Since the death of George Floyd in 2020, communities across the United States have reexamined how their police officers use force. But there remain many open questions about the root causes of excessive use of force in police departments.

In a paper in the American Economic Journal: Economic Policy, authors Justin E. Holz, Roman G. Rivera, and Bocar A. Ba provide a better understanding of use of force by showing that, shortly after a police officer is injured on the job, the officer’s peers are more likely to resort to force when interacting with civilians. 

Their findings come from a difference-in-difference analysis based on administrative data from the Chicago Police Department, comparing officers who had a former academy classmate injured on the job to other officers who worked in the same district but did not have a former peer injured.

Figure 7 shows the likelihood that an officer uses force before and after a peer’s on-duty injury.

Figure 7 from Holz et al. (2023)

The y-axis indicates the propensity to use force, and the x-axis indicates the number of weeks relative to the injury of a former peer. The vertical dashed red line represents the time of the officer injury, and the vertical bars represent 95 percent confidence intervals.

The black circles show that there was no significant change in the propensity to use force among officers who did not experience a peer injury. However, the gray squares show an increased likelihood to use force in the weeks immediately following the injury of a peer, which then drops back down to baseline levels soon after. 

Overall, the authors estimate that on-duty injuries of a peer increased the probability of officers using force by 7 percent in the following week.

The researchers argue that this pattern of evidence suggests that an officer injury causes an emotional shock, leading to more aggressive policing and more dangerous interactions between officers and civilians. They say that such knock-on effects should be considered when evaluating policies that can lead to an increase in officer injuries, such as police militarization.