Watching a video of a harmful or violent act being committed can provide useful evidence of the circumstances surrounding the action. But new research shows that watching that same video in slow motion can often cause viewers to see something that may not be there: intentionality.

According to a new paper titled "Slow Motion Increases Perceived Intent," viewing an action in slow motion compared to regular speed can cause viewers to perceive an action as more intentional.

"We show that when viewers see a replay in slow motion, rather than real time, they are more likely to see the action as intentional," says Eugene Caruso, associate professor of behavioral science at the University of Chicago Booth School of Business and a lead author of the paper. "This is because slow motion gives the false impression that the actor had more time to think before acting."

"In criminal trials, seeing intention in an action can mean the difference between first-degree and second-degree murder, in some cases between life and death, so any benefits of video replay should be weighed against its potentially biasing effects," Caruso says.

Caruso, together with colleagues Zachary Burns of the University of San Francisco and Benjamin Converse of the University of Virginia, conducted a series of experiments involving real surveillance footage from a murder to test whether slow motion biased viewers to see intentionality.

One analysis showed that nearly four times as many juries would unanimously vote for a first-degree murder verdict if made up of participants who saw the video in slow motion rather than of participants who saw the video at regular speed.

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"Even when we made sure that participants knew how much clock time had actually passed, they still could not get over the feeling that things unfolded more slowly and therefore tended to see more intentionality in the actions," according to Burns.

"Judges have an extremely difficult job: They have to decide based on intuition alone whether slowing a video is more prejudicial than probative. As behavioral scientists, we have the opportunity to collect empirical evidence. And the evidence for possible bias is clear," said Converse.

In another experiment, the researchers used a sports video to test whether this slow motion intentionality bias applies not just to horrific criminal actions, but also to more mundane transgressions. They used a video of a prohibited helmet-to-helmet tackle from an NFL game.

"We found the same results as in the shooting video. Participants who saw slow motion thought the offending tackler was trying harder to strike the other player's helmet, that he had more of a plan to do so, and that he had more of an opportunity to avoid doing so," said Burns.

In additional experiments with the murder video, the researchers found that allowing viewers to see both regular speed and slow motion replay mitigates the bias, but does not eliminate it. "Seeing both speeds helps a little, but does not eliminate the bias," said Caruso.

The researchers hope that work like this will increase legal and scientific interest in understanding the effects of video and video editing on social judgment. "Understanding of the effect of slow motion on mental state attribution should inform the life-or-death decisions that are currently based on tacit assumptions about the objectivity of human perception," the paper states.

Though slow motion may provide a better look at real-time events that happened quickly or in a chaotic environment, the researchers recognize that it can skew perception.

"We recognize that slow motion is intuitively appealing for this reason, and we are sure that it is helpful in some cases for allowing viewers to determine what actually happened. But when it comes to judging people's intentions, slow motion may sometimes do more harm than good," adds Converse.

The researchers argue that what viewers "see" in slow motion that could not be "seen" at regular speed is more time, or more specifically, an actor who seems to have had more time to form an act of intention.