RT Journal Article SR Electronic T1 Variability in prior expectations explains biases in confidence reports JF bioRxiv FD Cold Spring Harbor Laboratory SP 127399 DO 10.1101/127399 A1 Pablo Tano A1 Florent Meyniel A1 Mariano Sigman A1 Alejo Salles YR 2017 UL http://biorxiv.org/content/early/2017/09/09/127399.abstract AB Confidence in a decision is defined statistically as the probability of that decision being correct. Humans, however, display systematic confidence biases, as has been exposed in various experiments. Here, we show that these biases vanish when taking into account participants' prior expectations, which we measure independently of the confidence report. We use a wagering experiment to show that modeling subjects' choices allows for classifying individuals according to their prior biases, which fully explain from first principles the differences in their later confidence reports. Our parameter-free confidence model predicts two counterintuitive patterns for individuals with different prior beliefs: pessimists should report higher confidence than optimists, and, for the same task difficulty, the confidence of pessimists should increase with the generosity of the task. These findings show how systematic confidence biases can be simply understood as differences in prior expectations.