My theory....its not that pot-smokers are so much smarter than the populous as a whole....it's that smart people understand the value of smoking pot! Therefore you get smart smokers.

As far as the stereotype, I think that probably started from government propaganda years and years ago. "Smoking drugs kills brain cells"...or remember this one..."this is your brain, now this is your brain on drugs, any questions?"---that kinda shit. Oh well there's my two cents.