Crime guru James Q. Wilson surveys the evidence for why violent crime rates have dropped so dramatically over the past two decades. The state of the economy, he says, seems to have little to do with it:
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.
....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....A 2007 study by the economist Jessica Wolpaw Reyes contended that the reduction in gasoline lead produced more than half of the decline in violent crime during the 1990s in the U.S. and might bring about greater declines in the future.
....Another shift that has probably helped to bring down crime is the decrease in heavy cocaine use in many states....Drug use among blacks has changed even more dramatically than it has among the population as a whole....Among 13,000 people arrested in Manhattan between 1987 and 1997, a disproportionate number of whom were black, those born between 1948 and 1969 were heavily involved with crack cocaine, but those born after 1969 used very little crack and instead smoked marijuana.
So if I can put words into Wilson's mouth, the decline in crime is perhaps one-quarter due to increased incarceration, one-quarter due to reduced cocaine use, and one half due to reductions in blood lead levels in children. Better policing might be part of it too, though the evidence is spotty. Oddly, though, Wilson's own summary is different: "At the deepest level, many of these shifts, taken together, suggest that crime in the United States is falling [...] because of a big improvement in the culture." Aside from the reductions in cocaine and crack use, however, none of this sounds all that cultural to me. It sounds like we cleaned up the environment and built a lot of new prisons. It's hard to see an awful lot of room for cultural explanations here.