Nationwide, the prison population grew by 25,000 last year, bringing it to almost 1.6 million. Another 723,000 people are in local jails. The number of American adults is about 230 million, meaning that one in every 99.1 adults is behind bars.

Incarceration rates are even higher for some groups. One in 36 Hispanic adults is behind bars, based on Justice Department figures for 2006. One in 15 black adults is, too, as is one in nine black men between the ages of 20 and 34.

Common sense tells us that crime should increase during hard times. After all, more than 90 percent of the serious "index" crimes reported each year in the government's Uniform Crime Reports involve some kind of financial remuneration. And we've all seen examples of people taking desperate actions when they are cold, broke, and hungry, whether through real-life, firsthand observations or through fictional characters like Tom Joad in The Grapes of Wrath. Yet there is much evidence that crime rates and economic indicators often diverge.