From the pull of a trigger to the swing of a fist, a lot can happen in a fraction of a second. And gauging what's going through the minds of those involved during such dramatic slivers of time can be incredibly difficult, if not impossible. That's why law enforcement agents and prosecutors are increasingly turning to video. Those digital records don't just replay quick, life-altering events—they can be slowed down so that the slightest movements can be dissected. This, the logic goes, clarifies not just what happened, but helps explain what an alleged criminal intended to happen. But, according to a new study, slow motion might actually muddle our view.

Viewers who watch videos in slow motion—as opposed to regular speed—are more likely to feel that the people filmed act with a willful, deliberate, and premeditated intention, researchers report. The elongation of events, it turns out, gives viewers the impression that people in video clips have more time to think over and plan out what they are doing. The findings, published this week in the Proceedings of the National Academy of Sciences, suggest that jurors who view slow motion footage of an alleged crime may assign more responsibility to the accused than they would have otherwise.

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

The authors, Eugene Caruso of the University of Chicago, Zachary Burns of the University of San Francisco, and Benjamin Converse of the University of Virginia, point to the 2009 murder trial of John Lewis. During the trial, prosecutors showed slow motion surveillance video of Lewis fatally shooting a Philadelphia police officer during an armed robbery. The stretched footage was key in convincing the jury that the shooting was premeditated—counting as first-degree murder, punishable by death—rather than a reflexive, second-degree murder with the possibility of life in prison. Lewis is currently on death row.

To see if artificially lengthened footage can alter perceptions, the researchers showed 489 volunteers a similar five-second video clip of an armed robbery that ended with a robber shooting a sales clerk. Participants watched the clip at either regular or slowed speed. Those who watched the slowed version were significantly more likely to believe the shooter intended to kill. When the researchers plugged their data into a simulation of 1,000 12-person juries, they estimated that if all 1,000 juries watched the regular speed video, 39 would give a unanimous guilty verdict. But, if the juries watched the slow motion version, 150 of them would give a unanimous guilty verdict.

These results held up in three additional experiments. In the second experiment, the researchers wanted to see if the extra time it takes to watch a slow motion clip gave the audience more time to mull over the events. Perhaps that extra time was the reason viewers altered their perceptions of intent. So, the researchers got 580 volunteers to watch a video of a football tackle involving a prohibited helmet-to-helmet hit. Viewers either watched a slow motion version of the clip or a regular speed version that had a single frame frozen. The frame was frozen for long enough to even out the duration of the two clips. Volunteers who watched the slow motion version were still more likely to think the player intended to knock helmets.

Next, the researchers gathered another 410 volunteers to repeat the first experiment, but this time those that watched the slow motion clip were repeatedly reminded that it was slow motion. Researchers wondered if verbally correcting for the time distortion on the video would reverse the perception bias. Volunteers were cued to look at the time stamp during the slowed clip and were repeatedly told how much actual time had elapsed in the clip. This didn’t change the results.

Lastly, the researchers wondered if watching both the slow and the regular speed versions of a clip—as is often the case in trials—would erase or diminish any bias derived from watching the slowed version. Another 905 volunteers watched a clip at either regular, slow, or both speeds. Watching both speeds, the researchers found, could reduce the bias, but not eliminate it.

“The present investigation cannot determine whether slow motion replay makes viewers more or less accurate in judging premeditation in these situations,” the authors note. “But it does demonstrate that slow motion can systematically increase perceptions of premeditation itself.”

Proceedings of the National Academy of Sciences, 2016. DOI: 10.1073/pnas.1603865113 (About DOIs).