Cameras worn on police uniforms have been lauded as a possible solution to many of the problems facing officers in the line of duty, from violence against law enforcement to the unnecessary use of force. The US Department of Justice recently announced a plan to spend $20 million on body cameras for cops in 32 states.

The cameras are controversial, as all surveillance technology tends to be. And until recently, there’s been little hard evidence about how effective body cameras actually are. According to new research from the University of Cambridge, which studied seven police forces in the US and the UK, the answer is that they are transformative in at least one way.

Researchers used complaints against police as a proxy for the effect of the cameras, hypothesizing that one major reason for complaints is that cops behaved in a negative, avoidable way. (There are other reasons for complaints, the researchers acknowledge, given the emotionally charged nature of many interactions with police.)

Compared to the previous year when cameras were not worn, complaints across the seven regions fell by 93% over the 12 months of the experiment. The study encompassed nearly 1.5 million officer hours across more than 4,000 shifts.

“I cannot think of any [other] single intervention in the history of policing that dramatically changed the way that officers behave, the way that suspects behave, and the way they interact with each other,” Barak Ariel, the lead researcher, told the BBC.

The theory is that cameras make police officers more accountable for their actions, because people tend to change their behavior when they believe they are being observed. At the same time, this also limits non-compliance from people with whom the police interact.

“It seems that knowing with sufficient certainty that our behavior is being observed or judged affects various social cognitive processes: We experience public self-awareness, become more prone to socially acceptable behavior, and sense a heightened need to cooperate with rules,” the researchers write.

They also noted that there was a reduction in the amount of complaints against officers who didn’t wear cameras but were working in the same forces among those who did. The researchers called this “contagious accountability.” All officers were acutely aware of being observed more closely, whether they were wearing a camera or not.

Correction: An earlier version of this article stated that complaints fell by 98%. It was in fact 93%.