Why do gun murders have a higher clearance rate than gunshot assaults?

• Date: 01 August 2019
• Published date: 01 August 2019
• Author: Lisa M. Barao,Brandon S. Turchan,Philip J. Cook,Anthony A. Braga
• DOI: http://doi.org/10.1111/1745-9133.12451

DOI: 10.1111/1745-9133.12451

RESEARCH ARTICLE

SHOOTING CLEARANCE RATES

Why do gun murders have a higher clearance rate

than gunshot assaults?

Philip J. Cook1Anthony A. Braga2Brandon S. Turchan2

Lisa M. Barao2

1Duke University

2Northeastern University

Correspondence

PhilipJ. Cook, Sanford School of Public Policy,

DukeUniversity, 218 Rubenstein Hall, Box

90312,Durham, NC 27708-0245.

Email:pcook@duke.edu

Thisresearch was supported by funds pro-

videdby the U.S. Bureau of Justice Assis-

tance (Award#2011-DB-BX-0014) and the

Rappaport Institute forGreater Boston. We

wouldlike to thank Boston Mayor Martin

Walsh,Boston Police Commissioner William

Gross,former Boston Police Commissioner

WilliamEvans, Deputy Chief of Staff Desiree

Dusseault,Super intendent-in-Chief Gregory

Long,Capt ain DetectiveJames Hasson, Lieu-

tenantDetective Darr in Greeley,Sergeant

DetectiveCatherine Doher ty,and the Boston

PoliceDepar tmenthomicide unit and district

detectivesfor their valuable assistance in the

completionof this research. We would also like

tot hank John MacDonald and Greg Ridgeway

fortheir helpful comments on an earlier draft of

this article. Points of viewin this document are

those of the authors and do not necessarily rep-

resentt he official position of the U.S.Bureau

ofJustice Assistance, City of Boston, or the

BostonPolice Department.

Research Summary: The prevailing view is that follow-

up investigations are of limited value as crimes are primar-

ily cleared by patrol officers making on-scene arrests and

through the presence of eyewitnesses and forensic evidence

at the initial crime scene. We use a quasi-experimental

design to compare investigativeresources invested in clear-

ing gun homicide cases relative to nonfatal gun assaults in

Boston. We find the large gap in clearances (43% for gun

murders vs. 19% for nonfatal gun assaults) is primarily a

result of sustained investigative effort in homicide cases

made after the first 2 days.

Policy Implications: Police departments should invest

additional resources in the investigation of nonfatal gun

assaults. When additional investigative effort is expended,

law enforcement improves its success in gaining the coop-

eration of key witnesses and increases the amount of foren-

sic evidence collected and analyzed. In turn, the capacity

of the police to hold violent gun offenders accountable,

deliver justice to victims, and prevent future gun attacks is

enhanced.

KEYWORDS

clearance, detectives, homicide, investigations,shootings

Fatal and nonfatal gunshot assaults are similar with respect to victim characteristics and circumstances,

yet the clearance-by-arrest rates tend to be considerably higher for fatal cases. One plausible reason

for this disparity is that police departments give higher priority to the fatal cases, which in larger cities

are assigned to homicide detective squads. In comparison with the detectives who are assigned aggra-

vated assault cases, homicide detectives tend to have a lighter case load and greater access to forensic

Criminology & Public Policy. 2019;18:525–551. wileyonlinelibrary.com/journal/capp © 2019 American Society of Criminology 525

526 COOK ET AL.

evidence. A second possibility is witnesses are more inclined to cooperate with a police investigation

if the victim dies.1

The followingcontrast between police investigations of fatal and nonfatal gunshot cases is potentially

useful both for basic research on police science and for informing policy:

• From a research perspective, the contrast provides evidence on the extent to whicht he success rates

of police investigations of serious violent crimes may be affected bythe resources expended in these

investigations.

• From a policy perspective, the contrast provides a basis for assessing the value of increasing police

resources allocated to investigation of gun violence, in particular, fornonfatal gunshot cases. For the

prevention of future gun attacks, clearing nonfatal gunshot assault cases is as important as clearing

homicides.

In this article, we use 5 years of data (2010–2014) from the Boston Police Department (BPD) that

include all gun homicides and a sample of criminal cases in which the victim survived a gunshot

wound. In particular, we analyze the 204 shooting cases that included at least one homicide, as well

as a representative sample of 231 shooting cases in which no one died. Detailed data were collected

from police investigation files, forensicevidence databases, and interviews with detectives. Data items

included information on the circumstances and medical outcome of the shooting, characteristics of the

victim, measures of the amount of evidence collected during the investigation bythe BPD, and detective

reports of the key reasons for the success or failure of the investigation.Our quasi-experimental analysis

of these data results in the following findings relevant to the research and policy:

• The two groups of cases, fatal and nonfatal, are statistically indistinguishable with respect to cir-

cumstances, with the sole exception of whether the shooting was indoors or outdoors. The former is

more lethal.

• The clearance-by-arrest rate for gun homicide cases wasmore than twice as high as t he corresponding

rate for gunshot assault cases (43% and 19%, respectively). That difference would possibly be still

larger except forthe fact that homicide ar rests(but not assault cases) were subject to prior review for

probable cause by the district attorney.

• For both fatal and nonfatal cases, the likelihood of arrest is higher for cases involving personal dis-

putes or domestic violence than for cases arising from gang- and drug-related disputes. But regardless

of circumstance, fatal cases have a higher clearance rate compared with nonfatal.

• The arrest rates during the 2 days immediately following the shooting, however, are the same for fatal

and nonfatal cases, 11%. The fatal–nonfatal difference in arrest rates emergesin the much higher rate

of homicide arrests that occur subsequently, sometimes months after the shooting. We tentatively

interpret the equal success rate during the first 2 days as reflecting the importance of a latent dimen-

sion of shooting cases, which we label the “intrinsic difficulty of solution.” Easy cases are solved

quickly and without the need for the extra resources deployed in homicide investigations. Given the

similarity in fatal and nonfatal case distributions, we expect that the prevalence of easy cases would

be about the same in homicides as in nonfatal cases.

• As in other cities, BPD’s homicide unit has lighter caseloads and priority access to the crime lab and

other units of the police force. Based on a variety of indicators, the initial crime-scene investigations

yielded more evidence of various sorts. The commitment of additional resources to homicide cases

is also evident from comparing the amount of evidence collected outside of the crime scene.

• Most successful investigations have as one key source of evidence the information provided by a

cooperating eyewitness. Excluding exceptional clearances, that source was named as the key in