Chicago Mayor Rahm Emanuel announced his support today for a city ordinance instituting a new curfew for children under twelve. If approved, the measure would institute an earlier curfew for kids under twelve and apply Chicago's existing curfew of 10 p.m. on weeknights and 11 p.m. on weekends to kids between twelve and 16. Emanuel argues that instituting curfews is a matter of matching the city’s laws to sensible parenting, but he’s careful not to oversell their impact. “It doesn’t mean just because you have it, kids are going to be safe,” he cautioned. As it turns out, Emanuel is smart to keep expectations low.

According to a 2003 article in The Annals of the American Academy of Political and Social Science, there is little evidence that curfews actually reduce juvenile crime. The study’s author, Kenneth Adams of Indiana University’s School of Public and Environmental Affairs, acknowledged that there is a common-sense logic to the efficacy of curfews: “Juveniles are less likely to commit crimes and to be victimized if they are not on the streets.” But Adams reviewed several studies on curfews—which, he acknowledges, aren’t as rigorous as they ideally could be—and found “mixed results” that “[failed] to demonstrate that curfews produce a decrease in juvenile crime.” Still, Adams avoided jumping to conclusions. He noted that many public officials believe strongly in the usefulness of curfews and suggested his findings may indicate that they are poorly implemented, rather than inherently flawed. There was some evidence, he wrote, to suggest that “short-term, highly focused, and geographically limited” curfew enforcement can have an impact on juvenile crime. Like the logic for curfews, this conclusion also appeals to common sense: If a law’s not going to be enforced, why pass it at all?