Carusoa, Burns & Converse (2016)

Slow motion increases perceived intent

Eugene M. Carusoa, Zachary C. Burns, and Benjamin A. Converse


“To determine the appropriate punishment for a harmful action, people must often make inferences about the transgressor’s intent. In courtrooms and popular media, such inferences increasingly rely on video evidence, which is often played in “slow motion.” Four experiments (n = 1,610) involving real surveillance footage from a murder or broadcast replays of violent contact in professional football demonstrate that viewing an action in slow motion, compared with regular speed, can cause viewers to perceive an action as more intentional. This slow motion intentionality bias occurred, in part, because slow motion video caused participants to feel like the actor had more time to act, even when they knew how much clock time had actually elapsed. Four additional experiments (n = 2,737) reveal that allowing viewers to see both regular speed and slow motion replay mitigates the bias, but does not eliminate it. We conclude that an empirical 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.”

Contribution Of Present Research

Our results show that after viewing slow-motion video, the apparent speed of subsequently viewed slow-motion videos appears relatively natural. Carusoa, Burns & Converse (2016) also show that after adaptation to slow-motion videos the perceived duration of time intervals is extended. These two effects in combination have important consequences for intentionality judgements, with potentially life-or-death consequences: After jurors (or sports officials, for example) have viewed slow motion replays of an offence a number of times, their perception of the speed of the action will alter so that it appears more natural, and it may also appear to take place over a longer period of time. Actors may therefore be judged to have taken more time to assess the situation, decide on their course of action, and execute the act. Consequently the court’s judgement of the intentionality and premeditation of the act may be biased due to the natural process of perceptual adaptation to action speed.

In our studies, normalisation took place over a relatively short time frame involving 30 seconds of initial exposure with a series of top-up exposure periods of 5 seconds during testing. In order to minimize the possibility that speed normalization intrudes on judgements of intentionality, slow motion video clips should be viewed for the minimum period possible; repeated viewing of slow motion videos should be avoided so as to prevent the build-up of adaptation and consequent changes in apparent action speed.


Caruso, E M, Burns Z C, Converse B A (2016). Slow motion increases perceived intent. Proceedings of the National Academy of Sciences, Early Edition, 1–39.

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