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title: "Do More Police Reduce Crime? Evidence From Natural Experiments" abstract: "One of the central debates in criminal justice policy concerns whether increasing the number of police officers and patrol presence reduces crime. Observational studies are confounded by the fact that high-crime areas tend to have more police. Natural experiments — events that changed police presence for reasons unrelated to crime — have allowed economists and criminologists to identify causal effects. The evidence consistently finds that more police reduce violent crime, though the mechanisms and magnitude remain debated." topic: criminal-justice author: nonacademicresearch.org Editorial date: 2026-05-09

Do More Police Reduce Crime? Evidence From Natural Experiments

Abstract

One of the central debates in criminal justice policy concerns whether increasing the number of police officers and patrol presence reduces crime. Observational studies are confounded by the fact that high-crime areas tend to have more police. Natural experiments — events that changed police presence for reasons unrelated to crime — have allowed economists and criminologists to identify causal effects. The evidence consistently finds that more police reduce violent crime, though the mechanisms and magnitude remain debated.

Background

The simplest theory of policing and crime is deterrence: a potential offender calculates the probability of arrest, the severity of the resulting punishment, and the swiftness of its application. Increasing police presence raises the probability of arrest, increasing the cost of crime and deterring it. But this is not the only mechanism — police may also directly interrupt crimes in progress, improve community cooperation and information gathering, or affect crime through their relationship with community trust.

The measurement challenge is fundamental: cities with high crime rates tend to hire more police. This endogeneity makes it impossible to directly estimate the effect of police on crime from cross-sectional data — the correlation between police presence and crime levels would be positive even if police reduce crime, simply because more police are deployed where crime is highest.

Researchers have addressed this by identifying settings where police presence changed for reasons unrelated to crime — political decisions, terror alerts, natural experiments — and using these as proxies for the true causal effect.

The Evidence

Election Cycles and Police Hiring

Levitt (1997, American Economic Review) observed that police hiring in U.S. cities is sensitive to election cycles: mayors facing re-election tend to increase police hiring more than mayors not running. This gave him a way to identify exogenous variation in police levels. Using this instrument, he estimated that each additional police officer reduced the number of violent crimes by approximately 10 per year. The effect was smaller and less precisely estimated for property crime.

This study is widely cited but has faced methodological criticism related to the validity of the election cycle instrument. McCrary (2002, American Economic Review) challenged the approach, reducing confidence in the precise point estimates, though the qualitative finding of crime-reducing effects survived the critique.

Terror Alerts in Buenos Aires and Washington, D.C.

Two highly cited natural experiments exploit changes in police deployment triggered by terror threat levels — changes clearly unrelated to local crime conditions.

Draca et al. (2011, American Economic Review) analyzed changes in police deployment in central London following the July 2005 bombings. Additional officers were deployed to specific areas; crime in those areas fell during the period of enhanced deployment and rose when deployment returned to normal, with an estimated 15–20% reduction in crime during the peak of additional deployment.

Di Tella and Schargrodsky (2004, American Economic Review) used the deployment of police protection to Jewish institutions in Buenos Aires following a 1994 terrorist bombing as a natural experiment. Streets that received extra police had significantly lower vehicle thefts than adjacent streets. The estimates implied that each additional police car reduced vehicle thefts by approximately one theft every 22 days in the assigned block.

The Koper Curve: How Long Does Patrols Presence Need to Be?

Research on police patrol strategy has examined not just whether police presence deters crime but how it should be allocated. Koper (1995, Criminology) conducted an experimental study of police patrol duration in crime "hot spots" and found that patrol presence for approximately 15 minutes produced crime-reduction effects that persisted after officers left. Shorter patrols and longer patrols were less effective — a "Koper Curve" suggesting that presence needs to be long enough to be noticed but that continuous presence is not necessary.

Hot spots policing — concentrating patrol resources in small geographic areas with disproportionate crime — has since been evaluated in multiple randomized experiments. A meta-analysis by Weisburd et al. (2016, Journal of Experimental Criminology) found consistent crime-reducing effects without significant displacement of crime to adjacent areas.

Does More Police Presence Harm Community Trust?

A distinct question from crime reduction effects is whether increased police presence harms community trust and cooperation, particularly in communities with historical tensions with law enforcement. Ang et al. (2021, NBER Working Paper) found that in the United States, federally funded police grant programs increased violent crime clearance rates (arrests) but also increased use-of-force incidents, with the latter disproportionately affecting Black communities.

Desmond et al. (2016, American Journal of Sociology) found that publicized police violence incidents reduced 911 call rates in Milwaukee, suggesting that enforcement actions can reduce community willingness to cooperate with police — potentially undermining crime reduction effectiveness over time.

Counterarguments

Some criminologists argue that policing strategy matters more than the number of officers: community policing, procedural justice approaches (treating people fairly and respectfully), and problem-oriented policing focused on specific crime conditions may produce crime reduction without the trust costs of aggressive enforcement strategies.

The magnitude of police crime-reduction effects is also contested. Sherman (2013, Justice Quarterly) argues that the evidence supports concentrated deployment to hot spots as highly effective, with potential to reduce violent crime substantially with existing officer numbers if deployed optimally. This reframes the debate from "how many officers" to "how they are deployed."

What We Can Conclude

The evidence from natural experiments consistently finds that police presence reduces crime, particularly violent crime. The estimates vary but suggest that each additional officer generates meaningful reductions in violent crime — perhaps 10 to 15 incidents per officer per year in U.S. contexts.

The more nuanced evidence suggests that how police are deployed matters as much as how many there are. Hot spots policing has the strongest experimental evidence base. Aggressive enforcement strategies may generate short-term crime suppression at the cost of community trust and cooperation that undermines long-run effectiveness — an important trade-off that crime-focused metrics alone do not capture.

References

• Ang, D., et al. (2021). Police violence and public safety. NBER Working Paper 28202. https://www.nber.org/papers/w28202
• Desmond, M., Papachristos, A.V., & Kirk, D.S. (2016). Police violence and citizen crime reporting in the Black community. American Sociological Review, 81(5), 857–876. https://doi.org/10.1177/0003122416663494
• Di Tella, R., & Schargrodsky, E. (2004). Do police reduce crime? Estimates using the allocation of police forces after a terrorist attack. American Economic Review, 94(1), 115–133. https://doi.org/10.1257/000282804322970733
• Draca, M., et al. (2011). Panic on the streets of London: Police, crime, and the July 2005 terror attacks. American Economic Review, 101(5), 2157–2181. https://doi.org/10.1257/aer.101.5.2157
• Koper, C.S. (1995). Just enough police presence: Reducing crime and disorderly behavior by optimizing patrol time in crime hot spots. Justice Quarterly, 12(4), 649–672. https://doi.org/10.1080/07418829500096201
• Levitt, S.D. (1997). Using electoral cycles in police hiring to estimate the effect of police on crime. American Economic Review, 87(3), 270–290.
• McCrary, J. (2002). Using electoral cycles in police hiring to estimate the effect of police on crime: Comment. American Economic Review, 92(4), 1236–1243. https://doi.org/10.1257/00028280260344777
• Weisburd, D., et al. (2016). Does hot spots policing have displacement effects? A systematic review and meta-analysis. Journal of Experimental Criminology, 12(3), 295–328. https://doi.org/10.1007/s11292-016-9250-9