Abstract. We test several claims about the relationship between unanimity rule and Pareto optimality. Buchanan and Tullock (1962), Mueller (2003), and other scholars argue that unanimity rule is more capable of producing Pareto optimal outcomes than other voting rules, such as majority rule, because unanimity rule passes an alternative only if it makes everyone better off. Majority rule can pass alternatives that make some individuals worse off. Dougherty and Edward (2008), in contrast, claim that majority rule is at least as likely to select Pareto optimal outcomes as unanimity rule in finite games if proposals are random, sincere, or strategic. We test the two sets of conjectures in a two dimensional framework using laboratory experiments. Our results suggest: 1) majority rule enters the Pareto set more quickly than unanimity rule, 2) majority rule leaves the Pareto set at the same rate as unanimity rule, and 3) majority rule is more likely to select a Pareto optimal outcome than unanimity rule in the final round of play. Our results also suggest that proposers do not behave observationallyrational in the final round and complete information does not affect the primary result. * All authors: Department of Political Science, University of Georgia, Athens, GA 30602. Corresponding author: (706) 542-2989, .Throughout the paper, we use the term "majority rule" to refer to simple majority rule 1 (i.e. a proposal passes if and only if the yeas exceed the nays) and "unanimity rule" to refer to simple unanimity rule (i.e. a proposal passes if and only if the yeas > 0 and the nays = 0). See Dougherty and Edward (2004) and Riker (1982) for more details.Pareto optimality, also known as Pareto efficiency, is the most widely accepted criteria 2 for evaluating efficiency. An alternative is Pareto optimal if no alternative is Pareto preferred to it. That is, no other alternative can make at least one individual better off without making another individual worse off.A k-majority rule requires at least k individuals to vote in favor of a proposal in order for 3 the proposal to pass, where N/2 < k # N and N is the number of individuals; otherwise the status quo is chosen. Two special cases of k-majority rule are majority rule (k = N/2) and unanimity rule (k = N). See Dougherty and Edward (2004) for more precise definitions when non-voters and "votes to abstain" are permitted.