Two-ballot versus plurality rule: an empirical investigation on the number of candidates

Duverger claimed more than 50 years ago that the number of candidates in elections should be a function of electoral rules. Both his ?law?and ?hypothesis?suggest the number of candidates vying for seats in elections to be tightly linked to characteristics of the elec- toral process such as its degree of proportionality and the presence of runo¤s. Here we test the validity of Duverger?s claim using data from municipal elections in Brazil. Our study di¤ers from others in the ?eld in two important dimensions. First, by using municipal data we avoid the usual problems that plague statistical analysis using cross-country data. Sec- ondly, we have a truly exogenous source of variation due to a change in electoral legislation introduced by the constitutional reform of 1988: simple plurality remained the rule only in municipalities with less than 200,000 voters, and a second-ballot became mandatory for the others above that threshold. This allows for a neat identi?cation strategy using panel data. Our main ?nding is that elections with runo¤s lure greater numbers of candidates in municipalities with su¢ ciently high levels of heterogeneity.