Does the number of candidates increase turnout? Causal evidence from two-round elections

A long-lasting question in comparative politics is whether the number of candidates/parties increases turnout. Existing observational studies on the topic find mixed results. We thus apply a regression discontinuity design to data 13,910 legislative and cantonal electoral districts in France since 1978. In the two-round system used in these elections, the candidates who pass a certain vote threshold in the first round can participate in the second round. We use this discontinuity to estimate the causal effect of having a third candidate in the second round: it increases turnout by 3.5% points and the share of valid votes by 7.3% points. We confirm these findings with survey data from the 2012 legislative election. Further, we investigate the mechanism and find evidence supporting the alienation theory, according to which individuals whose preferences do not resonate with the preferences of any of the candidates are likely to abstain.