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 cand idates. 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 Duv erger's propositions at the presidential level. Unlike previous cross-natio nal studies, I also test for the effect of the presence or absence of an in cumbent presidential candidate. The presence of an incumbent has a signific ant reductive effect on the effective number of candidates.