does prison deter crime, or just delay it?

Joel Waldfogel takes on a new study.

In Florida during the years in question, Lee and McCrary found, the probability of being sentenced to prison for an offense jumped from 3 percent to 17 percent at exactly age 18. This tees up the answer to the economists' main question: How does the tendency to commit crimes vary around the 18th birthday, when the odds of a prison-sentence punishment jump? The answer is, hardly at all. While the probability of being arrested each week falls steadily from age 17 to age 19, there is no sizeable decrease in the arrest rate that corresponds to the bump up to an adult penalty in the weeks before and after people turn 18. To an economist, this is odd. At the grocery store, in weeks that Coke is on sale and Pepsi is not, consumers respond immediately. Coke sells out while Pepsi languishes on the shelf.

If the prospect of longer prison sentences does not deter young Floridians from committing crimes, prison still prevents some crime via the more mundane channel of locking them up—incapacitating rather than deterring them, in the lingo of criminal justice theory. Lee and McCrary see this in the re-arrest data they study. One-fifth of the people arrested the week before their 18th birthday were rearrested within a month. By contrast, only a tenth of the people arrested a week after their 18th birthday were rearrested within the same time period. The reason? The 18-year-old offenders spent more of the month behind bars (because they received longer sentences, on average) and therefore were not free to commit the crimes that would have gotten them re-arrested.