Everybody knows that crime rates rise in recessions. When legitimate work dried up, potential criminals found it more attractive to turn to illegal work. Indeed, neighborhoods with higher poverty often have elevated crime rates.

So the Great Recession should have produced a crime wave. Instead, the doubling of the unemployment rate did nothing of the sort to violent crime, which reached a 40-year low in 2010. What gives?

James Q. Wilson has four explanations: (1) More criminals in prison; (2) Better police tactics for finding and patrolling crime hotspots; (3) Better home security technology; (4) Fewer drugs, including lead in our blood and cocaine. I find the last explanation the most fascinating:

Tests have shown that the amount of lead in Americans' blood fell by four-fifths between 1975 and 1991. 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 economist, Rick Nevin, has made the same argument for other nations.

Another shift that has probably helped to bring down crime is the decrease in heavy cocaine use in many states. Measuring cocaine use is no easy matter; one has to infer it from interviews or from hospital-admission rates. Between 1992 and 2009, the number of admissions for cocaine or crack use fell by nearly two-thirds. In 1999, 9.8% of 12th-grade students said that they had tried cocaine; by 2010, that figure had fallen to 5.5%.

Read the full story in WSJ.