Do policing and punishment deter crime? Maybe not. Here's Joel Waldfogel in Slate:

The Irrational 18-Year-Old, by Joel Waldfogel, Slate: Crime control is one of the oldest problems facing social science... Two basic tools for controlling crime are policing and imprisonment...

Both the prospect of getting caught and the prospect of spending time in prison are supposed to deter forward-looking, rational potential offenders from criminal activity, encouraging more-constructive pursuits like staying in school or at least making French fries. More mechanically, prison also prevents crime by simply caging dangerous people. Deterrence has long been an article of faith among economic theorists and, more recently, economists who do empirical work, too. But now a series of careful studies by economists at Columbia and the University of Michigan are calling into question whether either policing or punishment successfully deters crime.

With the traditional tools of social science, the deterrent effect of policing and punishment is hard to measure. Usually, empiricists infer an effect if crime is lower in circumstances with stiffer punishments or more policing. The problem is that tougher policies don't occur randomly. Cities and states add police or lengthen sentences as a frustrated response to crime waves. So, crime affects policing and punishment as much as the other way around. This is one of the classic conundrums of empirical social science.

If social scientists ran the criminal justice system, it would be easy for them to measure the deterrent effect of longer sentences. They'd find a group of potential offenders and lengthen prison sentences the group would face if convicted. The scientists would make sure ... likely delinquents knew about the change, and then ... track whether they committed fewer offenses following the date their criminal penalties would increase. ...

David S. Lee of Columbia and Justin McCrary of Michigan ... noted that when kids turn 18, they suddenly face much stiffer adult sanctions. Then they got access to data on all felony arrests in Florida between 1989 and 2002. Each arrest links to an individual, whose birth date is included in the data. ...

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. ...[T]here 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... Lee and McCrary see this in the re-arrest data... 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)...

The conclusion that prison time prevents crime through incapacitation rather than deterrence raises questions about the effect of policing. What benefit do cities and states get from putting more cops on the street? In earlier work, McCrary ... concluded that existing data do not allow us to "learn about the causal effect of police on crime."

It would be premature to discard literally decades of scientific research based on one or two studies. Still, these studies should keep the debate going. ...