Research by US academics has found that viewers watching a slow motion video of a crime were more likely to say the crime was intentional rather than impulsive, even if they also saw a regular speed version of events unfolding as well.

In their study Slow motion increases perceived intent for the Proceedings of the National Academy of Sciences, Eugene Caruso, Zachary Burns and Benjamin Converse report on four experiments involving real surveillance footage from a murder or broadcast replays of violent contact in professional football.

They say the results 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," they say.

Four additional experiments revealed that allowing viewers to see both regular speed and slow motion replay mitigates the bias, but does not eliminate it.

Discussing the significance of their findings in a legal context, the authors say that when determining responsibility for harmful actions, people often consider whether the actor behaved intentionally.

"The spread of surveillance cameras, 'on-officer' recording devices, and smart-phone video makes it increasingly likely that such judgements are aided  by video replay. Yet, little is known about how difference qualities of the video, such as replay speed, affect human judgement," they say.

"We demonstrate that slow motion replay can systematically increase judgements of intent because it gives viewers the false impression that the actor had more time to premeditate before acting. In legal proceedings, these judgements of intent can mean the difference between life and death."