“Are Idle Hands the Devil’s Workshop? Incapacitation, Concentration, and Juvenile Crime” by Brian A. Jacob and Lars Lefgren, in The American Economic Review (Dec. 2003), 2014 Broadway, Ste. 305, Nashville, Tenn. 37203.

Getting kids “off the street” is a time-honored recipe for reducing juvenile crime and a commonsense rationale for everything from an extended school year to “midnight basketball” programs. But there’s a tradeoff involved, warn Jacob, a professor of public policy at Harvard University’s Kennedy School of Government, and Lefgren, an economist at Brigham Young University.

In analyzing data from 29 cities, ranging in size from Minot, North Dakota (pop. 36,657) to Austin, Texas (pop. 656,562), they found a surprise. The level of vandalism and other property crimes in the community did decline, by about 14 percent, on days when school was in session. But on those same days, assaults and other violent crimes—mostly among the kids themselves—increased by about 28 percent. Any parent could tell you why: Putting a bunch of kids together in one place increases the chance that some kind of mayhem will break out.

In a hypothetical city of 120,000, the authors calculate, lengthening the school year by a day would lead to a decrease of only 0.29 property crimes and an increase of only 0.25 violent crimes. Of course, there are other reasons for increasing the amount of time kids spend in school, but if keeping them out of mischief is the goal, Jacob and Lefgren conclude, it would be better to place them in summer jobs, small after-school programs, or other venues where their numbers don’t reach critical mass.