John Lewis was convicted of fatally shooting a police officer in a Philadelphia Dunkin’ Donuts in 2007. On appeal, his attorney argued that the jury had been swayed by the repeated slow-motion replay of the shooting, which gave them the impression of intent where there had only been reflex.

The appellate court rejected this rationale, but science suggests Lewis’s attorney had a point. We read more intent into a person’s actions when we see them in slow motion, according to research by Chicago Booth’s Eugene M. Caruso, University of San Francisco’s Zachary C. Burns, and University of Virginia’s Benjamin A. Converse.

“We first noticed the phenomenon in sports replays,” says Caruso. “Often a replay shown of something like a violent collision would seem worse in slow motion than it did in real time. We realized that any systematic bias that slow motion may introduce could have implications for the use of video evidence in courtrooms.”

The researchers ran a number of experiments to investigate the effect. In one study, participants watched video of a robbery that ended in a shooting. Some watched the video at regular speed, but those who watched the slow-motion version were more likely to feel that the perpetrator acted intentionally rather than reflexively.

Caruso, Burns, and Converse extrapolated the findings to see how this inclination might correspond to jury decisions. In 1,000 simulations of 12-person juries, “slow motion video quadrupled the odds that jurors would begin the deliberation phase ready to convict,” they write. Had jurors watched the video at regular speed, 39 juries would have unanimously decided to convict. But had they watched the slow-motion video, 150 juries would have made that decision, the researchers estimate.

As video slows, viewers’ perception can change
After watching video of a robbery that ended with a shooting, study participants assessed the shooter’s intentions.

Even when participants were told how much time had elapsed in the video, they still sensed more intent when the video was shown in slow motion. The effect attenuated, but not completely. “This reminder did not eliminate the difference in how much time participants felt the actor had,” says Caruso. “Slow-motion viewers still felt the actor had more time to act, and these perceptions translated into greater judgments of intent among slow-motion viewers—even when they knew how much time actually passed.”

Of course, video can help juries determine a timeline of events, such as when someone reached for a gun. But jurors can never be completely sure of a person’s intentions. If a jury considers video to be particularly objective, “its biasing potential may be especially pernicious,” the researchers write.

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