10 January 2020

Warriors and Vigilantes as Police Officers: Evidence from a Field Experiment with Body Cameras in Rio de Janeiro

By Beatriz Magaloni, Vanessa Melo, Gustavo Robles

This paper assesses the effects of body-worn cameras (BWC) on police behavior through a randomized control trial implemented in Rio de Janeiro. Results show that BWCs significantly reduced the use of lethal force and diminished the number of police written reports.


The excessive use of force by police officers is a pervasive problem in many democratic societies. One policy intended to decrease officers’ excessive use of force is the adoption of body-worn cameras (BWC) that can, among others, monitor police behavior and provide footage that could be used to sanction misconduct. BWCs have been adopted by many police departments in the US and in other countries. However, are BWCs effective at reducing unnecessary violence and increasing police compliance?


More than one-fifth of Brazil’s 2016 police killings occurred in Rio de Janeiro, where police killed close to 8,500 people in the past decade. The levels of officer-involved killings are associated with the militaristic approach of policing the favelas. In anticipation of the 2014 World Cup and the 2016 Olympics, the government of Rio de Janeiro instituted a far-reaching reform with the establishment and deployment of Pacifying Police Units (UPPs), a form of community-oriented policing. 

Details of the Intervention

The study was conducted between December 2015 and November 2016, and involved 470 police officers from several units in the UPP Rocinha. Within each type of unit, we randomly assigned units (with 3 to 7 police officers per shift) into treatment and control groups. Treated units received body-worn cameras according to different protocols. Control units were not assigned cameras. To make comparisons within groups, we reassigned units to treatment and control groups at different stages of the study. This allowed us to compare camera usage and the use of force between units and to compare officers within the same unit at different points in time.


  • Protocol compliance is a critical problem. Despite the fact that the random assignment of BWCs was strictly conducted, many police officers refused to turn on their cameras.
  • We find that BWCs had substantial effects on police officers’ behavior (excessive and necessary). 
  • BWCs inhibited police activity. Our results suggest that officers wearing BWCs reduced their regular policing activities and engaged in fewer interactions with the community, including stops-and-searches and other aggressive behaviors (slap in the face, frisk, hitting suspects) and arrests. 
  • When we randomly assigned cameras to supervisors, police increased the number of "occurrences" and their interactions with residents. 


The study generates an intriguing conclusion: BWCs effectively reduce police officers’ aggressive behaviors --and there is also an indication that community aggression toward the police also decreased -- even when protocol compliance (e.g., turning on the cameras during interactions with civilians) is low. 

  • Institutional Engagement. Gaining support from high-rank superiors is not enough to successfully implement and manage BWCs. Institutional engagement must be achieved at all strategic levels within the agency to improve BWC effectiveness.
  • Infrastructure. Police departments should anticipate the infrastructural requirements needed to host, operate, and maintain BWC equipment physically. Physical infrastructure entails but is not limited to dock stations, cameras, wires, appliances, and a reliable internet connection
  • Design of protocols. To effectively manage BWCs, police corporations must design and publicize in advance protocols for camera usage.

Footage Management. It is imperative to develop a protocol to manage the images collected by BWCs during different types of daily interactions.