Estimating the effect of police on crime is more difficult than it sounds because places with a lot of crime tend to have a lot of police and vice-versa. As a result, naive analyses tend to find that police cause crime! Jon Klick and I, following the amazing Steve Levitt, have what we think is a pretty clever solution. We look at what happens to crime in Washington DC when the terror alert level rises from elevated to high. During a high-alert period the police put on extra shifts, monitor closed circuit cameras on the National Mall and in general step-up policing. We find that crime falls a lot during these high-alert periods. Our new paper is, Using Terror Alert Levels To Estimate the Effect of Police on Crime. Comments welcome.
Here is the abstract:
We argue that changes in the terror alert level set by the Department of Homeland Security provide a shock to police presence in the Mall area of Washington, D.C. Using daily crime data during the period the terror alert system has been in place, we show that crime drops significantly, both statistically and economically, in the Mall area relative to the other areas of Washington DC. This provides strong evidence of the causal effect of police on crime and suggests a research strategy that can be used in other cities.