Multiple votes,ballot truncation and the two-party system: an experiment

Duverger’s law states that Plurality Voting tends to favor a two-party system. We conducted laboratory experiments to study whether voting procedures other than Plurality Voting could favor a two-party system as well. Plurality Voting is compared with Approval Voting and Dual Voting, both of which allow to vote for multiple candidates, but differ in whether voters are required to cast all their votes. We find that in most elections held under Plurality Voting and Approval Voting, at most two candidates are viable. By contrast, three candidates are viable in a sizable number of elections held under Dual Voting due to strategic voting. Our evidence suggests that Approval Voting may encourage a two-party system, whereas Dual Voting may encourage multipartism. The voters’ ability to truncate ballots (i.e., not cast all their votes) is essential for supporting the two-party system under Approval Voting.