If the New York Times is to be believed, the crime rate has nearly halved since 1980 [1]. Obviously it needn't follow that the increase in the prison population caused this, but the author's unwillingness to even explore the idea seems awfully incurious.
[1] See the "In the U.S." tab on this graphic http://www.nytimes.com/interactive/2008/04/22/us/20080423_PR...