There is abundant empirical evidence that the plurality rule constrains party competition and favors two-party systems. This reduction of party system fragmentation may be due to parties deciding not to enter elections for which they are not viable and/or voters voting strategically. Yet, no prior research has attempted to estimate the respective role of parties and voters in this process. To fill this gap, we conducted a unique laboratory experiment where some subjects played the role of parties and others played the role of voters, and where the two were able to respond to each other just as in real-life elections. We find that the reduction due to party strategic exit is higher than that due to strategic voting. We conclude that parties play a key role in the effect of the plurality rule on party system fragmentation.
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