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Sunday, March 20, 2022

The prejudices of expert evidence

Chin, J., Cullen, H. J., & Clarke, B. 
(2022, February 14).


The rules and procedures regulating the admission of potentially unreliable expert evidence have been substantially weakened over the past several years. We respond to this trend by focusing on one aspect of the rules that has not been explicitly curtailed: unfair prejudice. Unfair prejudice is an important component of trial judges’ authority to exclude evidence, which they may do when that unfair prejudice outweighs the evidence’s probative value. We develop the concept of unfair prejudice by first examining how it has been interpreted by judges and then relating that to the relevant social scientific research on the characteristics of expertise that can make it prejudicial. In doing so, we also discuss the research behind a common reason that judges admit expert evidence despite its prejudice, which is that judicial directions help jurors understand and weigh it. As a result, this article provides two main contributions. First, it advances knowledge about unfair prejudice, which is an important part of expert evidence law that has received relatively little attention from legal researchers. Second, it provides guidance to practitioners for challenging expert evidence under one of the few avenues left to do so.


What should courts do about the prejudices of expert evidence?

While we recognise that balancing probative value with unfair prejudice is fact-specific and contextual, the analysis above suggests considerable room for improvement in how courts assess putatively prejudicial expert evidence. Specifically, the research we reviewed indicates that courts do not fully appreciate the degree to which laypeople may overestimate the reliability of scientific claims. But, more than that, the judicial approach has been myopically focused on the CSI Effect (and in at least one case, significantly misconstrued it), rather than other well-researched expert evidence stereotypes and misconceptions. Accordingly, we recommend that judges apply the discretions to exclude evidence in sections 135 and 137 of the UEL in a way that is more sensitive to empirical research. For example, courts should recognise that experts, or counsel that emphasise the expert’s status and years of experience, also feed into that evidence’s prejudicial potential. Moreover, technical jargon and the general complexity of the evidence can serve to heighten that prejudice, such that these features of expert evidence may build upon each other in a way that is more than additive.

The expert evidence jurisprudence is even more insensitive to research on the factors that make evidence difficult or impossible to test. For example, we struggled (as others have) to find decisions acknowledging that unconscious cognitive processes and associated biases invite prejudice because the unconscious is difficult to cross-examine. Moreover, the closest decision we could find acknowledging adversarial imbalance as a limit on adversarial testing was a US decision in obiter.  And troublingly, courts sometimes simply mistake previously admitted evidence with evidence that has been adversarially tested. With evidence that defies testing, the first step for courts is to acknowledge this research on prejudice and incorporate it into the exclusionary calculus in sections 135 and 137. The next step, as we will see in the following part, is to use this knowledge to better understand the limitations of judicial directions aimed at mitigating prejudice – and perhaps craft better directions in the future.