Mock jurors’ evaluation of firearm examiner testimony.

Abstract

Objectives: Firearms experts traditionally have testified that a weapon leaves "unique" toolmarks, so bullets or cartridge casings can be visually examined and conclusively matched to a particular firearm. Recently, due to scientific critiques, Department of Justice policy, and judges' rulings, firearms experts have tempered their conclusions. In two experiments, we tested whether this ostensibly more cautious language has its intended effect on jurors (Experiment 1), and whether cross-examination impacts jurors' perception of firearm testimony (Experiment 2). Hypotheses: Four hypotheses were tested. First, jurors will accord significant weight to firearm testimony that declares a "match" compared to testimony that does not (Experiments 1 and 2). Second, variations to "match" language will not affect guilty verdicts (Experiment 1). Third, only the most cautious language ("cannot exclude the gun") would lower guilty verdicts (Experiment 1). Fourth, cross-examination will reduce guilty verdicts depending on specific language used (Experiment 2). Method: In two preregistered, high-powered experiments with 200 mock jurors per cell, participants recruited from Qualtrics Panels were presented with a criminal case containing firearms evidence, which varied the wording of the examiner's conclusion and whether cross-examination was present. These variations include conclusion language used by practitioners, language advised by government organizations, and language required by judges in several cases. Participants gave a verdict, rated the evidence and expert in all conditions. Results: Guilty verdicts significantly increased when a match was declared compared to when a match was not declared. Variation in conclusion language did not affect guilty verdicts nor did it affect jurors' estimates of the likelihood the defendant's gun fired the bullet recovered at the crime scene. In contrast, however, a more cautious conclusion that an examiner "cannot exclude the defendant's gun" did significantly reduce guilty verdicts and likelihood estimates alike. The presence of cross-examination did not affect these findings. Conclusion: Apart from the most limited language ("cannot exclude the defendant's gun"), judicial intervention to limit firearms conclusion language is not likely to produce its intended effect. Moreover, cross-examination does not appear to affect perceptions or individual juror verdicts. Public Significance StatementThis study addresses mounting legal concerns regarding the overstated conclusions experts reach using firearms comparison, one of the most commonly used forensic disciplines in criminal cases. Bradley D. McAuliff served as Action Editor.

Human Factors in Forensic Science: Psychological Causes of Bias and Error

“…Cross-examination presents a potential safeguard against unreliable forensic testimony (see Edmond et al, 2014), but research has yielded mixed findings as to its efficacy. Some have found that crossexamination had little or no effect on a forensic expert's credibility (Garrett et al, 2020;Koehler, 2011), which others found that some strategies were more (e.g., emphasizing the subjectivity of the expert's opinion; Thompson & Scurich, 2019) or less (e.g., attacking the expert's credentials; Lieberman et al, 2008) effective than others. Future work should continue to examine which cross-examination strategies enable jurors to rightly discount dubious forensic testimony without creating a blanket skepticism that undercuts even valid forensic evidence.…”

Beyond CSI: Calibrating public beliefs about the reliability of forensic science through openness and transparency

“…This research has occurred largely in the context of mock juror studies. These studies, focusing on feature comparison practices like firearms [62], bitemark [32,63], and fingerprint analysis [18,32,64,65], tend to find only a modest relationship between the stated limitations of the evidence and mock jurors' assessment of that evidence (and the associated expert). Mock jurors will, for example, take into account a forensic examiner's proficiency when assessing their evidence and expertise [32,64].…”

“…Converging with these findings, other work finds only a weak relationship between expressions of uncertainty in an expert's opinion and the weight mock jurors accord those opinions:[18,62,65]. On the importance of careful reporting generally, see[66].…”

Surveying practicing firearm examiners

“…The PCAST report concluded that the method was not sufficiently validated on the basis of this single study. In response to these types of concerns, judicial rulings, and new U.S. Department of Justice Guidelines, firearm examiners in some jurisdictions have tempered the description of their conclusions in criminal court testimony [ [7] , [8] ].…”