Usually, we behave better when we know we’re being watched. According to decades of research, the presence of other people, cameras or even just a picture of eyes seems to nudge us toward civility: We become more likely to give to charity, for example, and less likely to speed, steal or take more than our fair share of candy.

But what happens when the cameras are on the chests of police officers? The results of the largest, most rigorous study of police body cameras in the United States came out Friday morning, and they are surprising both police officers and researchers.

For seven months, just over a thousand Washington, D.C., police officers were randomly assigned cameras — and another thousand were not. Researchers tracked use-of-force incidents, civilian complaints, charging decisions and other outcomes to see if the cameras changed behavior. But on every metric, the effects were too small to be statistically significant. Officers with cameras used force and faced civilian complaints at about the same rates as officers without cameras.

“These results suggest we should recalibrate our expectations” of cameras’ ability to make a “large-scale behavioral change in policing, particularly in contexts similar to Washington, D.C.,” concluded the study, which was led by David Yokum at the Lab @ DC, a team of scientists embedded in D.C. government, and Anita Ravishankar at D.C.’s Metropolitan Police Department (M.P.D.).