Evidence that police reduce crime and violence

University of Nebraska Omaha criminologist Justin Nix said: “The evidence that proactive policing works is pretty solid.” A new article for Vox by German Lopez sets out some of that evidence.

Noting the surges in homicides across major American cities — up a staggering 131 percent in Minneapolis in 2021 over the same period last year — Lopez writes:

American policymakers now want answers on this surge. One approach has good evidence behind it: the police.

There is solid evidence that more police officers and certain policing strategies reduce crime and violence. In a recent survey of criminal justice experts, a majority said increasing police budgets would improve public safety. The evidence is especially strong for strategies that home in on very specific problems, individuals, or groups that are causing a lot of crime or violence — approaches that would require restructuring how many police departments work today.

“I know people don’t want to hear this, and I empathize with that,” Anna Harvey, a public safety expert at New York University, told me. “[But] as far as the research evidence goes, for short-term responses to increases in homicides, the evidence is strongest for the police-based solutions.”

Lopez then explores some of the evidence that police reduce crime and violence:

A 2020 study published by the National Bureau of Economic Research concluded, “Each additional police officer abates approximately 0.1 homicides. In per capita terms, effects are twice as large for Black versus white victims.”

A 2005 study in the Journal of Law and Economics took advantage of surges in policing driven by terror alerts, finding that high-alert periods, when more officers were deployed, led to significantly less crime.

A 2016 study published in PLOS One looked at what happened when more New York City police officers were deployed in high-crime areas as part of an effort called “Operation Impact,” concluding these deployments were associated with less crime across the board.

This research suggests some methods the police could use. One is ‘hot spot policing’, which:

…focuses on problem areas, even down to specific city blocks, with disproportionate levels of crime and violence. Police departments send officers to these places with a goal of deterring further disorder. In some versions of this approach, police don’t even have to take action against people on the block, focusing on surveillance instead. The idea is that the mere presence of police should prevent people from committing crimes — a sort of scarecrow effect.

A 2019 review in the Journal of Experimental Criminology looked atdozens of studies and found hot spot policing reduced crime without merely displacing it to other areas, and, in fact, there was evidence of “diffusion” in which crime-fighting benefits actually spread to surrounding areas. The review relied on several strong studies, including randomized controlled trials (generally the gold standard of research), suggesting that the findings were based on solid ground.

Another method is ‘problem-oriented policing,’ which:

…homes in on a chronic issue — say, shootings in a community — and brings together local resources and agencies, beyond the police, to address that problem. This uses a “scanning, analysis, response, assessment” model, also known as “SARA,” that detects the problem, analyzes the solutions, executes a response, and evaluates those efforts to iterate on them. The goal is not just to treat the problem in the short term but hopefully cure it in the longer term. Depending on the specific problem and the ensuing analysis, police might play a major role or more of a supplementary one.

A 2020 review of the evidence from the Campbell Collaboration, which conducts policy research reviews, estimated that problem-oriented policing produces a nearly 34 percent reduction in crime and disorder relative to control groups. This was based on a few fairly strong studies, including randomized controlled trials — suggesting the research base here is, like hot spot policing, on strong footing.

Another method, with some caveats regarding the research methods, is ‘focused deterrence’:

With this strategy, police focus on specific individuals and organizations, particularly gangs, and deliver a clear message: You must stop engaging in violent or criminal activity, and the community will provide resources to make that easier, or the police will come down on you with serious charges. As part of this, the police tend to partner with other groups in and out of government to provide a carrot — job training, education, government benefits, and so on — to help people get out of a criminal life along with a stick in the threat of punishment. Both the carrot and stick, experts said, are crucial to the idea.

As a 2019 review of the evidence from the Campbell Collaboration found, the studies focused on deterrence are largely positive. The problem, the review cautioned, is these studies tend to be of lower quality — there still are no randomized controlled trials, as far as I can tell, on the strategy as a whole.

Given that lower-quality research in criminal justice tends to find more favorable results for the studied intervention, the results are promising but should be taken with some caution. “My personal view is we just don’t know if [focused deterrence] works,” Jennifer Doleac, director of the Justice Tech Lab, told me, acknowledging that other experts disagree.

There is more depth and nuance to the findings of this body of research than I can do justice to here. Suffice it to say that the current push to ‘defund’ the Minneapolis police is likely to have very negative consequences.