ANN ARBOR—Young offenders aren’t necessarily deterred from crime after they turn 18 even when they know they could be slapped with a much longer prison sentence, a new study co-authored by a University of Michigan researcher suggests.

Offense rates should decline at 18 but they are essentially the same as offense rates just before 18, said Justin McCrary, a professor at the Gerald R. Ford School of Public Policy and the Department of Economics.

"Our results suggest that offenders are highly impatient and impulsive people. It is hard to deter people with these characteristics from committing crime, just by threatening them with longer sentences," said David S. Lee, who, with McCrary, co-wrote "Crime, Punishment, and Myopia," which appears on the National Bureau of Economic Research website. Lee is an economics professor at the University of California-Berkeley.

A natural policy implication, McCrary said, would be to spend less money on prison expansion and more money on policing. "Even with highly impatient or myopic criminals, doubling the odds of punishment will double the effective price of crime."

The study uses data from all felony arrests from 1995 to 2002 in Florida, where the criminal age of the majority is 18. Both researchers emphasized the importance of the paper’s methodology for answering how responsive individuals are to prison length.

"We adopted this strategy hoping for an apples-to-apples comparison," McCrary said. "By focusing on offense rates a handful of weeks before and after the 18th birthday, all factors aside from the punitiveness of punishment are being held constant. Typically, the individuals facing long prison sentences are just not very comparable to the individuals facing short sentences, and differences in offending behavior could be attributable to a number of different factors and have nothing to do with sentence length."

A copy of the paper is available at http://www.nber.org/papers/w11491.

For more information on McCrary, visit http://www-personal.umich.edu/~jmccrary/

Information on Lee can be found at http://emlab.berkeley.edu/users/dslee/index.shtml.