The Effect on Mock-Juror Decision-Making of Power-of-Speech Within Eyewitness Testimony and Types of Scientific Evidence

Abstract

This study examined whether manipulating power-of-speech within eyewitness testimony (EWT) and the type of scientific evidence (deoxyribonucleic acid [DNA] or fingerprint) had an effect on mock-juror decision-making. A between-participants design was utilised with jury-eligible participants randomly assigned to one of eight online experimental conditions. Participants were presented with a scenario followed by one type of scientific evidence and EWT. After each piece of evidence, participants made judgements of the probability of defendant guilt, judgement confidence and evidence strength. Lastly, participants chose a verdict and, if applicable, a sentence length. It was found that although powerful EWT was rated as more indicative of guilt (and indeed stronger than powerless EWT), only scientific evidence type affected the final verdicts with DNA evidence being associated with a greater number of guilty verdicts. Results suggest a biasing effect of DNA that outweighs the effect of other types of evidence including EWT.