Also, far be it for me to claim that I know better than the results of any scientific study, but I'd be lying if I said I didn't find this cannabis psychosis concept counter intuitive. I've been getting stoned on nearly a daily basis for close to 15 years and I like to think that I've still got a pretty firm grasp on reality.
I've also known a lot of other people getting stoned regularly. Never saw anyone go nuts from it. I've seen a lot of people that "smoked themselves stupid", but most of them weren't exactly NASA candidates to begin with (and I thank my lucky stars that I, apparently, had a little more to work with up stairs than they did at the onset of my drug use). But out of the dozens of potheads that have been my friends/acquaintances over the years, I have yet to see a single one display any "psychotic" personality traits.
Then again, I'm not a trained psychologist, and for all I know there's a good deal of shit that passes for psychotic that I would personally consider innocuous.
Two words.
Jared Loughner.
His experiences were a bit different than yours.
Is there evidence that Loughner committed his acts based on marijuana use? Is there evidence that he did not use other drugs? Or are you simply saying that no matter what other factors may be involved, if someone does something crazy and is a pot-smoker, that must be the cause?