In addition to supporting materials for two studies in the final paper, this site includes four studies from earlier versions of the paper that do not appear in the final published version. Paper abstract: Overprecision—the excessive certainty that one knows the truth—is both the most durable and least understood form of overconfidence. I propose a new theory, which holds that overprecision in judgment is caused by individuals’ neglect of all the ways in which they could be wrong. Confidence ought to be proportional to the evidence for a given answer divided by the evidence for all other answers. But when the number of possible answers is infinite, it is impossible to consider them all. Overconfidence in one’s favored answer is the likely consequence.