Votes Can Be Confidently Bought in Instant Runoff Elections, and What to Do About It

44 Pages Posted: 18 Mar 2023

See all articles by Jack Williams

Jack Williams

Massachusetts Institute of Technology (MIT) - Department of Political Science, Massachusetts Institute of Technology

Samuel Baltz

Massachusetts Institute of Technology (MIT) - Department of Political Science, Massachusetts Institute of Technology

Charles Stewart III

Massachusetts Institute of Technology (MIT) - Department of Political Science

Date Written: March 13, 2023

Abstract

We show that in Instant Runoff Voting (IRV) elections it is often possible to violate the secret ballot. There are so many ways to rank even a handful of candidates that most possible votes will not be cast in a typical IRV election, so a vote buyer could purchase these votes, and then use the announced election results to verify that they were successfully cast. We examine the feasibility of this attack both theoretically and empirically. Theoretically, we show that when voters can rank a reasonably large number of candidates, most possible votes in an IRV election will be unique. Under very conservative assumptions, we show that vote buyers should expect a very low probability that a vote they purchase will be coincidentally cast by someone other than the colluding voter. We then verify this finding using empirical data from 36 IRV contests, including Cast Vote Records from San Francisco. While there is no evidence that this vote-buying scheme has ever been used, its existence has immediate implications for the administration and security of IRV elections: this scheme is more feasible when more candidates can be ranked in the election, and when election results are reported at lower levels of aggregation.

Keywords: Instant Runoff Voting, Ranked-Choice Voting, Election Security, Vote Buying

Suggested Citation

Williams, Jack and Baltz, Samuel and Stewart III, Charles, Votes Can Be Confidently Bought in Instant Runoff Elections, and What to Do About It (March 13, 2023). Available at SSRN: https://ssrn.com/abstract=4387782 or http://dx.doi.org/10.2139/ssrn.4387782

Jack Williams

Massachusetts Institute of Technology (MIT) - Department of Political Science, Massachusetts Institute of Technology ( email )

30 Wadsworth Street (Rm 470)
Cambridge, MA 02142
United States

Samuel Baltz (Contact Author)

Massachusetts Institute of Technology (MIT) - Department of Political Science, Massachusetts Institute of Technology ( email )

30 Wadsworth Street (Rm 470)
Cambridge, MA 02142
United States

Charles Stewart III

Massachusetts Institute of Technology (MIT) - Department of Political Science ( email )

77 Massachusetts Avenue
Cambridge, MA 02139
United States

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