Quantifying the Discrepancy between Realworld Data and Theoretical Predictions in a Theory of Strategic Voting

Abstract

The single non-transferable vote system leads political parties’ electoral strategy and electorates’ strategic voting to a characteristic equilibrium, where M+1 viable candidates compete for M seats in a district. This “M+1 rule” further implies that each winning candidate gets the same share of the total votes in a district and, in the Duvergerian equilibrium, the rest of the votes concentrates on the runner-up. Existing studies have tested this theory using the effective number of candidates. In this paper, I point out the problems of this convention, propose new measures, and apply them to the post-war election data in Japan. The results demonstrate that the equality of vote shares among winning candidates first increased and then fluctuated, whereas the concentration of votes on runner-ups rose substantially with a temporary decline. The findings suggest that political contexts and strategic reactions of political actors determine how the “M+1” rule works.