doi: 10.3389/fpsyg.2012.00381 The influence of initial beliefs on judgments of probability

This study aims to investigate whether experimentally induced prior beliefs affect process-ing of evidence including the updating of beliefs under uncertainty about the unknown probabilities of outcomes and the structural, outcome-generating nature of the environ-ment. Participants played a gambling task in the form of computer-simulated slot machines and were given information about the slot machines ’ possible outcomes without their asso-ciated probabilities. One group was induced with a prior belief about the outcome space that matched the space of actual outcomes to be sampled; the other group was induced with a skewed prior belief that included the actual outcomes and also fictional higher out-comes. In reality, however, all participants sampled evidence from the same underlying outcome distribution, regardless of priors given. Before and during sampling, participants expressed their beliefs about the outcome distribution (values and probabilities). Evaluation of those subjective probability distributions suggests that all participants ’ judgments con-verged toward the observed outcome distribution. However, despite observing no support-ing evidence for fictional outcomes, a significant proportion of participants in the skewed priors condition expected them in the future. A probe of the participants ’ understanding o