Rialto’s police officers also used force nearly 60 percent less often — in 25 instances, compared with 61. When force was used, it was twice as likely to have been applied by the officers who weren’t wearing cameras during that shift, the study found. And, lest skeptics think that the officers with cameras are selective about which encounters they record, Mr. Farrar noted that those officers who apply force while wearing a camera have always captured the incident on video.

As small as the cameras are, they seem to be noticeable to civilians, he said. “When you look at an officer,” he said, “it kind of sticks out.” Citizens have sometimes asked officers, “Hey, are you wearing a camera?” and the officers say they are, he reported.

But what about the privacy implications? Jay Stanley, a senior policy analyst at the American Civil Liberties Union, says: “We don’t like the networks of police-run video cameras that are being set up in an increasing number of cities. We don’t think the government should be watching over the population en masse.” But requiring police officers to wear video cameras is different, he says: “When it comes to the citizenry watching the government, we like that.”

Mr. Stanley says that all parties stand to benefit — the public is protected from police misconduct, and officers are protected from bogus complaints. “There are many police officers who’ve had a cloud fall over them because of an unfounded accusation of abuse,” he said. “Now police officers won’t have to worry so much about that kind of thing.”

Mr. Farrar says officers have told him of cases when citizens arrived at a Rialto police station to file a complaint and the supervisor was able to retrieve and play on the spot the video of what had transpired. “The individuals left the station with basically no other things to say and have never come back,” he said.

The A.C.L.U. does have a few concerns about possible misuse of the recordings. Mr. Stanley says civilians shouldn’t have to worry that a video will be leaked and show up on CNN. Nor would he approve of the police storing years of videos and then using them for other purposes, like trolling for crimes with which to charge civilians. He suggests policies specifying that the videos be deleted after a certain short period.