The disunity of unanimity

Unanimity is the optimal voting rule in a world of zero transactions costs, when side payments are impossible. When side payments are available and transactions costs are zero, the voting rule is irrelevant to the ultimate outcome. In the more realistic situation where side payments are allowed but transactions costs are positive, a unanimity voting rule creates situations where the collective choice may fail a proposed measure even if all members favor the measure in principle. This evidences a disunity between unanimity rules and unanimous outcomes. Constitutional design should focus on rules leading to unanimous outcomes, as opposed to unanimity rules.