I've fallen into the trap of thinking my drug abuse led to my schizophrenia and looking back I was quite seriously disturbed and definitely experienced states of mind akin to psychosis during childhood and adolesence (sp?).
Cannabis for me was self medication for years that helped me (I haven't smoked a joint in so many years I can't even remember when the last one was).
Its not just the Ganja, it definitely contributes, but there are so many factors involved in why a person cracks up that its impossible to blame just one thing.
Personally I feel that it was just the sheer fucking stress of living in this crappy capitalist, uncaring, selfish society that sent me over the edge.
I smoke a joint now though and you can book me a bed on the psych ward if you can find me, I'll probably be so paranoid I'll be hiding down a drain.
My son smoked it at 12, was sectioned at 13 with drug-induced psychosis. He's nearly 16 now, clean for 2.5 years, won't touch the stuff. Wants to try other things when he's 18 and at uni and I'm not around to stop him(!) but totally against ganga.