Policing, as City Journal readers know, has become more disciplined over the last two decades; these days, it tends to be driven by the desire to reduce crime, rather than simply to maximize arrests, and that shift has reduced crime rates. One of the most important innovations is what has been called hot-spot policing. The great majority of crimes tend to occur in the same places. Put active police resources in those areas instead of telling officers to drive around waiting for 911 calls, and you can bring down crime. The hot-spot idea helped make the New York Police Department’s Compstat program—its planning and accountability system, which, using computerized maps, pinpoints where crime is taking place and enables police chiefs to hold precinct captains responsible for targeting those areas—so effective.

Researchers continue to test and refine hot-spot policing. For instance, criminologists Lawrence Sherman and David Weisburd, after analyzing data from more than 7,000 police arrivals at various locations in Minneapolis, showed that for every minute that an officer spent at a spot, the length of time without a crime there after the officer departed went up—until the officer had been gone for over 15 minutes. After that gap, the crime rate went up. The police can make the best use of their time by staying at a hot spot for a while, moving on, and returning after 15 minutes have elapsed…

There may also be a medical reason for the crime decline. For decades, doctors have known that children with lots of lead in their blood are much more likely to be aggressive, violent, and delinquent. In 1974, the Environmental Protection Agency required oil companies to stop putting lead in gasoline. At the same time, lead in paint was banned for any new home (though old buildings still have lead paint, which children can absorb). Tests have shown that the amount of lead in Americans’ blood fell by four-fifths between 1975 and 1991. A 2000 study by economist Rick Nevin suggested that the reduction in gasoline lead produced more than half of the decline in violent crime during the nineties. A later study by Nevin claimed that this also happened in other nations. Another economist, Jessica Wolpaw Reyes, has made the same argument. (One oddity about this fascinating claim has yet to be explained: why the reduction related to lead-free blood included only violent crime, not property offenses.)