On Plurality Voting and Runoff Elections: Information Aggregation under Divided Majority

This article compares Plurality Voting (PV) and two forms of Runoff Elections (RE) in a setting in which (i) there are two majority-preferred alternatives, (ii) a strong minority backs a third alternative which would make the majority strictly worse off, and (iii) some of the majority voters are uninformed about the "correct" majority alternative. I show that in the informative equilibrium in Majority Runoff Elections (MRE), uninformed majority voters vote randomly with strictly positive probability, achieving partial information aggregation, while they always abstain in Automatic Runoff Elections (ARE), achieving full information aggregation and strictly improving the majority's welfare. However, uninformed majority voters do not abstain in PV, resulting in less information aggregation than in both MRE and ARE.