The great policy challenge to cannabis legalization is discouraging problem use. Most consumers have no trouble keeping their consumption within reasonable bounds, but 10 to 15 percent lose control for months at a time, and some of them develop chronic problems. At any one time, there are a couple of million Americans who self-report that they’re trying and failing to cut down on their cannabis use. That hasn’t kept zealous advocates from convincing themselves and others that “pot isn’t addictive.” No one intends to become habituated to cannabis, any more than anyone intends to develop any other bad habit. Abuse creeps up on some people because their long-term desire to avoid habituation, and to function well in school, at work, and at home, has a hard time competing with the short-term allure of getting high.