How should groups make decisions? The authors provide an original evaluation of 9 group decision rules based on their adaptive success in a simulated test bed environment. When the adaptive success standard is applied, the majority and plurality rules fare quite well, performing at levels comparable to much more resource-demanding rules such as an individual judgment averaging rule. The plurality rule matches the computationally demanding Condorcet majority winner that is standard in evaluations of preferential choice. The authors also test the results from their theoretical analysis in a behavioral study of nominal human group decisions, and the essential findings are confirmed empirically. The conclusions of the present analysis support the popularity of majority and plurality rules in truth-seeking group decisions.Human societies rely on groups to make many important decisions. There is a deep-seated belief that groups are more accurate and more just than individuals. This belief is based on the commonsense notion that a group has more problem-solving resources than any individual member because "several heads are better than one." The belief also depends on the assumption that the group process is effective at eliciting and integrating its members' beliefs and preferences. The most popular decision rule in groups of all types is the majority rule. The majority rule has many virtues: It is "transparent" and the easiest of all social decision rules to execute; it is based on a simple principle of equal participation and equal power; it encourages the expression of sincere personal beliefs, rather than conformity; and it yields more effective problem solutions than typical (and sometimes even most accurate) members could achieve. The present article is an exploration of the capacity of the majority rule and the closely related plurality rule to produce accurate judgments.The majority rule is popular across the full spectrum of human groups from hunter-gatherer tribal societies (Boehm, 1996;Boyd & Richerson, 1985;Wilson, 1994) to modern industrial democracies (Mueller, 1989). Certainly, in ad hoc Western groups, it seems to be the decision rule most frequently adopted to make formal social choices in popular elections, legislatures, and committees. The second most popular explicit procedure in committees is an autocratic "leader decides" rule (e.g., Smith & Bliege-Bird, 2000). However, when no explicit decision rule is adopted by a group or when a designated leader appears to make the group's decision, the implicit decision rule is usually still essentially a majority rule (cf. Davis, 1973Davis, , 1982Kerr, Stasser, & Davis, 1979;Stasser, Kerr, & Davis, 1980). Even when a small group has an explicit rule other than a majority rule, the largest initial faction, that is, the plurality, is usually the ultimate winner (cf. There are several versions of majoritarian rules used in human societies. The most popular version in America is a majority-plurality rule in which the candidate that receives the most votes wins ...