Militarization fails to enhance police safety or reduce
crime but may harm police reputation
Jonathan Mummoloa,b,1
a

Department of Politics, Princeton University, Princeton, NJ 08544; and b Woodrow Wilson School of Public and International Affairs, Princeton University,
Princeton, NJ 08544

The increasingly visible presence of heavily armed police units in
American communities has stoked widespread concern over the
militarization of local law enforcement. Advocates claim militarized policing protects officers and deters violent crime, while
critics allege these tactics are targeted at racial minorities and
erode trust in law enforcement. Using a rare geocoded census
of SWAT team deployments from Maryland, I show that militarized police units are more often deployed in communities with
large shares of African American residents, even after controlling for local crime rates. Further, using nationwide panel data on
local police militarization, I demonstrate that militarized policing
fails to enhance officer safety or reduce local crime. Finally, using
survey experiments—one of which includes a large oversample
of African American respondents—I show that seeing militarized
police in news reports may diminish police reputation in the mass
public. In the case of militarized policing, the results suggest that
the often-cited trade-off between public safety and civil liberties
is a false choice.
police militarization   public safety   crime   race and policing  
bureaucratic reputation

A

s thousands marched in Ferguson, MO to protest the police
shooting of Michael Brown in 2014, many Americans were
surprised and alarmed by the character of law enforcement’s
response. For days, national news networks broadcast images
of armored vehicles, snipers taking aim at unarmed Black and
brown civilians, and officers clad in battle armor, deployed by
state and local police agencies (1).
To some people, American police appeared to have suddenly
transformed into a wartime occupying force. But to scholars of
race and policing, and perhaps to many citizens of color, these
images were less surprising. More than half a century earlier,
James Baldwin described urban police as “occupying forces” in
Black communities (2). And decades of research in the intervening years have documented the ways in which policing efforts like
“stop and frisk” and the “war on drugs” have served to maintain
race- and class-based social hierarchies (3–6). In part due to this
history, critical race scholars have characterized police militarization as another means by which the state exercises social control
over racial minorities (7).
But despite a prolonged and vigorous national debate, there
is little systematic evidence demonstrating the consequences of
militarized police tactics or whether they are more prevalent in
communities of color. Because of heterogeneity in the way thousands of local law enforcement agencies in the United States
document the presence and activities of their militarized units
(if they document them at all), the study of police militarization
has been hampered by data constraints (8, 9). In the absence
of scientific analysis, the arguments of both advocates and critics are largely informed by anecdotal and journalistic accounts.
Proponents argue that militarized police units enhance officer
safety and deter violent crime (10), while critics allege that these
tactics are disproportionately applied in the policing of racial
minorities (11–13), potentially eroding the already-anemic levels
of trust between citizens and law enforcement in highly policed
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communities (14). The implications of police militarization for
civil rights, public safety, and the exercise of state power depend
crucially on the empirical validity of these claims.
This study leverages previously unavailable data to describe
the communities affected by militarized policing and to estimate its effects on crime, officer safety, and public perceptions of
police. I first use a rare census of “special weapons and tactics”
(SWAT) team deployments in Maryland to characterize the ways
in which militarized police units are used and the characteristics
of the communities in which they deploy. I show that militarized
police units are more often deployed in communities with high
concentrations of African Americans, a relationship that holds
at multiple levels of geography and even after controlling for
social indicators including crime rates. I then use an original
nationwide panel measuring the presence of active SWAT teams
in roughly 9,000 US law enforcement agencies, as well as the
Maryland SWAT deployment data, to test whether militarized
policing lowers crime rates and promotes officer safety. Using
within-agency comparisons that greatly mitigate concerns over
omitted variable bias, I find no evidence that obtaining or deploying a SWAT team reduces local crime rates or lowers the rates at
which officers are killed or assaulted.
Finally, using survey experiments that randomly assign images
of police officers in news reports, I show that seeing more
militarized officers—relative to traditionally equipped police—
can inflate perceptions of crime and depress support for police
Significance
National debates over heavy-handed police tactics, including
so-called “militarized” policing, are often framed as a tradeoff between civil liberties and public safety, but the costs
and benefits of controversial police practices remain unclear
due to data limitations. Using an array of administrative
data sources and original experiments I show that militarized
“special weapons and tactics” (SWAT) teams are more often
deployed in communities of color, and—contrary to claims
by police administrators—provide no detectable benefits in
terms of officer safety or violent crime reduction, on average.
However, survey experiments suggest that seeing militarized
police in news reports erodes opinion toward law enforcement. Taken together, these findings suggest that curtailing
militarized policing may be in the interest of both police and
citizens.
Author contributions: J.M. designed research, performed research, analyzed data, and
wrote the paper.
The author declares no conflict of interest.
This article is a PNAS Direct Submission.
This open access article is distributed under Creative Commons AttributionNonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Data deposition: All replication materials have been deposited in Harvard Dataverse
(https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/VYPUVC).
1

Email: jmummolo@princeton.edu.y

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1805161115/-/DCSupplemental.

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Edited by John Hagan, Northwestern University, Evanston, IL, and approved July 2, 2018 (received for review March 24, 2018)

 funding and presence. This analysis includes a large oversample
of African American respondents—an important feature given
the high rate at which militarized police units deploy in Black
neighborhoods.
On average, militarized police units do not appear to provide the safety benefits that many police administrators claim.
And police may suffer reputational damage when they deploy
militarized units. These results suggest that the often-cited tradeoff between public safety and civil liberties is, in the case of
militarized policing, a false choice.
Defining Police Militarization
Police militarization is a continuum defined by a combination of
equipment, tactics, and culture that centers on violent conflict (7,
15, 16). In recent decades, local police agencies have militarized
their departments to varying degrees, adopting weapons, attire,
tactics, and organizational structures developed for theaters of
war. The proliferation of militarized policing is due in part to
an expansion of the war on drugs and federal initiatives that
supplied localities with excess military equipment and funds to
purchase arms (17, 18). Heterogeneity in agency capacity makes
it difficult to precisely code police agencies as “militarized” or
not. Recently publicized data on military gear disbursements
have been used in some studies to estimate the effects of militarization on police violence, crime, and officer safety (19–21).
But these data convey only the receipt of equipment from one of
several programs that help supply agencies with militarized gear
(22). The data also appear incomplete (see SI Appendix, section
1A for details).
As an alternative approach, this paper analyzes the effects of
a substantively important threshold on the militarization continuum: the use of SWAT teams. Both popular and scholarly
debates over police militarization have focused on the activities of SWAT teams, their pronounced role in conducting the
drug war (17), and their high-profile crowd-control efforts (23).
SWAT teams often receive advanced combat training and exhibit
a command structure modeled on military special forces units
(15). In general the formation of a SWAT team represents a
heightened commitment to the use of militarized equipment and
tactics.
While it is plausible that SWAT deployments deter violent
crime—and, in turn, improve officer safety—previous research
on other heavy-handed tactics, including some that were much
more widely applied such as stop and frisk, has found little evidence of resulting crime reductions (24–26). It is therefore crucial to empirically evaluate the assumed benefits of militarized
policing.
Data and Methods
To characterize where and why SWAT teams deploy, I obtained
data on every SWAT deployment in the state of Maryland over a
5-y period via a public records request. These data exist because
of an unusual statute requiring every Maryland agency to uniformly record all SWAT activity. Because the statute has since
sunset, the data represent a rare, complete accounting of militarized police units’ activities and contain the date, postal zip
code, and agency of each SWAT deployment between FY2010
and FY2014, as well as the reasons for and outcomes of each
deployment (n ≈ 8,200 deployments).
To estimate the effects of police militarization on crime and
officer safety, I use a nationwide panel measuring the presence of active SWAT teams. I generated the national panel by
merging (27) three waves of the federal Census of State and
Local Law Enforcement Agencies (CSLLEA) surveys—which
measure whether agencies supply SWAT services—with FBI
data on violent crimes and the FBI’s Law Enforcement Officers Killed and Assaulted (LEOKA) database. The resulting
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panel includes roughly 9,000 agencies, each observed in 2000,
2004, and 2008. About 29% of agencies vary on SWAT status during this period (SI Appendix, Table S2). I then use an
agency-month panel in Maryland to test whether the increased
deployment of SWAT teams affects crime and officer safety
outcomes.
To estimate the effects of militarized policing on public
perceptions of law enforcement, I conducted two survey experiments: one using a convenience sample from Amazon’s Mechanical Turk (n= 1,566) and one conducted by Survey Sampling
International (SSI) (n= 4,465). (Survey experiments were
approved by Stanford University’s Institutional Review Board,
protocol no. 32534. All respondents supplied informed consent.)
These experiments provided brief news articles accompanied
by a randomly assigned image that conveyed different levels
of militarization, allowing for an estimate of the causal effect
of seeing militarized police on attitudes toward law enforcement relative to seeing more traditionally equipped police forces.
Because of the prevalence of SWAT deployments in Black communities, the SSI data include an oversample of roughly 1,850
African Americans to test whether treatment effects varied with
respondent race.
Where and Why SWAT Teams Deploy
The rare census of SWAT activity available in Maryland offers
a valuable opportunity to study the stated reasons for, and geographic correlates of, militarized policing. Maryland also exhibits
large variation in the racial composition of localities [e.g., the
percentage of Blacks in zip-code tabulation areas (ZCTAs)
ranges from 0% to more than 95%], allowing for an evaluation
of the relationship between militarized tactics and neighborhood
racial composition with minimal extrapolation.
Before examining that relationship, Table 1, generated using
the pooled Maryland SWAT census, first displays the reasons
SWAT teams are deployed. Table 1 shows that roughly 90%
of SWAT deployments in that state over 5 fiscal years were
conducted to serve search warrants. Previous work has shown
that the use of SWAT teams to serve warrants, a practice
which escalated as a result of the war on drugs (17), is an
extremely disruptive event in the lives of citizens and often
involves percussive grenades, battering rams, substantial property damage, and in rare cases deadly altercations stemming
from citizens’ mistaken belief that they are experiencing a home
invasion (28, 29). Table 1 also shows that less than 5% of
deployments involved a “barricade” scenario, which typically
involves an armed suspect refusing to surrender to police. Violence to people and animals is rare, and gun shots are fired
1.2% of the time—roughly 100 deployments during this period.
While the data suggest that indiscriminate violence is less common than some anecdotal reports suggest, they also show that
the vast majority of SWAT deployments occur in connection
with nonemergency scenarios, predominately to serve search
warrants.
Table 1. SWAT deployments, Maryland FY2010–2014
Reason, legal authority
Search warrant
Barricade
Other
Arrest warrant
Exigent circumstances

%

Outcome

91.06
4.92
2.67
0.89
0.45

Property taken
Forcible entry
Arrest
Shot fired
Person injured
Officer injured
Animal killed
Animal injured
Person killed

%
84.38
68.36
63.69
1.20
1.15
0.42
0.17
0.12
0.11

Mummolo

 Critics allege that militarized tactics are more often applied
in communities of color, a pattern that would be in line with
decades of evidence indicating disparate treatment of racial
minorities by police (4, 6, 30). To test this claim, Fig. 1 shows
the relationship between the percentage of Black residents in
a geographic unit and the volume of SWAT deployments per
100,000 residents in that unit during FY2010–2014, both logged.
Fig. 1, Top Left and Top Right is generated using ZCTAs in
Maryland and shows a strong positive correlation, even after
controlling for local unemployment, education, and household
income levels (Fig. 1, Top Right). Fig. 1, Middle Left and Middle Right is generated using data from three large agencies—the
Baltimore City Police Department and the Prince George’s
County and Montgomery County Police Departments—which
publish incident-level crime data that can be mapped to ZCTAs.
Even after controlling for the aforementioned social indicators
and local crime rates (Fig. 1, Middle Right), the percentage of
Blacks still strongly predicts the volume of SWAT activity in a
ZCTA. The slope in Fig. 1, Middle Right (β = 1.05, P < 0.001;
SI Appendix, Tables S8 and S9) implies that a 10% increase in
the percentage of Blacks in a ZCTA is associated with a 10.53%
increase in SWAT deployments per 100,000 residents during this
period. Fig. 1, Bottom Left and Bottom Right shows a similar pattern at the county level, although the slope is not statistically
significant in the county model with controls. This analysis comes
from only one state, so caution is warranted when generalizing beyond Maryland. We also cannot confidently infer a causal
relationship from these cross-sectional comparisons. However,
these results are consistent with the descriptive claim that Black
residents face a pronounced risk of experiencing militarized
policing.
Effects on Crime and Officer Safety
Proponents claim that militarized tactics deter violent crime and
protect police (10). To test these claims, I use a nationwide panel
Mummolo

Fig. 2. Obtaining a SWAT team does not reduce crime or improve officer safety. Shown are coefficients from regressions estimating the effect of
obtaining a SWAT team on crime and officer safety. Outcomes are logged.
Bars are 95% confidence intervals.

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Fig. 1. SWAT deployments are more frequent in areas with high concentrations of African Americans. (Top row) Maryland ZCTAs. (Middle row)
ZCTAs from three large Maryland agencies. (Bottom row) Maryland counties. Left column shows bivariate relationships. Right column controls for
social indicators in corresponding geographic unit. Locally weighted and
linear regression fits are shown. Variables are plotted on log scales.

to estimate the effects of acquiring a SWAT team on violent
crime and officer fatalities and assaults and an agency-month
panel in Maryland to test whether the increased deployment
of SWAT teams affects these outcomes. I focus on violent
crime because of the asserted link between militarized policing
and public safety. To gauge robustness, I estimate two models per outcome: a generalized difference-in-differences (DID)
estimator containing agency and time period fixed effects and
a second model including fixed effects but also allowing each
agency to trend uniquely over time. By comparing agencies to
themselves over time, both approaches greatly mitigate concerns
over omitted variable bias that plague cross-sectional comparisons (31). All outcomes are logged to reduce the influence of
extreme values (see SI Appendix, sections 2–4 for alternative
specifications).
Fig. 2 displays estimates of the effect of obtaining a SWAT
team on violent crime and officer safety measures. The top
coefficient implies a statistically significant 6.5% increase in
within-agency violent crimes, on average. This result is consistent with anecdotal evidence of suspects reacting violently to
SWAT teams (29) or with militarized policing lowering trust in
police, thereby hindering criminal investigations and promoting
crime (14). However, once we allow agencies to trend differentially over time, the effect shrinks to a statistically insignificant
4.3%. Estimates for officer deaths, both accidental and felonious,
are precise and near zero, partly because they are so rare (i.e.,
there is little variation in these outcomes to explain). Estimates
for assaults are noisier, and one shows a statistically significant 3.2% increase in noninjurious assaults, although this result
misses statistical significance in the time trends specification. In
sum, estimated effects are generally positive and often indistinguishable from zero, and there is no evidence that acquiring a
SWAT team lowers crime or promotes officer safety.

 image randomly varied across respondents (Fig. 4). Images were
randomly assigned within racial groups of respondents in the second survey (which contained an oversample of African American
respondents), with equal probability. The control image (Fig. 4A)
featured five male, traditionally uniformed officers (e.g., blue
uniforms, brimmed caps, and standard side arms). The “low”
militarization conditions (Fig. 4B and Fig. 4C) showed five male
officers with “riot gear” and batons or with body armor and
assault rifles. The “high” militarization condition (Fig. 4D) added
an armored vehicle. (Because it elicited very similar treatment
effects to the other low militarization image in Fig. 4C, the Fig.
4B image was dropped from the SSI survey to enhance statistical
power.)
In each case, the caption beneath the photo read, “Above: Five
city police officers stand guard during a local protest.” All images
were tightly cropped to ensure that any differences in responses
were due to the appearance of police officers and not the surrounding area in which they were deployed. Following the article,
respondents answered questions measuring perceived crime levels, support for police spending, and confidence in police. All
images and text were borrowed or adapted from real online news
content (32–36).
Experimental Results. Fig. 5 shows average differences in

Fig. 3. SWAT team deployments exert no detectable effects on crime or
officer safety. Shown are coefficients on log(#SWAT Deployments + 1) from
panel regressions using data from Maryland, FY2010–2014. All outcomes are
logged. Bars are 95% confidence intervals.

Fig. 3 displays the effects of increased SWAT deployments in
Maryland on crime and officer safety. Fig. 3, Left shows results
from models using all deployments, summed within agency
months and logged, while Fig. 3, Right uses deployments only
for barricade situations to test whether using SWAT in emergencies might be especially effective. (I thank an anonymous
reviewer for this suggestion.) (Officer fatalities are omitted as
an outcome here because they almost never occur in these data;
SI Appendix, Table S7.) As Fig. 3 shows, point estimates are
negative but near zero, none are statistically significant, and all
estimates imply a 10% increase in deployments exerts changes
smaller than 1% in all outcomes. Some small declines in officer
assaults are detectable if the data are weighted by agency size (SI
Appendix, Table S35), suggesting the Maryland results are more
sensitive to model specification than the national analysis which
makes it difficult to draw firm conclusions. Using the available
data, the benefits of increased deployments appear to be either
small or nonexistent.
The adoption and deployment of SWAT teams is not randomly assigned, and we should therefore make causal interpretations with some caution (see SI Appendix, section 2B, for placebo
tests assessing the risk of endogeneity). The Maryland results
may also not generalize to other states. But the lack of any robust
association between militarized policing and public and officer
safety shown here calls the validity of these claimed benefits into
question.
Effects on Police Reputation
I conducted two survey experiments to estimate effects on public perceptions of police, the first one via Amazon’s Mechanical
Turk (M-Turk) and the second one via SSI. Respondents read a
mock news article concerning an unnamed police chief seeking a
budget increase. Text remained constant while the accompanying
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responses between each treatment condition and the control condition in the unweighted samples. All outcomes were
rescaled to range from 0 to 100 so effects can be interpreted as
percentage-point increases on each outcome’s respective scale.
Militarized images in the M-Turk survey caused clear increases
between roughly 8 points and 15 points in perceived levels of
crime in the vignette city. The high militarization condition in
the SSI survey caused a statistically significant 2.2-point increase
in the perceived level of crime in the vignette city and, strikingly,
a 3.2-point drop in respondents’ desire for more police patrols in
their own neighborhoods.
The high militarization treatment also caused support for
police funding in the United States to fall by roughly four points
in the M-Turk survey and two points in the SSI survey. Support

A

B

C

D

Fig. 4. Experimental manipulation [from Top Left (A) to Bottom Right
(D)]. Reprinted with permission from Reuters Pictures/Jessica Rinaldi, Shutterstock/JPL Designs, Associated Press/Jeff Roberson, and The New York
Times/Redux Pictures/Whitney Curtis.

Mummolo

 Fig. 5. Survey experimental results: seeing militarized police may tarnish
police reputation. Shown are effects of militarized images in M-Turk (blue)
and SSI (red) surveys. Bars are 95% confidence intervals.

for funding the department in the news article also fell. A
close reading and text analysis of open-ended responses suggest
that treated respondents were less supportive of police funding because militarized equipment gave the impression that the
agency was already well funded (see SI Appendix, Fig. S10 and
discussion in SI Appendix, section 2E).
Effects on confidence in police in the United States are generally negative but not statistically significant, although the high
militarization treatment in the SSI survey did lower confidence
in the police portrayed in the experimental vignette, suggesting
some of militarized images’ effects may be confined to the agency
in question and not law enforcement at large. The effects of
the high militarization treatment are also substantially smaller
in the SSI sample, and the low militarization treatment effects
from the M-Turk sample did not replicate, so we should be cautious about endorsing the large effect sizes from the M-Turk
survey. Despite these limitations, the SSI results generally replicate the M-Turk results in terms of direction, and all effects
appear after only a single brief exposure to militarized images.
Repeated exposure to similar news items over time could help to
cement negative views of law enforcement in the mass public.

Discussion and Conclusion
Aggressive policing strategies have historically been disproportionately applied to citizens of color in ways that serve to preserve
race- and class-based social hierarchies (3). The normalization of
militarized policing in the United States (15, 16) has raised concerns that a new, heavy-handed policing strategy is being used in
similar ways and is eroding public opinion toward law enforcement, but law enforcement administrators defend the tactics
claiming they can deter violent crime and protect police. This
study marshals an array of data sources and analytical techniques
to systematically evaluate these claims.
Consistent with anecdotal evidence (11), militarized police
units are more often deployed in areas with high concentrations
of African Americans, even after adjusting for local crime rates

Effects by Race of Respondent. Fig. 6 shows treatment effects in

the SSI survey estimated on non-Hispanic white and African
American respondents (with respondent race/ethnicity measured
via self-reports) separately, as well as the difference in effects.
In some cases the group effects have opposite signs, such as the
effects of high militarization on perceived crime in the vignette
city, but this difference is not statistically significant. In general,
these results reveal little evidence of heterogeneous responses.
Larger samples may allow for the detection of disparate effects,
but the small point estimates in Fig. 6 suggest that any differences
in effects are likely to be small in magnitude.
This relatively uniform pattern of response is surprising given
extensive prior work indicating disparate treatment of Black
Mummolo

Fig. 6. No heterogeneous effects by race of respondent. Bars are 95%
confidence intervals.

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Americans by police and depressed levels of trust in police
in communities of color. Indeed, in the control condition in
the SSI data, when asked how much confidence they had in
police in the United States, the mean response among Black
participants was 21 percentage points lower than among white
respondents (P < 0.001). Why then, do we observe such similar
effects among the two groups? One explanation is that different
mechanisms are operating within each group to produce effects
of similar magnitude. For example, white respondents may react
negatively to militarized images because they clash with their
baseline perception of law enforcement, while Black residents
may react negatively because they conjure memories of discrimination. These results are also consistent with previous work
showing that perceived discrimination is not well predicted by the
increased probability of exposure to racial hostility (37). Adjudicating between these explanations is beyond the scope of this
study. But what we learn from the present analysis is that militarized policing can impose reputational costs on law enforcement,
likely in unintended ways. This is troubling, since prior work
shows that negative views of police inhibit criminal investigations
and are associated with stunted civic participation (38).

 and other community traits. But I find no firm evidence that
SWAT teams lower an agency’s violent crime rate or the rates at
which officers are killed or assaulted. Using survey experiments,
I show that citizens react negatively to the appearance of militarized police units in news reports and become less willing to
fund police agencies and less supportive of having police patrols
in their own neighborhoods.
Given the concentration of deployments in communities of
color, where trust in law enforcement and government at large
is already depressed (14, 38), the routine use of militarized
police tactics by local agencies threatens to increase the historic tensions between marginalized groups and the state with

no detectable public safety benefit. While SWAT teams arguably
remain a necessary tool for violent emergency situations, restricting their use to those rare events may improve perceptions of
police with little or no safety loss.

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ACKNOWLEDGMENTS. I thank Justin Grimmer, Alexandra Blackman,
Courtenay Conrad, Ted Enamorado, Andrew Guess, Jens Hainmueller,
Andrew Hall, Dorothy Kronick, Amy Lerman, Neil Malhotra, Steve Mello, Tali
Mendelberg, Naomi Murukawa, Terry Moe, Erik Peterson, Jonathan Rodden,
Gary Segura, Mike Tomz, Sean Westwood, Vesla Weaver, Lauren Wright, and
Yiqing Xu for their guidance throughout this project. I thank Helene Wood,
Madeleine Marr, and Grace Masback for research assistance. This work was
funded by Stanford University, Princeton University, and National Science
Foundation Grant 15-571.

Mummolo