Tennessee Representative Steve Cohen made a comment (sarcastic I think) that illustrates one of the most common decision making errors - correlation is not causation. People call marijuana a gateway drug because people who use cocaine or other hard drugs often did marijuana first. But because they used it first (correlation), doesn't mean the use of marijuana CAUSED the hard drug use. If that were the case, then milk (up to age 8?) would be a gateway drug to juice (ages 5-12?), which would lead to soda (10-15), then to marijuana or maybe beer (I'm not sure which is easier to get in high school now) and then on to hard drugs. Maybe I should start this chain at infant formula?
Because legalization or at least decriminalization of marijuana has so many good features (for example), it would be nice to know for sure if it is or if it isn't. But with so many studies in both directions, it is impossible for anyone to know for sure. If someone tells you they know, they are succumbing to some other decision making fallacy (Read this to know more about DM fallacies. Great book and a quick read. But it will scare you about your own ability.).
I would like to see the same approach to this that we are taking in several other areas. Let's encourage a few states to decriminalize, with perhaps a 3-5 year sunset provision. Then we can see what happens. It won't be a perfectly controlled scientific study, because people will cross state lines to get it and other confounding factors. But I suspect we would still learn a lot. And on the off chance that it really does significantly reduce organized crime, overcrowded prisons, and other possible benefits - WOW.