A study co-authored by professors of psychology and law at the University of New South Wales examined the factors that jurors are likely to consider when they decide whether an expert’s opinion is persuasive. The goal was to determine how jurors “differentiate witnesses who offer genuinely expert opinions from those who do not.”

The study was motivated in part by the consistent use of questionable forensic expert evidence to prove guilt in criminal cases. In Australia as in the United States, innocent defendants are convicted when prosecutors bolster weak cases with expert opinions that are founded on unreliable science — or no science at all.

Understanding how juries might evaluate expert evidence should help lawyers as they select experts and prepare them to testify. An understanding of jury psychology may also help lawyers cross-examine experts to expose testimonial weaknesses that are most likely to matter to jurors. Experts can also benefit from the study by shaping their reports and testimony in ways that are the most persuasive to jurors.

Methodology

After examining existing scholarship in the areas of persuasion generally and jury decision-making specifically, the authors proposed “eight broad attributes that are logically relevant to the merit-based assessment of an expert opinion.” They termed those attributes the “Expert Persuasion Expectancy (ExPEx) Framework.”

The ExPEx Framework suggests that jurors consider these factors when they evaluate expert evidence:

• Foundation for opinions — whether the field of study upon which the expert relies is sufficiently valid to support the expert’s conclusions
• Field expertise — whether the expert has sufficient training and experience in the field of study to merit trust in the expert’s opinions
• Specialty of expert — whether the expert’s training and experience in the field addresses the specific specialty that forms the basis for the expert’s opinions
• Ability of expert — whether the expert states opinions accurately and reliably
• Opinion expression — whether the expert’s opinions are stated clearly and with necessary qualifications
• Support — whether the expert supports opinions with evidence
• Consistency — whether the expert is consistent with other experts in the field
• Trustworthiness — whether the expert is personally reliable as a source

The study did not examine actual jurors who consider the testimony of actual witnesses in actual trials. Rather, the authors provided different study participants with different versions of an expert report: a control version plus versions that weakened or strengthened one of the attributes identified above.

In one experiment, the control report was strong as to every attribute. Each of the remaining eight reports weakened a single attribute but made no change to the rest of the report. In a second experiment, the control report was weak while each of the other reports strengthened a single attribute.

In each experiment, roughly fifty participants reviewed the control report, another fifty reviewed a report with a weakened or strengthened attribute, another fifty reviewed a report with a different weakened or strengthened attribute, and so on. About 450 people participated in each experiment.

The reports concerned gait analysis, which purportedly allowed the expert to identify a person on a video recording by the way the person walked. The expert in the strong report was a podiatrist. The expert in one of the weak reports was a hand surgeon. Other attributes were manipulated in similar ways.

Test subjects were asked questions to measure the persuasiveness of the various reports. The questions asked participants to rate “the credibility of the witness, the value of their evidence, and the weight they would give to the opinion” on sliding scales of 1 to 100.

The three ratings were closely correlated to each other. The authors thus combined them into a single “persuasiveness” rating.

Study Results

In the first experiment, the participants regarded the control report as highly persuasive. Weakening the Support attribute made no difference in perception of the report’s strength. Weakening the other attributes had some impact on perceptions of persuasiveness, but perceptions of persuasiveness were significantly affected only by four attributes: Ability, Trustworthiness, Consistency, and Opinion.

In the second experiment, the participants regarded the control report as unconvincing. Strengthening the attributes for Ability or Consistency significantly improved perceptions of persuasiveness. Strengthening one of the other attributes of the weak report had no significant impact on how participants viewed the report.

Perhaps unsurprisingly, the study suggests that jurors are most likely to be persuaded when an expert in the relevant field who is trained and unbiased provides an empirically supported, confident opinion derived from methods that have been endorsed by other experts in the field.

Conversely, jurors are less likely to be persuaded when they consider an empirically unsupported, doubtful, contentious opinion from a novice “hired gun” who is testifying outside his or her field of expertise.

According to the authors, a “strong expert opinion was significantly undermined by a high likelihood of error, disagreement among experts, and questionable integrity. Conversely, a weak expert opinion was significantly improved by a low likelihood of error and agreement among the experts.”

Selecting and Preparing Experts

Perhaps the most surprising finding is that participants were not particularly concerned with whether an expert supported an opinion with evidence. Instead, “Ability, Consistency and Trustworthiness may be particularly influential attributes.”

Judges in a Daubert regime, of course, focus largely upon the Support attribute. Admissibility depends upon whether the evidence upon which the expert relies is supported by sufficient facts and a reliable methodology.

The study suggests that trial outcomes will be best when the evidence assures jurors that an expert is honest (Trustworthiness), that the expert is stating opinions accurately (Ability), and that the expert’s opinions are consistent with those of other experts in the field (Consistency). Witness preparation that focuses on those attributes might persuade a jury to believe an expert’s opinions.

The authors also acknowledge that decision-making is a subtle art, and that the interplay of the defined attributes might not always be obvious. For example, Confidence and Trustworthiness are both important, but other studies suggest that an expert can enhance Trustworthiness by acknowledging reasons to doubt the expert’s opinion. Since Trustworthiness is a more important attribute than Confidence, acknowledging weaknesses in an opinion might actually strengthen its persuasive value.