Jurors' use of probabilistic evidence

Mock jurors' use of probabilistic evidence was examined in a fractional factorial design manipulating 7 variables: strength of nonstatistical evidence; quantification of nonstatistical evidence; strength of statistical evidence; combination of 2 pieces of statistical evidence; instruction in use of Bayes' theorem; and presentation of fallacies (both prosecutor's and defense attorney's) concerning use of statistical evidence. One hundred eighty-nine subjects viewed 1 of 16 videotapes presenting a condensed mock trial. Subjects completed dependent measures after each of 4 witnesses and at the end of trial. The strength of both nonstatistical and probabilistic evidence affected verdicts; the other manipulations did not. Overall, subjects slightly underused the probabilistic evidence, as compared to their individualized Bayesian norms, and subjects did not succumb to fallacies. However subjects greatly varied in over- or underutilization even after Bayesian instruction. Future research should examine use of weak nonstatistical evidence, and should test different probabilistic instructions.