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The theory: putting more police on the streets prevented crime

The case for: More police make it easier for law enforcement to respond to crimes and apprehend criminals. If they’re visibly patrolling, they can also help prevent crime by deterring would-be wrongdoers. Research on specific areas, as well as the US as a whole, found that hiring more police helped decrease crime.

The case against: But while the number of police can affect crime rates, crime rates also affect the number of police. When crime rises, cities hire more police in response. It’s extremely tricky to isolate the effect of police on crime from the effect of crime on police.

The bottom line: A small effect. Steven Levitt, in a 1997 paper later revised in 2002, tried to fix this problem by looking at the timing of local and state elections. The logic is that an upcoming election would have an effect on police hiring, but wouldn’t affect crime on its own — so it could show cases where more police did have an impact on crime. Brennan Center researchers evaluated his findings and others to show that a small part of the drop in crime in the 1990s was caused by hiring more police. As with incarceration, though, it looks like hiring more police has more of an effect on reducing property crime than violent crimes.