The paper proposes a way to measure mechanical and psychological effects of majority runoff versus plurality electoral systems in candidate elections. Building on a series of laboratory experiments, we evaluate these effects with respect to the probability of electing a Condorcet winner candidate. In our experiment, the runoff system very slightly favours the Condorcet winner candidate, but this total effect is small. We show that this is the case because the mechanical and psychological effects tend to cancel each other out. Compared to plurality, the mechanical effect of runoffs is to systematically advantage the Condorcet winner candidate, as usually assumed; but our study detects an opposite psychological effect, to the disadvantage of this candidate.
TSE Working Paper, n. 12-296, April 2012