Electoral laws and the effective number of candidates in presidential elections

This study employs data from 33 countries over a 50-year period to analyze the institutional determinants of the effective number of presidential candidates. I find the presidential electoral formula-plurality versus majority runoff-to have a strong and significant impact on the effective number of candidates, thereby providing the most substantial support thus far for Duverger's propositions at the presidential level. Unlike previous cross-national studies, I also test for the effect of the presence or absence of an incumbent presidential candidate. The presence of an incumbent has a significant reductive effect on the effective number of candidates.