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CSAFE Researchers Study Effect of Variations in Firearms Examiners Conclusion Language on Mock Jurors

Closeup of a bullet in a lab

A study of mock jurors’ evaluation of firearm examiner testimony showed that many judicial and prosecution-driven interventions to limit conclusion language for firearms testimony are not likely to produce the intended effect.

The study, published in Law and Human Behavior, was led by Center for Statistics and Applications in Forensic Evidence (CSAFE) researchers Nicholas Scurich, professor of Criminology, Law and Society and vice chair of the Department of Psychological Science at the University of California, Irvine; Brandon Garrett, CSAFE co-director and the L. Neil Williams, Jr. Professor of Law and director of the Wilson Center for Science and Justice; and William Crozier, research director of the Wilson Center for Science and Justice at Duke University.

Traditionally, firearm and toolmark examiners have testified that a firearm leaves unique marks on bullets and cartridge casings. These unique marks permit an examiner to make a source identification conclusion. While scientific organizations have called this sort of categorical assertion into question, jurors still place a great deal of weight on a firearms examiner’s testimony.

Figure 1 from the study shows the proportion of guilty verdicts and mean likelihood ratings that the defendant fired the gun (with 95 percent confidence intervals) in each experimental condition.
Click on image to enlarge. Figure 1 from the study shows the proportion of guilty verdicts and mean likelihood ratings that the defendant fired the gun (with 95 percent confidence intervals) in each experimental condition.

To examine the weight jurors place on these testimonies, Scurich, Garrett and Crozier conducted two studies: the first evaluated if using more cautious language influenced jurors’ opinions on expert testimony, and the second measured if cross-examination altered these opinions.

In the first study, 1,420 participants read a synopsis of a criminal case that included the testimony of a firearms examiner. The examiner gave one of seven specifically worded conclusions, ranging from a “simple identification” to a “reasonable degree of ballistic certainty” to a more cautious “cannot be excluded.” The participants then decided whether they would convict based on the testimony.

In study two, 1,260 participants read the same criminal case, but with two important changes. First, the examiner’s conclusion had only three possible conclusions: “inconclusive,” “identification” or “cannot be excluded.” Second, some of the participants read a cross-examination of the firearms examiner. Again, the participants were asked whether they would convict the defendant.

The researchers found that guilty verdicts significantly increased when a match was declared compared to when a match was not declared. Variations in the examiner’s conclusion language did not affect guilty verdicts. There was one exception that did appear to have an impact on jurors’ perception of firearm evidence. The more cautious conclusion that an examiner simply “cannot exclude the defendant’s gun” did affect guilty verdicts. The researchers also found that the cross-examination did not lead jurors to consistently discount firearms conclusions.

The researchers noted that the results can be viewed with a mix of optimism and pessimism. They write: “On the one hand, forensic experts can use wording that more cautiously describes their conclusions without harming their credibility or conviction rate. Thus, there is little to no cost in avoiding misleading or overstated conclusion language that implies a categorical association between evidence. On the other hand, it is unfortunate that the more cautious and tempered conclusion language did not result in higher credibility ratings for the expert or reliability and scientific ratings for the evidence.”

The researchers suggest that until there is further research into the validity and reliability of firearms comparison methods, more forceful judicial instructions may be needed.

Figure 2 from the study shows the proportion of guilty verdicts (with 95 percent confidence intervals) in each experimental condition.
Click on image to enlarge. Figure 2 from the study shows the proportion of guilty verdicts (with 95 percent confidence intervals) in each experimental condition.

 

Download the journal article and read insights from this study at https://forensicstats.org/blog/2020/12/22/insights-mock-jurors-evaluation-of-firearm-examiner-testimony/.

Scurich, Garrett and Crozier discussed this study during a CSAFE-hosted webinar, Mock Juror Perceptions of Forensics. Watch it at https://forensicstats.org/blog/portfolio/mock-juror-perceptions-of-forensics/.

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