THE first time I talked to Mark Kleiman, a drug policy expert at U.C.L.A., was in 2002, and he explained why legalization of marijuana was a bad idea. Sure, he said, the government should remove penalties for possession, use and cultivation of small amounts. He did not favor making outlaws of people for enjoying a drug that is less injurious than alcohol or tobacco. But he worried that a robust commercial marketplace would inevitably lead to much more consumption. You don’t have to be a prohibitionist to recognize that pot, especially in adolescents and very heavy users, can seriously mess with your brain.