Juror Certainty about Firearms Evidence: Examination Effects

Firearms examiners routinely compare tool-marks on suspect ammunition with those on ammunition test-fired by a suspect weapon to evaluate if they can be associated. The discipline has been subject to criticism, including by the National Academy of Sciences, but the testimony of firearms examiners is routinely admitted as expert evidence in the United States (US). Jurors must determine the weight of expert evidence, which opposing and proffering lawyers can aim to, respectively, discredit on cross-examination and rehabilitate on redirect-examination. The authors investigated the effect of both cross and redirect- examination on potential US jurors’ certainty about expert firearms evidence using a series of online vignettes. Participants (n=114) were asked to rate their certainty (on a scale of 0-100) about three expert statements – Very Certain (an exact match), Certain (a match to a reasonable degree of certainty), and Uncertain (evidence is unsuitable for comparison) – when assigned to one of three conditions. These conditions were a judicial instruction about weighing the evidence (control condition); a cross-examination referencing criticism of firearms evidence; and a redirect-examination (following the cross-examination) referencing the routine admission of firearms evidence. Analysis was undertaken both between groups and between the statements given to each group. Results suggest that experts conveying high certainty create higher certainty in jurors, cross-examination has a detrimental effect on this certainty, but redirect-examination does not reduce this detrimental effect.