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.
Mock Jurors’ Evaluation of Firearm Examiner Testimony

Journal: Law & Human Behavior
Published: 2020
Primary Author: Brandon L. Garrett
Secondary Authors: Nicholas Scurich, William E. Crozier
Type: Publication
Research Area: Implementation and Practice
Related Resources
How do Labs Ensure Quality? A Nationwide Review of SOPs for Latent Print Examination
This presentation is from the 108th International Association for Identification (IAI) Annual Educational Conference, Reno, Nevada, August 11-17, 2024. Posted with permission of CSAFE.
Statistics and its Applications in Forensic Science and the Criminal Justice System
This presentation is from the 2024 Joint Statistical Meetings (JSM), Portland, Oregon, August 3-8, 2024.
Silencing the Defense Expert
In the wake of the 2009 NRC and 2016 PCAST Reports, the Firearms and Toolmark (FATM) discipline has come under increasing scrutiny. Validation studies like AMES I, Keisler, AMES II,…
Demonstrative Evidence and the Use of Algorithms in Jury Trials
We investigate how the use of bullet comparison algorithms and demonstrative evidence may affect juror perceptions of reliability, credibility, and understanding of expert witnesses and presented evidence. The use of…