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Evidentiary instructions improve mock juror assessment of feature-comparison evidence

Feature-comparison evidence has been introduced in court without sufficient scientific validation and has been at the heart of numerous miscarriages of justice. Juror assessment of such evidence and the efficacy of evidentiary instructions were examined through a mock jury experiment with case reports featuring either central or peripheral feature-comparison evidence. In a case-control design (N = 174), the test group was exposed to an evidentiary instruction about the ear print evidence presented in the first case report (adapted from R v Dallagher [2002] EWCA Crim 1903) whereas the control group did not receive such an instruction. The provision of this instruction resulted in a significant decrease in verdict severity with a large effect size. For the second case report (based on R v George (Barry) [2007] EWCA Crim 2722), all subjects were asked to return verdicts based on circumstantial evidence, gunpowder residue evidence and an evidentiary instruction about that gunpowder residue evidence. Verdict severity increased significantly after the provision of gunpowder residue evidence, followed by a subsequent reduction in verdict severity after the introduction of an evidentiary instruction. Furthermore, there was a significant difference in verdict severity between the test and control group, suggesting that the test group exhibited a scepticism effect brought about by the initial evidentiary instruction about ear print evidence. This study demonstrates that although mock jurors consider feature-comparison evidence a convincing indicator of guilt, the provision of an evidentiary instruction has the potential to educate jurors about the limitations of such evidence.