Can Jurors Disregard Inadmissible Evidence? Using the Multiphase Optimization Strategy to Test Interventions Derived from Cognitive and Social Psychological Theories

Inadmissible evidence generally biases jurors toward guilty verdicts; jurors who hear inadmissible evidence are more likely to convict than jurors not exposed to inadmissible evidence-even when <i>admissible</i> evidence is constant. When inadmissible evidence is introduced, the common legal remedy is judicial instructions to jurors to disregard it. Appeals courts repeatedly affirm instructions to disregard as a sufficient safeguard of defendants' constitutional rights, despite research finding that jurors do not disregard when instructed. The goals of this research were to (1) test the main and interactive effects of four theory-driven candidate strategies to help jurors disregard inadmissible evidence (i.e., inducing suspicion, giving a substantive reason for disregarding, committing to disregarding, advising future jurors) and identify an optimized intervention package, and (2) evaluate whether adding the optimized intervention package showed more favorable effects than judicial instructions only. Study 1 used a 2⁴ full factorial randomized controlled trial to evaluate the four candidate intervention strategies. A synergistic interaction among the candidate components suggested an optimized intervention package comprising all four interventions. Study 2 used a parallel four-arm randomized controlled trial to compare conviction rates in the same hypothetical murder trial under four conditions: (1) no exposure to inadmissible evidence, (2) exposure to inadmissible evidence without objection, (3) exposure to inadmissible evidence + judicial instructions ("standard practice"), and (4) exposure + judicial instructions + optimized intervention package. Across both studies, mock jurors who received the optimized intervention package returned significantly lower conviction rates than comparison conditions. These findings show early promise that novel intervention strategies may assist jurors in disregarding inadmissible evidence. Interpretation, limitations, and calls to action are discussed.