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 electoral process such as its degree of proportionality and the presence of runoffs. Here we test the validity of Duverger’s claim using data from municipal elections in Brazil. Our study differs from others in the field in two important dimensions. First, by using municipal data we avoid the usual problems that plague statistical analysis using cross-country data. Secondly, 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 identification strategy using panel data. Our main finding is that elections with runoffs lure greater numbers of candidates in municipalities with sufficiently high levels of heterogeneity.