What makes an expert witness persuasive to a jury? Is it their background and training? The work that they did on the case? Their communication skills in teaching the jury? The research suggests that expert influence depends on a variety of factors, and a scale even exists to measure these factors: the Expert Persuasion Expectancy (ExPEx) framework. That framework includes many of the variables that experts and the attorneys who sponsor them would expect: Foundation, Field, Specialty, Ability, Opinion, Support, Consistency, and Trustworthiness. Even as the items are potentially unsurprising, it is still helpful to keep the whole list in mind.

Recent research (Martire, Edmond, & Navarro, 2020) shows that these factors matter, and also suggests that there may be a ‘Big 3’ in this list. Looking at experts focusing on “forensic gait”, or identifying a suspect by the way he or she walks, the research team from the University of New South Wales had subjects read either a control version of a report, or a version in which one of the eight traits was manipulated to be either weak or strong. They found that a strong expert report could be undermined by weak versions of Ability, Consistency, and Trustworthiness, and similarly found that a weak report could be helped by strong versions of these traits as well. All eight traits helped the expert fare better, but statistical significance was shown on three. More broadly, the fact that the researchers were able to succeed in these manipulations suggests that it helps to see these expert witness factors not as traits that an expert either has or doesn’t have, but as messages you can leverage and make salient during testimony. In this post, I will consider each of the eight factors and ways that an expert may want to emphasize them.

Ability

Does this expert provide their conclusions accurately and reliably? Does the expert know what they are doing? Competence is a basic dimension across many contexts, but for the expert witness, it is critical to demonstrate ability, and often that will mean finding ways to show and not just tell during testimony.

Consistency

Is this expert’s opinion endorsed by other experts? Are their views and methods within mainstream opinion and endorsed by the broader field? I would add the question, is the expert consistent within her or his own range of opinions? Experts need to show both external and internal consistency on not just the core opinions, but also across the board.

Trustworthiness

Is this expert’s opinion unbiased, honest, and conscientious? Can we trust what they are saying? Ultimately, what the expert is promoting and protecting is the notion that jurors can listen and believe what they’re hearing.

Foundation

Does the expert’s training and work support the conclusions they’re offering? Is there a solid foundation underneath these conclusions? This is a reason why effective communication focuses not just on conveying conclusions, but on walking jurors through the process, so that they can get to those conclusions on their own.

Field

Is the person an expert in the relevant field? Is it the right field? Has the expert’s interests, experience, and training prepared them to play the role they’re playing in this trial? Part of the expert’s testimony should include an explanation of how they became interested and engaged in this particular line of work. That’s part of the story.

Specialty

Is this expert’s training and experience specific to the question at hand? Within the general field, has the expert made a specific exploration that bears on the case at hand? It isn’t about applying general wisdom as much as it is about bringing specific work to bear on a specific question.

Opinion

Is this expert’s opinion clear? Does it include reasonable caveats, limits, or other exceptions? Is the expert staying within the realm of reasonability and avoiding over-reaching claims? Ultimately, the expert needs to stay an expert and not become an advocate.

Support

Are the expert’s conclusions backed up? Is the reasoning offered logical and well-founded? As much as character and credibility matters, jurors are still wanting to be guided by logic and rationality. Part of getting jurors to side with you means getting jurors to believe that they don’t just like you, but they also see that the better reasons are on your side.

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Martire, K. A., Edmond, G., & Navarro, D. Exploring juror evaluations of expert opinions using the Expert Persuasion Expectancy framework. Legal and Criminological Psychology.

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