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

Countries that elect their policy-makers by means of Plurality Voting tend to have a two-party system. We conduct laboratory experiments to study whether alternative voting procedures yield a two-party system as well. Plurality Voting is compared with Approval Voting and Dual Voting, both of which allow voters to vote for multiple candidates, but differ in whether voters are required to cast all their votes. We find that both Plurality and Approval Voting yield a two-party system, whereas Dual Voting may yield a multi-party system due to strategic voting. Voters’ ability to truncate ballots (not cast all their votes) is essential for maintaining the two-party system under Approval Voting.