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Forensic Science Testimony: What most  influences jurors? 

Monday, January 23, 2017
posted by Douglas Keene

We all want our expert witnesses to be influential with jurors. But when you have an expert testifying about forensic science (like fingerprint or DNA identification) what part of the testimony is going to influence jurors the most? Will it be the science? The technology used by the witness to interpret and understand the data? Or some characteristic of the witness? A new study tells us what jurors find most influential as they make decisions about your case.

You may find these results distressing (or you may breathe a sigh of relief over them). The researchers were interested in seeing how much the

“science” (i.e., how has the method the witness used to determine their findings been tested and validated) was persuasive, to what degree the

“technology” (i.e., is the technology older or is it new, whiz-bang technology of the latest findings) was persuasive, or, to what degree

“individual characteristics” of the witness (i.e., education and experience) was the most persuasive to jurors.

So was it the whiz-bang of the technology or the validation of the technology used? Nope. The individual background and experience of the expert witness was what most persuaded jurors. In other words, credibility turned on whether the jurors found the expert witness was qualified and had the experience base to understand the science and communicate their findings effectively. And in our experience, the likability and personal appeal of the expert is a significant additional factor that goes well beyond credentials.

Courts have long asserted standards for admissibility of scientific evidence and testimony. Jurors always insist that they need to understand the science in order to judge the merits of a technological or scientific dispute, but in truth most jurors get tired of trying to figure it out pretty quickly, unless they have a background that gives them a head start. Ultimately, the messenger is a crucial part of the evidence, and for those who struggle to understand it, the messenger (i.e., your expert witness) is a crucial factor.

Many of us also have beliefs that the latest technology is more persuasive than tired, old-fashioned and low-tech ways of interpreting data. But in this study, jurors compared fingerprint experts using either whiz-bang technology with computerized matching of prints or a visual scan of the fingerprints using the ACE-V (analyze, compare, evaluate, and verify). While the ACE-V method may “sound” good to jurors, the National Academy of Sciences (NAS) report in 2009 stated that the ACE-V method of fingerprint analysis was “not specific enough to qualify as a validated method”.

Regardless of the invalidation of the ACE-V method, the experienced expert won out over the technology. The researchers thought perhaps jurors were not confident that the witness using whiz-bang technology knew enough about how to interpret the results accurately.

So, what it came down to both times (across two experiments) was juror evaluations of the experience of the testifying forensic scientist. Researchers said the jurors “leaned on the experience of the testifying forensic scientist to guide their assessments of the soundness of his [sic] findings”. In another section of the paper, the researchers opine that witness “experience serves as a proxy for scientific validity”.

Even more disturbing, the researchers asked participants for the “total number of college and graduate level classes in science, math, and logic that you have completed”. They thought those who were more educated in scientific methods would focus more on the scientific validity of the analysis used by the testifying expert. This was not the case.

“It may be that jurors simply didn’t perceive a connection between the scientific validation of a forensic technique and its accuracy.”

From a litigation advocacy perspective, this speaks to the importance of educating jurors at a level they can understand about the science behind the expert’s interpretation. We have blogged before about using skepticism in direct examination and this approach (wherein you have your expert discuss the methods used by the other expert and why your expert’s strategy is more reliable and valid) would be a good strategy to discredit opposing counsel’s expert.

Overall, when your case relies on science and technology, use pretrial research to ensure jurors understand enough of the science to make educated and informed decisions about the evidence. If you do not make sure you are teaching at a level they understand, it is likely they will fall back, like the jurors in this research, on their intuition about witness experience and training (and probably on how likable, knowledgeable, confident, and trustworthy is the expert) rather than on whether the expert used credible methods used to analyze the evidence.

Koehler, J., Schweitzer, N., Saks, M., & McQuiston, D. (2016). Science, technology, or the expert witness: What influences jurors’ judgments about forensic science testimony? Psychology, Public Policy, and Law, 22 (4), 401-413 DOI: 10.1037/law0000103

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