The United States legal system relies on the ability of jurors to impartially consider complex psychological testimony despite research indicating that they are unable to distinguish between scientifically rigorous evidence and evidence based on less objective methods. The purpose of this study was to better understand how different forms of expert testimony and juror information processing mode influence decision making in a capital case. Mock jurors completed a measure identifying their preferences for processing information in a rational, logical manner (Rationality) and an affect-driven, emotional manner (Experientiality). Following this, participants were instructed to alter their cognitive processing through written instructions given by the judge. Specifically, participants were primed to employ a rational, analytical mindset or rely on their gut-instincts and intuition. Participants were then presented with a capital murder case vignette and expert testimony (clinical opinion or actuarial) concerning defendant likelihood for future violence (high or low) and psychopathy diagnosis (psychopath or no diagnosis). Finally, a sub sample of participants participated in a deliberations activity. Results suggest testimony type did not have a significant influence on juror ratings of future dangerousness or ultimate verdict. However, participants who reviewed clinical opinion testimony asserting the defendant was a psychopath perceived the defendant to be more psychopathic than participants who heard actuarial testimony making the same assertions. This effect was driven specifically by participants' higher ratings of the defendant's affective and interpersonal psychopathy traits. In addition, participation in the deliberations activity resulted in significant changes in perceptions of psychopathy. Specifically, participants who heard clinical opinion testimony asserting that the defendant was a psychopath perceived the defendant as significantly more psychopathic following the deliberations activity compared to their pre-deliberations ratings. Future research examining the relationship between psychopathy evidence and legal decision making should consider the relationship between individual information processing and perception of psychopathic traits. Further, these data support the importance of including deliberations in mock jury research. Finally, these data lend further support to the effectiveness of clinical opinion testimony, particularly in regards to its influence on juror perceptions of defendant psychopathic traits.
