Does an increased police presence decrease crime? That’s the seemingly simple and obvious question that Mark Easton poses on his BBC blog before explaining the difficulty in attempting to discern if a greater number of police helps to reduce crime.
To set the scene, Easton quotes from a Steven Levitt study (pdf) that attempted to answer this question by analysing crime fluctuations around electoral cycles (because, equally interestingly, the number of police increases around elections).
One of the most surprising empirical results in this literature is the repeated failure to uncover evidence that an increase in the number of police reduces the crime rate. Of the 22 studies surveyed by Samuel Cameron (1988) that attempt to estimate a direct relationship between police and crime using variation across cities, 18 find either no relationship or a positive (ie incorrectly signed) relationship between the two.
Easton does conclude, however, by saying that it “would be almost perverse to argue that more police has no effect on crime. But we don’t know how much impact they have or how long that impact lasts”.