Historically, measuring the effect of police on crime has been difficult, because it’s difficult to tease out cause and effect: areas with a lot of crime have a lot of police.
With ever-changing terror-alert levels though, we have ever-changing police levels not tied to local crime levels. Fom One Possible Cure for the Common Criminal:
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.’
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To make sure tourists were not just avoiding Washington during high alerts, the economists checked midday subway ridership and hotel vacancy levels, finding no significant difference. Nor did criminals appear to shift from District 1 to other parts of town. Crime was down throughout the city.A bigger police presence does affect some kinds of crimes more than others. The number of murders, for instance, does not change.
“If you think about what crimes you most expect to be affected by putting more police on the streets, well, it’s street crimes,” Professor Tabarrok said in the interview. “Theft from automobiles and automobile theft are the classic street crimes, and we found that they fell by a whopping 40 percent during these high-alert periods.” Burglaries were also down, by 15 percent.
Policing seems very cost-effective:
Using generally accepted cost estimates, Professor Tabarrok said, every $1 to add officers would reduce the costs of crime by $4. The authors did not identify a point of diminishing returns.“We estimate that if we had a 10 percent increase in police, crime would go down by about 4 percent,” he said, adding that researchers taking other approaches have come up with similar numbers. Nationally, he said, “that means about 700,000 fewer property crimes and 213,000 fewer violent crimes.”
As a back-of-the-envelope calculation, Professor Klick offered an even more striking suggestion. “It wouldn’t be unreasonable,” he said, “based on our estimates and based on conservative estimates of the costs of crime, to say it would be cost-effective to actually double the number of people working in police forces, which is pretty amazing.”