Roughly speaking, there is about half as much crime now in the U.S. and across Chicago as there was in 1991. How come?

Northwestern political scientist Wesley G. Skogan, best known for his studies of community policing in Chicago, calls this “one of the most significant social facts of the end of the 20th century,” (PDF) and “one of the least understood.” In plain language, more than 3,000 people are walking around today in Chicago who would have been murdered if the crime rate had stayed where it was 15 years ago.

Skogan is pretty sure it’s not because there are fewer young people (there aren’t), and it’s not due to economic boom times (at least in Chicago).

Is it that we’re putting more people in jail? Then why did crime continue to drop after 1999, even as rates of imprisonment fell?

Has gun control made a difference? But the number of crimes committed without guns is down too.

Skogan speculates that a decline this long and this large is probably due to more than one factor. Although data are hard to find, he suspects that incarceration of hard-core offenders in the early 1990s, community policing in the late 1990s, and smarter policing tactics since then may be the important ones.