Economists and political scientists have long debated the socially optimal proportion of a collective decision-making body whose consent should be required in order to reach a decision. In their classic contribution to this debate, Buchanan and Tullock [Buchanan, J.M., Tullock, G., 1962. The Calculus of Consent. Univ. of Michigan Press, Ann Arbor] argued that, in the absence of what they called ... [Show full abstract] `decision-making costs', the unanimity rule is socially optimal. The present paper shows that their approach actually leads to the conclusion that the unanimity rule is (almost always) suboptimal. In contrast, simple majority rule—the rule most commonly observed in collective decisions—is found to be socially optimal under plausible conditions.