Can collaboration reduce expert bias?

The human mind, although complex and intelligent, is prone to cognitive errors. We often overestimate our ability to think clearly and maintain an objective point of view, but the reality is that our unconscious biases permeate our decisions. These biases can develop naturally as we go about our lives, or as a more direct consequence of a situation we’re in. This is often the case for experts testifying in court who begin to identify more closely with the side that hired them, either prosecution or defense. This is known as adversarial allegiance and is problematic because it compromises the expectation of neutrality of expert testimony.


In “Testing the Waters: An Investigation of the Impact of Hot tubbing on Experts from Referral through Testimony”, Perillo and colleagues (2020) explore a possible solution to adversarial allegiance of experts. Concurrent expert testimony, or hot tubbing, consists of allowing opposing experts to interact throughout the testimony process. The experts find areas of agreement and disagreement and testify together but limit testimony to the disagreements only. This is believed to reduce bias in experts as they will more closely align with their identity as experts discussing their area of expertise, rather than their identities as prosecution or defense sided experts. This method has been implemented in Australia and other countries but there has not yet been a rigorous study evaluating its effectiveness against adversarial allegiance.


In this study, the researchers manipulated side (prosecution or defense) and testimony type (adversarial or concurrent) with a court appointed expert condition as a control group. They recorded the participants’ judgements (binary criminal responsibility) and how responsible they believed the defendant to be (scale ratings of criminal responsibility). It was hypothesized that 1. Those in the defense condition would give more not criminally responsible judgements and give lower ratings of criminal responsibility than those in the prosecution condition, 2. Adversarial allegiance effects would increase for experts in the adversarial condition and decrease for those in the concurrent condition, and 3. Participants in both adversarial and concurrent conditions would report more dissonance than court-appointed experts, and that dissonance would increase for adversarial experts, decrease for concurrent experts, and remain constant for court appointed experts. The results of the study showed support for the first hypothesis, but not for the second two. Experts did align more closely with their side as there was a significant difference between the ratings of criminal responsibility of the two sides, and this remained true across testimony type, indicating that there was no reduction of adversarial allegiance in the concurrent testimony condition. There were also no significant differences in dissonance between concurrent, adversarial, or court-appointed experts.


A possible explanation for why concurrent testimony did not lessen adversarial allegiance is the timing of when the experts first interact. It’s possible that by the time they meet, the experts have already committed to their side. Further discussion with another expert could also result in confirmation bias, further reinforcing commitment to their original position. However, it is also important to acknowledge the limitations of the study that may have led to these results. A portion of the participants were not clinicians, but doctoral students, which may affect the way concurrent experts interacted with one another. It is also possible that although the study was designed to be as realistic as possible, the participants weren’t as motivated to remain unbiased as they would in a real trial.


Although there has been support for the implementation of concurrent testimony, the current study did not find any evidence to suggest that it is an effective method to prevent adversarial allegiance or halt bias. There is a need for further empirical studies (including implementation earlier on in the testimony process) of this method before it can be implemented as a solution to adversarial allegiance in the American justice system. It is often easy to get excited about new and innovative solutions to problems in the justice system, however it is important that we rigorously, and scientifically, evaluate their true effectiveness.

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