The presence of irrelevant alternatives paradoxically increases confidence in perceptual decisions

Confidence in perceptual decisions often reflects the probability of being correct. Hence, we predicted that confidence should be unaffected or be minimally decreased by the presence of irrelevant alternatives. To test this prediction, we designed three experiments. In Experiment 1, participants had to identify the largest geometrical shape among two or three alternatives. In the three-alternative condition, one of the shapes was much smaller than the other two, being a clearly incorrect choice. Counter-intuitively, all else being equal, confidence was higher when the irrelevant alternative was present. We accounted for this effect with a computational model where confidence increases monotonically with the number of irrelevant alternatives, a prediction we confirmed in Experiment 2. In Experiment 3, we evaluated whether this effect replicated in a categorical task, but we did not find supporting evidence. Our findings stimulate the use of multi-alternative decision-making tasks to build a thorough understanding of confidence.