One obvious answer is that many more people are in prison than in the past. Experts differ on the size of the effect, but I think that William Spelman and Steven Levitt have it about right in believing that greater incarceration can explain about one-quarter or more of the crime decline. Yes, many thoughtful observers think that we put too many offenders in prison for too long. For some criminals, such as low-level drug dealers and former inmates returned to prison for parole violations, that may be so. But it’s true nevertheless that when prisoners are kept off the street, they can attack only one another, not you or your family…

Another possible reason for reduced crime is that potential victims may have become better at protecting themselves by equipping their homes with burglar alarms, putting extra locks on their cars and moving into safer buildings or even safer neighborhoods. We have only the faintest idea, however, about how common these trends are or what effects on crime they may have.

Policing 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 to increase the effectiveness of the New York Police Department’s Compstat program, which uses computerized maps to pinpoint where crime is taking place and enables police chiefs to hold precinct captains responsible for targeting those areas…

There may also be a medical reason for the decline in crime. 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).