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How Do Jurors Decide When a Witness is Telling the Truth?

Updated Thursday, 18 May 2023

Professor Fiona Leverick deciphers the mysteries of one of jurors' most important decisions.

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This content is part of our Jury Hub.


A photograph of a lie detector.

In a criminal trial, one thing that jurors often have to do is listen to competing accounts of events given by witnesses and try and work out which witness is telling the truth. These judgments are called credibility judgments. They often arise in trials for sexual offences, where the defendant and the complainant agree that sexual intercourse took place, but the defendant claims it was consensual and the complainant asserts that they did not consent.

Very little is known about how jurors make credibility decisions. The worry is that they might rely on irrelevant factors (such as body language) or on harmful prejudices (such as an assumption that genuine victims are always emotional when they are giving evidence). We could simply ask jurors how they make credibility judgments, or listen to their deliberations, but there are restrictions on conducting research into jury decision making that prohibit this. 

Research undertaken with mock juries has now shed some light on this issue. Mock jury research attempts to simulate a real criminal trial and asks members of the public to act as jurors. Our mock jury project – the Scottish Jury Research – involved 863 participants divided into 64 juries. They all watched a video of either a rape trial or a non-sexual assault trial (where the defendant claimed he acted in self-defence). Jurors were drawn from jury service eligible members of the public to reflect a representative sample of the local population. The trials were scripted with the help of legal professionals and performed by actors. In both trials, jurors were required to make credibility judgments to assess competing witness testimony. In the rape trial, the (male) defendant testified that the (female) complainant consented to sexual intercourse, but the complainant denied this. In the assault trial, the (male) defendant claimed that the (also male) complainant attacked him and he was only acting in self-defence, but the complainant disputed this.

Once they had watched the trial video, our jurors were sent away to deliberate and try and reach a verdict. We recorded and transcribed those deliberations. What we found was that the cues that jurors relied on could be divided into two different categories: the content of testimony and its delivery. 

In terms of the content of the testimony, the main things that jurors looked to were whether the witness had any motivation to lie, whether the witness’s account had any internal discrepancies or contradictions, whether the witness’s account was consistent with other evidence in the case, and the level of detail the witness was able to provide in response to questions. Although they are perhaps over-generalisations, these factors are – by and large – sensible cues to use in detecting deception.  

Jurors also sometimes relied on ‘life experience’ to judge a witness’s truthfulness. In the assault trial, some jurors used their experience of being involved in a fight to judge the credibility of various aspects of the witnesses’ evidence, including the fact that the defendant ran away and did not phone the police after the incident and the complainant’s claim that he picked up the knife after being stabbed. These insights were helpful to other jurors, sometimes leading them to reconsider their initial misplaced assumptions about how someone might react in a stressful situation. Juries here did exactly what they are supposed to do, bringing knowledge to decision making that would – most likely – be lacking if a professional judge were the fact finder. In the rape trial, however, the reliance on personal experience was less helpful. Biases about the extent to which a genuine victim would resist an attacker came into play, with (usually) male jurors drawing on their own experience of resisting attack to surmise that the female complainant could have resisted more forcefully, in contradiction to the extensive body of psychological literature establishing that ‘freezing’ or dissociative responses are common in response to a sexual assault.

In terms of the delivery of the testimony, the picture was even more concerning. In the psychological literature, body language has been shown to be a very poor indictor of truthfulness. Nonetheless, jurors regularly relied on a witness’s body language in trying to establish whether they were telling the truth. Averting the gaze was the most frequently mentioned indictor of an untruthful account – one juror doubted the complainant’s testimony because he “couldn’t make eye contact with anybody”. Another regarded the complainant as “shifty” because he “didn’t make eye contact”, contrasting that to the accused, who was seen as credible because he met the questioner’s gaze. One witness was doubted because he “kept looking away”. Other cues (wrongly) associated with lying were hand movements, fidgeting, licking the lips and, in one case, “the way [the witness] stood – he looks a bit dodgy”.

Jurors also looked to the degree of emotion displayed by the witness. This was especially common in the rape trial, where jurors assessed whether the level of emotion displayed by the complainant was consistent with having been raped. In our trial reconstruction, the actor playing the complainant was visibly upset when giving evidence and this was usually seen as an indication that she was telling the truth. One juror commented that “I don't think you could fake those kinds of emotions when she is going through it like that”. Another said that he found her “convincing” because “she was emotional and upset right from the start”. That said, the complainant was sometimes seen as not emotional enough to be a credible rape victim. One juror said that they did not believe her story as “she didn't come across distressed enough personally for me”. Another commented that “I felt there would have been a bit more anger and rage if he had done it”. These expectations were not placed on the male complainant in the assault trial, whose level of emotion was barely commented upon.

Finally, jurors looked to whether or not the witness seemed nervous, agitated or hesitant (or, by contrast, calm and/or confident). This too is problematic as witnesses can be nervous when testifying for all sorts of reasons unrelated to their truthfulness. 

What – if anything – can be done to improve juror credibility assessments is not an easy question. Detecting deception in another person is notoriously difficult, especially where that person is a stranger, and research has shown that people think they are much better at it than they actually are. Accuracy can be enhanced by detailed training, and by the use of specialist cognitive techniques, but these measures would be difficult and/or costly to implement in criminal trials. But informing jurors about the factors that research has shown are – and perhaps more importantly are not – associated with truthfulness would be a start and might serve at least to prevent the most glaring misconceptions.


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