There is an incredible longterm prospective study from a large population about the psychological effects of using "heavy" marijuana use (4x or more per week). Decreased IQ by about 1 standard deviation (a seemingly larger issue if starting with less), and greatly increased risk for schizophrenia etc. So, uh, yeah. It has incredibly socially expensive effects that we are already aware exist.
My point was more to the likes of traffic incidents, etc. By legalizing, use will increase giving a greater sample pool for data. Colorado and Washington are currently being watched. In other words, a giant petri dish for the world to watch as an experiment. Having just returned from Europe, they think us here fine 'Mercans are quite stupid (not really a change from normal). Also, sure, it has been utilized for a long time. But so has cocaine. And peyote. Etc. It gets back to that old adage about jumping off a bridge... and don't start with the BS natural crap. Everything is naturally part of the universe-at least at some level. We aren't exactly creating matter...