In late 2014 and early 2015, escalating tensions in New York City led to the NYPD staging a slowdown in which the department performed only its most essential duties. That might be expected to lead to an increase in crime, but a new analysis of official statistics shows the opposite: a significant drop in major crime for the period of the slowdown. Researchers are now arguing about what this tells us.

The slowdown developed in response to a sequence of events following the death of Eric Garner in July 2014, who died when placed in a chokehold by the police officers who were arresting him. This led to extensive protests, which continued after the decision of a grand jury not to indict the officers involved in Garner’s death. Two weeks after that ruling, NYPD officers Wenjian Liu and Rafael Ramos were fatally shot by an anti-police extremist, and the NYPD responded by informally and collectively stepping back their policing to the bare minimum.

This included fewer tickets and a huge drop in arrests. The action was partly attributed to precautionary measures, but there were also political motivations: “The act was a symbolic show of strength to demonstrate the city’s dependence on the NYPD,” write criminologists Christopher M. Sullivan and Zachary P. O’Keeffe in a paper in Nature Human Behaviour this week.

During the slowdown, police continued to respond to calls, and the arrest rate for major crimes (murder, rape, robbery, felony assault, burglary, grand larceny, and grand theft auto) remained constant. But the arrest rate for non-major crime and narcotic offenses dropped, as did the number of stop-and-frisk events. It took until mid-January for things to begin to return to normal.

Events like these provide rare opportunities to explore questions that couldn’t be tested experimentally, for practical or ethical reasons. So Sullivan and O’Keeffe looked at crime statistics for the duration of the slowdown, and they found something surprising: reports of major crime dropped during the slowdown period.

Competing explanations

Maybe people didn’t bother calling the cops during the slowdown? That’s possibly the most obvious explanation. Perhaps public trust in police was damaged after this well-publicized incident of police violence. Or perhaps people knew about the slowdown and didn’t think there was any point in reporting crimes—although it’s difficult to believe that people were likely to let things like burglary slide.

Regardless of possible motivations, the data suggests otherwise. The drop in major crime didn’t occur after the killing of Eric Garner, during the protests, or after the decision not to indict. In a similar seven-week window after the killing of Freddie Gray in April 2015, there was no comparable drop. So, public trust in police doesn’t seem to be the reason.

The drop also lingered for the first few weeks after the slowdown and after public statements reporting that it was over. That also undermines the idea that people weren’t bothering to report crimes because they thought there was no point. It’s not possible to show conclusively that under-reporting doesn’t explain the pattern, but it doesn’t seem like a particularly good explanation.

Another explanation could be that police violence serves as a ghoulish deterrent: that after a publicized incident of violence, people are less likely to engage in crime because they become more scared of the consequences. Again, the exact timing of the crime drop matches the slowdown rather than the incidents themselves, suggesting that this wasn’t the case.

Proactive policing and its discontents

Sullivan and O’Keeffe suggest that the absence of police activity itself is what led to the drop in crime. To explain this, they point to the idea of “proactive policing,” which suggests that police shouldn’t wait for crime to be reported but, rather, should be patrolling and maintaining order constantly through policing of low-level crimes, a strategy popularized as the “broken windows” model. The idea behind it is that aggressive, low-level policing, like stop-and-frisk and the issuing of citations, deters more serious crime.

But the authors argue that the data suggests the opposite is true. “The results,” they write, “imply that aggressively enforcing minor legal statutes incites more severe criminal acts.” Rather than proactive policing deterring major crime, Sullivan and O’Keeffe think it’s more likely that this kind of aggressive enforcement “disrupts communal life, which can drain social control of group-level violence.” In other words, overly aggressive policing brings a level of social disruption that actually leads to more crime—and the reduced proactive policing during the slowdown produced a calming effect.

There’s a lot to question here. As criminologist David Weisburd points out in a commentary in Nature News & Views, “proactive policing” can result in a huge array of different strategies, some of which have data indicating that they work. It’s difficult to take a finding from the exact strategy applied in New York, in a specific city in a very particular political climate, and generalize to other contexts. The natural experiment is “well-done social science, and, in this context, the results have strong weight,” Weisburd writes. But “we need experimental evidence,” he adds.

And other questions arise, too, like whether seven weeks would have been long enough for things to change—if the problem with proactive policing is the kind of social stress that Sullivan and O’Keeffe point to, is seven weeks enough time for life to start knitting back together?

The study is an important contribution to an ongoing question. Right now, some evidence points in favor of proactive policing, and some points against it. And the precise implementation of different proactive policing strategies will also need careful study. Given the monumental impact on many people’s lives, there’s an urgent need for more research to contribute to the pile of evidence.

Nature Human Behaviour, 2017. DOI: 10.1038/s41562-017-0211-5 (About DOIs).