In a recent issue of this journal, Baranski and Petrusic (1994) presented emperical data revealing overconfidence in sensory discrimination. In this paper, we propose an explanation of Baranski and Petrusic's results, based on an idiosyncrasy in the experimental setting that misleads subjects who are using an unwarranted symmetry assumption. Experiment 1 showed that when this hypothesis is controlled for, a large underconfidence bias is obtained with Baranski and Petrusic's procedure. The results of Experiment 2 confirmed that overconfidence is difficult to obtain in subject-controlled sensory discrimination tasks, even for a very low proportion of correct responses. The different results obtained in sensory and cognitive tasks suggest that one should not uncritically draw parallels between confidence in sensory and cognitive judgments.