The theory: broken-windows policing prevented serious crime

The case for: Three words: New York City. New York implemented a host of policing reforms in the early 1990s under Mayor Rudy Giuliani, but the most famous was “broken-windows policing” — an emphasis on enforcing minor “quality-of-life” offenses to make neighborhoods feel safer and encourage residents to feel pride (as well as deter people from committing more serious crimes). And sure enough, crime declined precipitously in New York during the Giuliani era.

The link between broken-windows policing and the crime decline has taken a beating among criminologists in recent years — so much so that two of its early champions (one of them current NYPD Commissioner Bill Bratton) wrote a defense of it in winter 2015. The essay says that other theories use “macro data sets” and “fail to grasp how crime is managed in dense, urban settings.” The implication: You can only tell that broken-windows policing works if you saw it up close.

The case against: There is a reason this explanation has fallen out of favor. First of all, crime declined in plenty of places that weren’t New York City during the 1990s — meaning New York’s trends probably weren’t totally the result of the NYPD. For another thing, broken-windows policing was just one aspect of the policing reforms implemented in New York or other places during this time; the crime-tracking system CompStat, for example, also came online in the mid-1990s. How can you separate how much of the crime drop was due to one thing versus another introduced at the same time?

The bottom line: Too difficult to tell. Ultimately, different departments define “broken-windows policing” differently and implement it in different ways — and, again, often alongside other changes. It’s true that it’s hard to tell why crime declines in cities, but that applies to broken-windows policing as much as it applies to other macro explanations.