Measuring the Effects of ShotSpotter on Gunfire in St. Louis County, MO

By Jillian Carr

Gunshot detection systems are used by policing agencies across the U.S. to detect incidents of firearms discharge. The most commonly used gunshot detection system is ShotSpotter, currently deployed in more than 100 cities across the U.S.1 Such systems typically use sensors, placed strategically throughout a particular area, to pinpoint the location of gunfire. Agencies that adopt the technology hope that it can help them reduce gun violence and make communities safer. They report that the technology can alert officers to gunshots that otherwise would not be reported, and that it can reduce officer response times by directing them to a more precise location. But the technology can be quite expensive—costing several hundred thousand dollars or more per year to maintain.

We found that in areas of St. Louis County that used ShotSpotter technology, police were alerted to four times as many gunshot incidents during the study period than in comparable areas without the technology. Despite responding to more calls related to gunfire, we found that reported assaults, which include gun-related assaults, fell by roughly 30 percent in areas with ShotSpotter. Moreover, the technology did not produce changes in the number or pattern of arrests. Because we did not find racially disparate effects on the prevalence of crime reporting or arrests from the adoption of the technology, ShotSpotter’s social costs appear minimal.

New York: Policing Project at New York University School of Law, 2021. 18p.