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. (The researchers also checked the data to make sure officers were turning their cameras on when they were supposed to, and found a very high level of compliance.) 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, 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 Mr. Yokum, the head of the Lab @ DC.
Finally, cameras may have had less impact in Washington, D.C., because the police department there has already had to confront excessive-force problems. After a devastating 1998 Washington Post series revealed that the city’s police department had shot and killed more people per resident in the 1990s than any other police force in a large American city, the Department of Justice entered into a memorandum of agreement with D.C. to reform its policing.
“We went through a transformation with regard to use of force when Justice came in here,” Chief Newsham said.
Cities that lack such accountability in their police culture may find cameras more effective, under this theory. (The Rialto Police Department had been reeling from a series of scandals when the Rialto study showed a large impact from cameras.)
Even if cameras do not reduce violent encounters, they can still offer other kinds of benefits: for training, or to hold a rogue officer accountable after the fact.
To Chief Newsham, the cameras’ primary benefit is to improve relations with the community. “The transparency and trust that the community has, knowing your department is recording the interactions, I don’t think you can undervalue that,” he said.