Even with some mensa correlation, I'm still very skeptical about the idea that cannabis makes people clever. In britan a similar survey was done with alcohol. The group with the highest IQ were those that drank regularly in moderation. Still, its not as if anyone is marveling in awe at the amazing brain enhancement properties of alcohol. In my opinion the only point this goes to prove is that the cleverest people are curious/open minded enough to try different things, yet have enough self awareness not to over indulge in them.