In saiban-in systems, both lay and professional judges deliberate and reach a verdict. Previous studies have suggested that the opinions of professional judges strongly affect those of lay judges owing to the legitimate power of the former. On the other hand, research on persuasion shows that “inoculation” contributes to inhibiting the effect of persuasion. This study examined the effects of closing arguments, treated as a form of inoculation, on the deliberations and decision making of lay judges. In Experiment 1, 24 undergraduates participated in a mock deliberation. Four undergraduates served as mock lay judges and a law school graduate served as a mock professional judge to constitute a mock jury. Before deliberation, participants read a trial scenario in which the main issue was the “intention to kill.” The closing argument in the scenario read by half the juries included only the defense attorney’s presentation of the defendant’s behavior (non-Inoculation group), whereas the remaining juries read this presentation as well as the legal standards for judging “intention to kill (inoculation group),” The results showed that the lay judges in the inoculation group were less likely to challenge the opinion of the professional judge than were those in the non-inoculation group. However, very few explicit references to the defense attorney were made during deliberation. Experiment 2, which involved the presentation of a self-defense scenario to 24 undergraduate and graduate students, revealed a tendency similar to that found in Experiment 1, but it was not statistically significant. These results suggest that inoculation in closing arguments can affect the reactions of lay judges to the implicit power sometimes wielded by professional judges.
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