A new study showed that arousal does reduce the ability to make accurate decisions. Peter Murphy and colleagues (2014) gave participants a perceptual decision-making task – they had to look at a cloud of dots and decide the direct of travel. This was designed to mimic the types of perceptual decisions we make in everyday life.
Just before doing the perceptual task the researcher’s measured the size of a participant’s pupils. Pupil size increases when a person’s autonomic nervous system (ANS) is activated (sympathetic arousal). Stress and anxiety are associated with this arousal, so a person who is anxious will have a larger pupil size.
Murphy et al. found that the larger the pupil was before the task, the worse the person subsequently performed. Applying these results to eyewitness testimony would suggest that a person who is scared would be less able to make perceptual judgements and thus would have a less accurate perception of the events of a crime. This would obviously affect their subsequent recall of events but not because their memories were affected, but because of initially poor perception.