Harmful actions may seem more sinister when viewed in slow motion, study finds


The camera lens may seem like an objective eye — but it’s the video playback where things can get tricky.

A new study finds that when viewers watched a video of a violent act, they were far more likely to ascribe ill intent to the person who did it if the video was played in slow motion.

The findings, described in the Proceedings of the National Academy of Sciences, show how differing presentations of the same video can significantly affect a viewer’s sense of events. In the courtroom, the authors say, this could have serious implications for the decisions by juries and the fates of defendants.


With businesses’ security cameras, police officers’ body cameras and ubiquitous smartphone cameras filming the world every day, violent acts are increasingly being caught on video and ending up in courtrooms. Such footage can be crucial evidence when testimony from a victim or witness isn’t sufficient to persuade a jury.

“Because video affords repeated viewings, it can augment the limits of human attention, visual processing and memory,” the study authors wrote. “Because video can be slowed down, it also provides the ostensible benefit of giving people ‘a better look’ at real-time events that happened quickly or in a chaotic environment.”

But while video might seem more objective than an individual’s recollection, humans still have to interpret what they see. And that interpretation could have life-or-death consequences.

Consider a 2009 murder case that eventually made its way to the Supreme Court of Pennsylvania.

During the trial, prosecutors showed jurors a surveillance video of defendant John Lewis fatally shooting a Philadelphia police officer during an armed robbery. The shots were fired about two seconds after Lewis saw the officer at the door.

At first, prosecutors presented the video footage in slow motion. Later, they showed it in real time as well.


Ultimately, the jurors found that the shooting was premeditated (making it first-degree murder), not reflexive (which would have made it second-degree murder). This opened the possibility for a sentence of death by lethal injection instead of life in prison.

The defense filed an appeal, arguing that the slowed-down video “artificially stretched the relevant time period, creating a ‘false impression of premeditation,’” the study authors wrote.

The Supreme Court, however, sided with the trial court, finding that the slow-motion footage was “more probative than prejudicial.”

Intuitively, that court decision may make sense; after all, the jurors were given all the facts of the case. But the study authors wondered whether the defense had a point. By slowing the tape, did it seem as though the suspect had more time — perhaps enough to plan out the act?

“The question of whether an actor had a ‘long enough’ window to assess and prepare to inflict the harm is likely to be central in many disputes,” the authors wrote.

To investigate this question, they set up a series of experiments. In the first one, participants were asked to act like jurors in a criminal trial. They were shown five seconds of a real surveillance video of an attempted robbery in which the assailant shot a store clerk. A digital display tracked the actual elapsed time. Some people watched the video at regular speed, and the others watched it at a rate that was 2.25 times slower than reality (about the same slow-down rate for the video that jurors saw in the John Lewis trial).


After viewing either version, they were asked whether the assailant shot the gun “with the intention to kill the victim,” as well as how willful, deliberate and premeditated the action was. They also were asked how much time they felt the shooter had to assess the situation.

The researchers found that 77% of those who saw the regular video thought the action was intentional, as did 86% of those who saw the slow-motion version. When they used that data to run simulations of 1,000 12-person juries, 39 juries shown the regular-speed video unanimously agreed that the assailant shot with the intention to kill. In sharp contrast, 150 juries presented with the slow-motion video came to the same conclusion.

The scientists wondered whether this time-distortion bias had the same effect in situations that didn’t involve criminal actions. So in a second experiment, they had participants watch video of an NFL football player making a prohibited helmet-on-helmet tackle. The effect held — the slow-motion viewers were more likely to think the player hit his helmet against his opponent’s on purpose.

In a third experiment, viewers were shown the robbery video and reminded, over and over again, of how many seconds had passed based on the digital time stamp.

This seemed to help reduce some bias seen in the first two experiments, but not all of it.

It did not significantly affect the difference in estimates of the time it felt like the shooter had, the study authors explained. In other words, the slow-motion viewers still felt that the shooter had more time to assess the situation before pulling the trigger.

In the fourth set of experiments, the researchers had some participants watch the robbery video at regular speed, followed by the slow-speed version. Although seeing both speeds seemed to mitigate the bias somewhat, it didn’t fully correct the problem, the authors found.

This doesn’t mean that slow-motion video shouldn’t be used in court, the authors emphasized, but it does mean that their potential benefit has to be weighed against the possible costs.


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

The authors emphasized that this is early research and that a lot more work has to be done to understand the effect of slowed-down video in light of other confounding factors. Among them: Does the viewing order matter? Does the number of times the video is seen affect perception? Perhaps the speed of playback matters. Perhaps if you slow down the video even more, it will cease to affect people’s perception of time.

“At a certain point of ‘superslow motion,’ actors may appear to be moving at nonhumanly slow speeds and seem less likely to possess any mental states, including intentions,” the study authors hypothesized.

In any case, they concluded, “it seems imperative that an empirical understanding of the factors that contribute to assessments of intent inform the life-or-death decisions that are currently based, in part, on the intuitions of lawmakers and their tacit assumptions about the objectivity of human perception.”

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