Susceptibility to Strategic Voting: A Comparison of Plurality and Instant-Runoff

• In Event: Electoral Systems as a Source of Bias

Abstract

Reformers and researchers often speculate that some voting systems are more susceptible to strategic voting than others, but existing measures of susceptibility are unsatisfying because they essentially ask how likely a voter is to regret a sincere vote, not how likely a voter is to anticipate that an insincere vote is beneficial. We propose a new approach to assessing susceptibility to strategic voting (and other voting system properties) under more realistic assumptions about uncertainty and voter behavior, and we use it to compare three-candidate plurality and instant-runoff (IRV) elections. Drawing on preference data from 160 electoral surveys, we estimate that the average voter's gain in expected utility from being strategic (rather than always voting sincerely) is between 5 and 35 times higher in plurality than in IRV. IRV is less susceptible to strategic voting both because casting an insincere vote is more risky in IRV and because strategic behavior by other voters tends to reduce the benefit of being strategic --- a previously unexplored type of negative feedback that we suggest deserves more attention.

Authors

• Andrew C. Eggers, Oxford University

• Tobias Nowacki, Stanford University