Contrary to what prohibitionists assume, the increase in marijuana-related problems following legalization may not be proportional to the increase in consumption. It’s plausible that people prone to excess are less likely to be deterred by prohibition than people of more moderate habits. Problem users may represent a smaller share of cannabis consumers after legalization than they did before, which means marijuana’s benefit-to-cost ratio would improve. A study in JAMA Psychiatry provides some evidence that as the number of cannabis consumers increases, the percentage who experience serious cannabis-related problems will decline.