Juror Certainty about Expert Firearms Identification Evidence and the Impact of Cross-examination

To evaluate association, firearms examiners compare tool-marks present on suspect ammunition to those present on ammunition test-fired by a suspect weapon. Examiners’ conclusions are generally admissible in US courts, yet the scientific underpinnings of the discipline have been subject to considerable criticism. Cross-examination can be used to bring such criticism to the attention of jurors, who determine the weight of expert evidence. The authors investigated the effect of such cross-examination on juror certainty about expert firearms evidence using online vignettes. A community sample of US participants (n=437) were asked to rate their certainty (0-100) of a forensic match for each of 4 expert statements of certainty, in 2 groups; either with or without a cross-examination highlighting limitations of the tool-mark discipline’s scientific underpinnings. Analysis was undertaken both between groups and between the statements given to each group. Results suggest that cross-examination can have a strong influence on juror decision-making, particularly when experts express their conclusions in certain terms