Researchers tracked use-of-force incidents, civilian complaints, charging decisions and other outcomes to see if the cameras changed behavior. But to researchers’ surprise, the effects were too small to be statistically significant.

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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 city government, and Anita Ravishankar at Washington’s Metropolitan Police Department.

After the public uprising in response to the 2014 police shooting of Michael Brown in Ferguson, Missouri, advocates and many police officials turned to cameras as a way to reduce violent encounters and build trust. By 2015, 95 percent of large police departments reported they were using body cameras or had committed to doing so in the near future, according to a national survey.

The cameras provide an independent, if sometimes ambiguous, record of police-civilian encounters.

Efforts to equip Seattle officers with body cameras have stalled.

U.S. District Judge James Robart, who is overseeing reform of the Seattle Police Department, approved the department’s body-camera policy in May, but the issue soon became entangled in contract discussions between the city and police union.

In July, a month after the police shooting of Charleena Lyles, then-Mayor Ed Murray issued an executive order directing the Seattle Police Department to equip patrol officers with body cameras. He expressed frustration over what he described as stalled negotiations on the issue.

About a week later, the Seattle Police Officers’ Guild, which represents about 1,300 officers and sergeants, filed an unfair-labor-practice complaint with the state of Washington’s employee-relations commission.

The two parties are in the process of scheduling a hearing with the commission, and it could be months before the commission makes a decision.

Until now, the most commonly cited study on police body cameras had suggested that cameras did indeed have a calming effect. That experiment took place in 2012 in Rialto, Calif., where officers were randomly assigned cameras based on their shifts. Over a year, shifts that included cameras experienced half as many use-of-force incidents (including the use of a police baton, Taser or gun) as those shifts without cameras. The number of complaints filed by civilians against officers also declined — a stunning 90 percent compared with the previous year.

The Rialto study had a big impact in policing. Axon (formerly known as Taser International) has sold more than 300,000 police cameras worldwide and cites the Rialto study on its website. A federal district judge also cited the study in 2013 when she ordered the New York City Police Department to conduct a yearlong pilot program using body cameras. (Results are due out this spring.)

But the Rialto experiment featured just 54 officers, compared with more than 2,000 in Washington, D.C. Officers in Washington captured five times as many hours of video. The larger sample size and the long-term way the cameras were assigned added to the reliability of the Washington results.

“This is the most important empirical study on the impact of police body-worn cameras to date,” said Harlan Yu from Upturn, a Washington, D.C., nonprofit consulting company that studies how technology affects social issues. It was not directly involved in the research. “The results call into question whether police departments should be adopting body-worn cameras, given their high cost.”

Chief Peter Newsham of the Metropolitan Police Department in Washington, D.C., said the results were surprising. “I thought it would have a difference on police and civilian behavior,” he said. “Particularly for officers — and this is the exception — who might be more inclined to misbehave.”

But Newsham said the cameras had a number of benefits not easily measured: more accurate investigations, better training and at least one case in which the footage exonerated an officer accused of shooting an unarmed suspect (who was in fact armed). Most important, he said, they bolstered the trust of the community. “You can’t underestimate the value these cameras bring to that,” he said.

Behavior modification has never been the sole argument for body cameras. Their most important function may be to create an independent record of police shootings and other encounters with the public. But in some of those areas, too, videos have proved ambiguous: In the courtroom, for example, they have repeatedly failed to convince juries.

Though body cameras are in greater use, their purpose is often left undefined, raising thorny questions about surveillance, privacy and other issues. “Police departments have been rushing to body cameras without sufficiently deciding what the goal is,” said Seth Stoughton, a former officer and a law professor at the University of South Carolina, who has studied the devices extensively. “When no one is sure what it is supposed to do, no one knows if it is working.”

So why didn’t the cameras change officer behavior?

One hypothesis is that officers got used to the cameras and became desensitized to them. But the researchers saw no difference in behavior during the initial phase, when the cameras were new. Another possibility is that officers without cameras were acting like officers with cameras, simply because they knew other officers had the devices.

An equally plausible explanation has to do with fear: In Washington, D.C., police officers are instructed to turn on their cameras whenever they answer a call or encounter the public in a law-enforcement context. The kinds of situations that might lead to civilian complaints or use-of-force incidents are high-stress encounters. When frightened, humans tend to act on automatic fear responses (or, in the case of good police officers in an ideal world, training).

“It’s a lot to ask, psychologically speaking, to not only remember the camera is on but to moderate your behavior,” said Yokum, the head of the Lab @ DC.