One of the most interesting studies about policing is a randomized comparison of different strategies for dealing with areas of Lowell, Mass., that were hot spots for crime. One was aggressive patrols, which included stop-and-frisk encounters and arrests on misdemeanor charges, like drug possession. A second was social-service interventions, like mental-health help or taking homeless people to shelters. A third involved physical upkeep: knocking down vacant buildings, cleaning vacant lots, putting in streetlights and video cameras. The most effective in reducing crime was the third strategy.