The technology is "surely promising, but we don't know that it's working," Ariel told me. The Food and Drug Administration doesn't approve drugs until they've been studied extensively, he explained, and governments should take a similar approach with body-worn cameras. It's a solution that has yet to be proven.

Ariel should know. He's currently researching the effects of body cameras on policing everywhere from Brazil to Ghana to Israel to Northern Ireland, and finding that some police departments (and police unions) love the idea and others hate it. Nearly all of these tests have yet to be completed, but Ariel recently co-authored a study on the practice in Rialto, California, where he found that police officers who weren't wearing cameras were twice as likely to use force as those who were. During the 12-month experiment, the police department also saw a reduction in citizens' complaints compared with previous years. The researchers concluded that the benefits of wearing cameras trumped the costs.

But Ariel insists that there isn't enough evidence so far to generalize the finding and assert that body-worn cameras offer a net benefit to community policing. The Rialto report, for example, was the world's first randomized-controlled trial involving the technology.

Consider the costs, both calculable and incalculable. There's the hefty price tag for the cameras (around $800 to $1,000 each) and the systems to store and sift through the data they collect. Equipping frontline police officers across the U.S. with cameras, and paying the monthly subscription fees for their use and upkeep, could cost billions of dollars, Ariel estimated.

There are also privacy concerns—the fear, as one editorial in Canada's Globe and Mail recently expressed it, that cameras "could turn every officer into a mobile, closed-circuit camera, hooked up to a database tracking and recording people’s movements across the city." People could refrain from reporting incidents to the police because they don't want to appear on camera. Ariel is less worried about increased surveillance—"I don't think there's a single street in London that isn't covered by CCTV [closed-circuit television]," he points out—than about what data is stored and how that evidence is used. Those are questions that each country's legal system will have to sort out.

There is another potential cost to widespread adoption of body-worn cameras: The credibility of police testimony against defendants could be discounted in the absence of footage to corroborate the officer's version of events. It's the legal equivalent of "pics or it didn't happen."

Ariel gave the example of a traffic cop who spots an infraction out of the corner of his eye, and then can't produce body-worn video of the episode. In that scenario, a judge could toss out the case for lack of evidence. "That has serious implications for policing and the status of officers' testimony and statements in the court of law," he said.