This paper uses exogenous variation in electoral rules to test the predictions of strategic voting models and the causal validity of Duverger's Law. Exploiting a regression discontinuity design in the assignment of single-ballot and dual-ballot (runoff) plurality systems in Brazilian mayoral races, the results indicate that single-ballot plurality rule causes voters to desert third placed candidates and vote for the top two vote getters. The effects are stronger in close elections and cannot be explained by differences in the number of candidates, as well as their party affiliation and observable characteristics.