One trait of jurors is that, at the start of a case at least, they are not already savvy about the case’s subject matter. So, how do they learn? Through expert testimony. They won’t necessarily just sign-on to an expert’s conclusions, but they are likely to be influenced by both the authority an expert possesses and the explanation the expert provides. So when that testimony is flawed, in the case of scientifically suspect forensic techniques, for example, it poses a problem. Based on the Daubert standard, judges are supposed to be the gatekeepers on the science, but judges can be as much in the dark as jurors are when it comes to the complexities of scientific methods and conclusions. So what are the options for debunking the testimony if the motion in limine fails? A good cross-examination.

An expert’s cross-examination can be a key process where the witness’s credibility can be drained. And there is research to show that it works. In a recent doctoral dissertation from Jacqueline Kirshenbaum, “Persuasion Factors that Influence the Effectiveness of Cross-Examination of a Forensic Expert” (Kirshenbaum, 2021), the author focuses on whether an effective cross actually cures false and misleading testimony from a forensic expert in a criminal context. Kirshenbaum manipulated four factors of credibility that are part of a model called the “Expert Persuasion Expectancy Framework” (Martire, Edmond & Navarro, 2020) in summarized case presentations to 563 mock jurors. Using 16 different conditions of expert cross-examination, varying the cross on the prosecution expert’s consistency, field, foundation, and trustworthiness, the study found that exposure to the cross-examination reduced the expert’s source and message credibility, and significantly decreased guilty verdicts as well. “The current study’s findings suggest that a cross-examination should help jurors better understand and use complicated testimony and is ultimately effective in exposing to the jury the subjectivity of forensic science and the inaccurate conclusions of an expert. An effective cross-examination should help jurors better critique unreliable forensic testimony – which, in turn, should serve as a buffer against unreliable science that is still admitted into court.”

It is, of course, no surprise to experienced attorneys that a good cross works. There is, however, some nuance to the study as well. For example, the author found that in some circumstances, higher credibility is worse: a high credibility source delivering a low credibility message cuts against the witness even more than a low credibility source delivering the same message. Apparently, there is a rebound effect in the violation of expectations: We expect the credibility of the source and the message to be consistent. This is a reminder for both witnesses and examiners to pay attention to both the wrapper and the contents.

The Wrapper: Source Credibility

Source credibility means the attitude toward the person delivering the message. In the study, two factors of source credibility were tested:

Field: Whether the expert has training, experience and study in the relevant field. The cross-examination on this focused on the witness’s education, training and experience as specific preparation for offering their opinions.

Tustworthiness: Whether the expert appears to be personally fair, impartial, and objective. The cross-examination on this focused on payment for testimony as well as a pattern of working only for one side. I encourage advocates to think beyond these somewhat overworked themes, though, and to focus instead on any mistakes you might be able to catch them on — in a previous post, I called it the “Large Internal Error,” mostly because that works well as an acronym.

The Contents: Message Credibility

Message credibility refers to the attitude toward the message itself. Independent of the source, does the testimony itself hold together and rest on solid footing? The study manipulated two aspects of message credibility.

Foundation: Whether there is evidence, study, training, or experience that provides a backup for the conclusions offered. In this case, the cross focused on the reliability and error rates of the method.

Consistency: Whether the expert’s claims and methods are consistent with other experts’ practices. In the study’s cross-examination, the focus was on different methods and a lower threshold than that used by other professionals in the field.

Interestingly, compared to source credibility, message credibility had the stronger association to ultimate verdicts. That means, all things being equal, it is probably more important for you to take apart the expert’s analysis than it is for you to take potshots at the expert’s character, training or motivation. A message-centered approach, however, requires you to dig in on the details.

The research shows that it is a bad idea to just stick to a surface cross-examination focused on credentials or bias. As Dr. Kirshenbaum notes, “a scientifically informed cross-examination highlights the weaknesses in the expert’s research, testing, or field, whereas a scientifically naive cross-examination tends to focus on characteristics of the expert (e.g., whether the expert was paid to testify).” Those source factors can be important, but the more jurors understand and are engaged in the content, the more they will be mostly interested in the substance of the message itself.

Go After Both in Cross-Examination

The dissertation cites research showing that with low involvement, people might focus on the surface characteristics of source credibility, but with increasing involvement and sophistication, they will focus on the message cues instead. In other words, the more they know, the more they focus on the contents and not just the wrapper. In a longer trial, where jurors have the chance to become subject-area experts themselves, they’re likely to care more about those aspects of cross-examination that bear on the message substance. But still, jurors are likely to have different levels of ability, interest, and engagement. So it makes sense to play to all audiences. When outlining your cross, focus on both the credibility of the source and the credibility of the message.