woah! I read this in one of the links:

Robin Murray, a psychiatrist at King's College London, is one of Britain's leading researchers in this area and his results are worrying. "The conclusion was that, if you took cannabis at age 18, you were about 60% more likely to go psychotic. But if you started by the time you were 15, then the risk was much greater, around 450%," he says.
http://www.guardian.co.uk/life/thisw...239984,00.html