Using more recent data, Bert Useem of Purdue University and Anne Piehl of Rutgers University estimate that a 10% increase in the number of people behind bars would reduce crime by only 0.5%. In the states that currently lock up the most people, imprisoning more would actually increase crime, they believe. Some inmates emerge from prison as more accomplished criminals. And raising the incarceration rate means locking up people who are, on average, less dangerous than the ones already behind bars.