From the body cameras of US police officers to mobile phone footage and grainy CCTV shots, video evidence is becoming increasingly important in courts around the world. However, researchers in the US have now issued a warning that such evidence could be skewing the outcome of trials.
When jurors are shown slowed-down footage of an event, the researchers said, they are more likely to think the person on screen has acted deliberately. While a slow-motion replay may allow jurors to see what is taking place more clearly, it also creates “a false impression that the actor had more time to premeditate” than when the events are viewed in real time.
According to the researchers, a calculated, rather than an impulsive, crime can be the difference between “lethal injection and a lesser sentence”. They pointed to the case of John Lewis, who is on death row after being found guilty of murdering a police officer in 2007 in an armed robbery. Lewis’s lawyers argued in an appeal that slowing down video evidence had made jurors more likely to think the killing was premeditated. Judges dismissed this argument, however, because jurors were shown both the full-speed and the slowed-down versions, and the tape had a digital display that showed the time elapsed.
However, in a series of experiments, behavioural scientists showed participants footage of an attempted armed robbery in which a shop assistant was shot dead. Those who watched the footage slowed down were three times more likely to convict the accused than those who watched it in real time. If participants were shown a slowed-down version and a real-time version of the footage, it went some way to “[mitigating] the bias, but [did] not eliminate it”, said the researchers.
So, what is going on? Cognitive neuropsychologist Ashok Jansari from Goldsmiths, University of London says the problem is that we are adding “perception information” to what we are being shown. He points to the theories of Daniel Kahneman, a Nobel prize-winning scientist who says we have two methods of decision-making: System 1, which is a fast, intuitive process; and System 2, which is slower and more deliberate. In the case of slowed-down video evidence, jurors feel that the criminal is using the more calculated decision-making process – even when the time taken shows this is unlikely.
Forensic psychologist Jacqueline Wheatcroft at Liverpool University is not surprised by the findings. Wheatcroft, who has been researching the effect of giving out warnings to jurors about putting too much weight on one particular piece of evidence, says that “even minor changes can affect perception”. Caution, and more evidence-based research, is needed, she says, “before we rush in and make changes that can have an impact on people’s lives”.
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