More Cops Reduce Crime!

Virginia Postrel, writing in the Times, delivers some real breakthrough material from the social sciences - more police on the streets can reduce street crime!

A cheap shot artist would cite this unsurprising result as confirming the need for a bit of political diversity in the halls of our nation's academia - surely this cannot be news.

But surely it can be!  Just last December, the good people of the great state of Minnesota were vexed by this puzzle, and here is an earnest pushback by a self-described "paleoconservatarian".  His theme - no one knows if more cops help or hinder a reduction in crime.

The problem, as the authors explain in their lead paragraph, is that using statistical methods to separate cause and effect has been a challenge that has confounded past studies:

Do police deter crime? A majority of studies surveyed found that either there is no relationship or increases in the number of police are associated with increases in the level of crime.(1)  Most economists are suspicious of these results. It is no surprise to find that places with an inordinate amount of crime tend to employ a large police force. Nor is it unreasonable to suspect that jurisdictions increase the size of their police forces when they witness or expect an increase in the level of crime. Thus, neither cross-sectional nor time-series analyses can credibly identify a causal effect of police on crime.

We will pass the baton to Ms. Postrel to see how this was overcome:

WHEN Jonathan M. Klick worked in Washington, he noticed a striking effect every time the terrorism alert level went from its usual yellow ("elevated") to the more urgent orange ("high").

"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.

...

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.

Changes in the terror alert level provided just the sort of natural experiment Professors Klick and Tabarrok needed, because the shifts in police deployment are big, making effects easier to spot. The alert levels - and hence the number of officers on the street - go up and down over time, providing multiple tests. And since the number of police officers fluctuates over days or weeks, rather than months or years, any new officers are unlikely to be there because of crime-related expansions of the force.

The two economists looked at daily crime statistics in Washington from March 12, 2002, to July 30, 2003. During that time, the terror alert level rose and fell four times. "On high-alert days," they wrote, "total crimes decrease by an average of seven crimes per day, or approximately 6.6 percent."

OK, now we know.

MORE:  Another point a cheap shot artist would raise - back when Bill Clinton was claiming credit for putting 100,000 new cops on the beat and helping to reduce crime, why was the "reality based community" silent on the lack of evidence supporting a causal relationship between more police and less crime?  Or did Clinton's involvement make that an acceptable faith-based initiative?

Also see Clinton's 1999 SOTU; or his 1993 SOTU.  Or any mainstream politican from recorded history (but I am cheap-shotting the silent academics, not the ignorant pols).

ALMOST DONE:  The Heritage Foundation puzzled over this in 2001, and had some seemingly snide comments here:

The theories of deterrence and incapacitation have been challenged by those who believe that police activities have little effect on crime since criminals rarely weigh the costs and benefits of illegal activities before they engage in them. For example, criminologists Michael R. Gottfredson and Travis Hirschi assert that to commit a crime, an "obvious opportunity coupled with a lack of self-control is all that is required."¹³ Further, the "offender sees a momentary opportunity to get something for nothing and he seizes it. These facts delineate the natural limits of law enforcement."¹⁴ These experts conclude that "no evidence exists that the augmentation of police forces or equipment, differential patrol strategies, or differential intensities of surveillance have an effect on crime rates."

Comments

[Dan]

Are crime rates calculated based on reported crimes, arrests or a combination of the two? If arrests are a component, you would think that more police = more arrests, therefore a higher crime rate. WRT the terror alerts, again, if arrests play are part in the stats, you might suspect the drop in the rate was due to policemen being focused on building or structure security vs. "normal" crime.

[BumperStickerist]

Philadelphia had Operation Sunrise a couple of years ago. The goal was to sweep into a neighborhood, clean out the crack houses, have a strong police presence for a time, and then move on to the next neighborhood.

Oddly enough, the people in a neighborhood that was cleaned out protested the absence of police officers who'd moved on to the next neighborhood.

It seems crime went down during the times there was increased police presence on the street and in patrol cars. Of course, most of these neighbors, perhaps all, were not social scientists so they may have mistaken correlation for causation.

But they thought having cops around kept criminals away.

[Crank]

The good thing about more cops is, they're a more direct deterrent. A guy who may not care the difference between a 10- and 20-year sentence because he thinks he won't get caught will still recognize that the cop on the street corner means a greater chance of getting caught.

It's the old formula: motive, means, opportunity. More cops = less opportunity.

[dsquared]

I think that the authors here are overstating the extent to which "more cops less crime" was not believed. It is true that it is hard to sort out a model in which cops and crime are exogenous from one another, but this is not the first time that it has been done. Steve Levitt did something similar ages ago to prove that Braxton's "broken window" approach to policing added zero value when one took into account the increase in manpower.

[Jon Klick]

The data we used was reported daily crimes, as opposed to arrests (we would have liked to examine arrests as well, but, amazingly, the DC police told us they did not have daily arrest counts to give us . . . I suspect if we had pushed a bit more, we might have gotten those numbers). I don't know that we overstated the degree to which people didn't believe more cops => less crime since we basically say in the JLE article that economists have uniformly discounted the possibility that the studies finding no/positive effect of cops on crime are successful in isolating a causal effect (though the criminology and sociology literature is less uniform in this respect). I think our contribution is that we offer a cleaner test than has been used in the past, and it offers more opportunities for replication (i.e., can be done for any city providing daily crime numbers and a description of how it changes its police staffing when the terror alert level changes). Additionally, as my co-author points out in the Times article, it's important not just to figure out the direction of the effect (which is fairly intuitive) but the magnitude as well, given how much we spend on policing and the cost of crime. Thanks for the comments.