When the FBI announced last week that violent crime in the U.S. had reached a 40-year low in 2010, many criminologists were perplexed. It had been a dismal year economically, and the standard view in the field, echoed for decades by the media, is that unemployment and poverty are strongly linked to crime. The argument is straightforward: When less legal work is available, more illegal "work" takes place.

The economist Gary Becker of the University of Chicago, a Nobel laureate, gave the standard view its classic formulation...