted against the Mexican population living in America.
Heavy propoganda agains marijuana began in order to pursuade other states to create progibition laws. False heavy advertising (and we know how much Americans love TV) made people believe Marijuana is going to make you go crazy, jump off the window, lead you to other drugs and destroy your life. The people made the government outlaw marijuana, and still today, they do believe Marijuana is the devil. Unbelievable. No science whatsoever can destroy dozens of years of heave, false advertising. And in democracy like in democracy, it’s the poeple who decide of the laws.