Atypical behaviours found in some mental health conditions negatively affect judgments of deception and credibility

Objective: Unusual behaviours are commonly perceived to indicate deception and low credibility. However, they may also characterise individuals with certain mental health or developmental conditions, thus making those individuals vulnerable to negative judgements. We examined the effect of four behaviours–gaze aversion, body movements, monologuing, and flat affect–on judgements of deception and credibility. Method: In an online experiment, we presented participants (N = 392) with videos of actors being interviewed about their involvement in stealing money. In each video, the actor was either instructed to display one of the four behaviours or was not instructed to display any particular behaviour (control condition). Participants were then asked to provide ratings of perceived deception and credibility. Results: There were significant effects of body movements and monologuing on perceived deception, and significant effects of monologuing and flat affect on the specific credibility dimension of perceived caring. Gaze aversion did not have a statistically significant effect on perceived deception or credibility. Conclusion: Body movements, monologuing, and flat affect negatively affected deception and credibility judgements. Populations who commonly display these behaviours, such as individuals with certain mental health or developmental conditions, may be vulnerable to unfair evaluations in the criminal justice system. KEY POINTS What is already known about this topic: Based on self-report measures, unusual behaviours are widely believed to be indicators of deception and low credibility. There is no empirical evidence that behavioural cues are reliable indicators of deception or credibility. Gaze aversion, repetitive body movements, monologuing, and flat affect are common among individuals with certain disabilities and mental health conditions. What this paper adds: This study provides experimental support for the effect of repetitive body movements, monologuing, and flat affect on judgements of deception and credibility. Individuals who present with repetitive body movements, monologuing, or flat affect due to a disability or mental health condition may be subject to unjust evaluations of deception and credibility. There is an important need for further research on how individuals with disabilities and mental health conditions are perceived in the criminal justice system.