Quantifying the presumption of innocence

The presumption of innocence is sacrosanct in Anglo-legal doctrine, yet how jurors interpret it remains unknown. This experiment manipulated the alleged crime (violent, child, or sexual assault) and the defendant’s physical appearance (good, mediocre, or bad). Following Savage (1954), uncertainty about the guilt of the defendant was conceptualized in terms of prospective jurors’ willingness to gamble about whether the defendant is guilty. For each case, participants were asked to choose between a standard lottery, with specified probability of winning $100 versus nothing, and a trial gamble involving the unspecified probability of guilt (win $100) and innocence (win nothing) for the case presented. Participants indicated a preference to either play the standard lottery or the trial gamble, or indifference between the two options. Once a participant indicated indifference, the probability of guilt was set equal to the probability of winning the standard lottery. Across all 3 types of crimes and all 3 categories of appearance, median participants’ prior odds of guilt were close to 0.50, indicating that prospective jurors generally believed the defendants were just as likely to be guilty as innocent priorto the introduction of any evidence. A main effect for appearance was detected. ‘Bad’ and ‘mediocre’ defendants were perceived to be more likely to be guilty than ‘good’ defendants. There were no differences between the various alleged crimes. Overall, male participants and self-identified Republicans were the most likely to have high (>0.65) prior probabilities of guilt. Implications for legal doctrine are discussed.