14 June 2023

"Warriors and Vigilantes as Police Officers: Evidence from a Field Experiment with Body Cameras in Rio de Janeiro", Cambridge Journal of Evidence-Based Policing

By Beatriz Magaloni, Vanessa Melo & Gustavo Robles


Director of PovGov, Professor Beatriz Magaloni, was awarded the prestigious Stockholm Criminology Prize 2023 by H.M. Queen Sylvia of Sweden. The jury cited the recently published paper "Warriors and Vigilantes as Police Officers: Evidence from a Field Experiment with Body Cameras in Rio de Janeiro" as a major contribution to the field of Criminology and as a key example of why Professor Magaloni was awarded the equivalent of the Nobel in the field. 

Research Question

Can requiring police to wear body-worn cameras (BWC) on duty restrain police misconduct in contexts such as a favela in Rio de Janeiro, where police use militaristic and highly aggressive tactics?


We collected quantitative and qualitative data on a wide range of behaviors, including police wearing BWC, turning on the BWC for recording citizen contacts, use of force by and against police officers, stop and search, responding to citizen requests for police assistance, and police supervisors wearing BWC. A total of 857 different police officers were tracked during the 1-year study, with a mean of 470 officers each month participating in the test of BWC across 52,000 officer shifts.


BWC status was randomly assigned by shifts to all officers in the shift, within five different kinds of police units. Analyses focused on intent-to-treat effects, with high compliance of wearing BWC but less than half of measured encounters recorded. Regression analyses provided estimates of different effects for officers who had previously been injured or had injured civilians.


Camera assignment, regardless of whether police turned cameras on, reduced stop-and-searches and other forms of potentially aggressive interactions with civilians. Cameras also produced a strong de-policing effect: police wearing cameras were significantly less likely to engage in any activity, including responding to calls and dispatch and street requests for help. These changes in police behavior occurred even when in 50% of the registered interactions with civilians, officers disobeyed the protocol that required them to turn their cameras on. Yet when officers’ supervisors wore cameras, policing activities and camera usage increased. Police surveys, interviews, and focus groups strengthen the findings.


The potential of BWC to reduce police abuse finds limitations where an organizational culture that perpetuates a lack of compliance with internal protocols and violence persists.

Magaloni, B., Melo, V. & Robles, G. Warriors and Vigilantes as Police Officers: Evidence from a Field Experiment with Body Cameras in Rio de Janeiro. Camb J Evid Based Polic 7, 2 (2023).