More Cops, Less Crime?
Sometimes the most seemingly obvious connections are the hardest to prove. My NYT column looks at work by Alex Tabbarok (of Marginal Revolution, one of my favorite blogs) and Jon Klick on whether adding police on the street really reduces crime:
"When the terror alert level went up," he recalled in an interview, "you all of a sudden saw zillions of cops around the Capitol and around the Mall."
The pattern gave Professor Klick, now a professor of law and economics at Florida State University, an idea for how to examine a tough social science question: Do more police officers in fact reduce crime?
The answer may seem obvious, but many social scientists have argued that the number of police officers has no effect on crime rates and may even increase them. "If you look at the studies, particularly in the criminology literature, it's either no effect or actually a positive effect," Professor Klick said.
Cities with more police officers have more crime. That is probably because cities with high crime rates hire more officers. But it is hard to separate cause and effect, and, assuming that the officers do deter crime, to figure out how big the effect is.
"We spend a huge amount on police," Alexander Tabarrok, an economist at George Mason University, said in an interview. "So we want to know not simply do police reduce crime, but by how much. Should we have more police?"
He and Professor Klick examine the question in a study published in the April 2005 Journal of Law and Economics, "Using Terror Alert Levels to Estimate the Effect of Police on Crime." (A copy of the article is available at http://mason.gmu.edu/~atabarro/TabarrokPublishedPapers.html.)
To separate cause and effect, researchers need a "natural experiment" - in this case, an event that changes the number of police officers for reasons having nothing to do with the crime rate. The crime rates before and after the change can then be compared.
Read the rest here.