Electoral Institutions and Party System Problems: the Run-Off Rule and Brazilian Parties Prepared for the October, 2010 Meeting of the Latin American Studies Association by Daniel J. Epstein Colgate University depstein@colgate.edu Abstract: This paper uses very basic statistical methods, including a quasi-experimental design, to test one of the tenets of Duverger's famous law regarding the effect that electoral rules have on the fragmentation of party systems, specifically, the effects of the majority runoff (or second ballot) rule, which is predicted to give rise to a multiparty system, while a simply plurality single-member district system is predicted to give rise to a two-party system. They paper takes an advantage of provisions of the Brazilian constitution that impose the runoff rule on mayoral elections in municipalities with over 200,000 voters, while those with fewer voters elect their mayors by simple plurality. By various selection methods meant to reproduce an experimental design as closely as possible (on the assumption that the “treatment” that occurs when the 200,000 th voter is added to a municipality's elector rolls is relatively random and arbitrary compared with the 199,999 th , the 175,000 th or the 212,121 st voter), the paper shows that the predicted effect of the addition of runoff rule, an increase in the number of parties, is nowhere to be found. DRAFT VERSION—PLEASE DO NOT CITE WITHOUT AUTHOR'S PERMISSION