Juries that watch slow-motion footage convict more often, study finds

Slow-motion footage appears to dramatically sway juries in favor of convicting people of first degree murder by giving the impression that the defendant thought through what they were going to do.

That's the conclusion of a study designed to explore how the speed of videos used as evidence can influence human judgement, and case outcomes.

The authors of the study — published this month in the US science journal PNAS — asked participants to imagine they were jurors while watching real surveillance footage of an assailant shooting a store clerk, and considering the question of intent to kill.

The experiment concluded that simulated 12-person juries, where all the members were shown the slow-motion footage, were four times more likely to start deliberations ready to reach a guilty verdict than those juries made up of members who had seen the images at regular speed.

The authors suggest that the difference stems from the way slow motion gives the impression that there is not only more time involved in a given action, but also more planning.

"In legal proceedings, these judgments of intent can mean the difference between life and death," the study concludes. "Thus, any benefits of video replay should be weighed against its potentially biasing effects."

Video evidence is increasingly used in courts around the world, thanks to the proliferation of surveillance cameras, body-cams on law enforcement officers, and smartphones. But, the researchers argue, not enough attention has been paid to the way its see-it-to-believe-it mystique can mould perceptions.

The study highlights the case of John Lewis, who is currently on death row following his conviction for murdering a police officer during an armed robbery in 2007. His appeal argues that slow-motion footage from a surveillance camera shown in court created an illusion of premeditation.

"Slow motion can be a better version of reality, sometimes it's very helpful for seeing how actions unfolded," Eugene Caruso, the lead author from the study from the University of Chicago, told the BBC. "But, at the same time, we found that it seems to have an effect on our perceptions of someone's inner mental state, and there it's really not so clear that slowing things down gives us a more accurate perception of what was going on in someone's mind at the time they were acting."

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