An empirical test of the adversarial allegiance effect when evaluating interrogation coerciveness

Adversarial allegiance is a bias that occurs when expert witnesses are unconsciously influenced by lawyers’ opinions before conducting their analysis. While this bias has been demonstrated empirically, the underlying mechanisms are unclear and, more importantly, the strategies required to mitigate its presence in legal settings are unknown. This study explores the impact of the adversarial allegiance effect on voluntariness decisions across two experiments. In Experiment 1, participants (N = 82) either read only a police interrogation, a police interrogation and prosecution materials, or a police interrogation with defence materials, and then answered questions related to the guilt of the suspect and the coerciveness of the interrogation. In Experiment 2 (N = 38), participants either read a police interrogation transcript with defence materials alone or with defence materials and information about the duty of expert witnesses, and then answered questions related to the guilt of the suspect and coerciveness of the interrogation. The results from Experiment 1, in line with previous research, indicated the presence of adversarial allegiance, with participants aligning with the expert/lawyer of the team to which they were randomly assigned. The results of Experiment 2 indicated that the intervention did not reduce adversarial allegiance. The implications of these findings are discussed.