Split Cycle: A New Condorcet Consistent Voting Method Independent of Clones and Immune to Spoilers Wesley H. Hollidayy and Eric Pacuitz y University of California, Berkeley (
[email protected]) z University of Maryland (
[email protected]) March 23, 2021 Abstract We propose a Condorcet consistent voting method that we call Split Cycle. Split Cycle belongs to the small family of known voting methods that significantly narrow the choice of winners in the presence of majority cycles while also satisfying independence of clones. In this family, only Split Cycle satisfies a new criterion we call immunity to spoilers, which concerns adding candidates to elections, as well as the known criteria of positive involvement and negative involvement, which concern adding voters to elections. Thus, in contrast to other clone-independent methods, Split Cycle mitigates both “spoiler effects” and “strong no show paradoxes.” Contents 1 Introduction 2 1.1 The Problem of Spoilers . .4 1.2 The Strong No Show Paradox . .7 1.3 Organization . 10 2 Preliminaries 11 2.1 Profiles, Margin Graphs, and Voting Methods . 11 arXiv:2004.02350v8 [cs.GT] 23 Mar 2021 2.2 Operations on Profiles . 13 3 Split Cycle 14 3.1 Three Main Ideas . 14 3.2 Defining Split Cycle . 16 3.3 Comparison with Beat Path and Ranked Pairs . 20 4 Core Criteria 22 4.1 Condorcet Criteria . 23 4.2 Monotonicity . 23 1 4.3 Independence of Clones . 24 4.4 Narrowing . 25 5 New Criteria 26 5.1 Immunity to Spoilers and Stability for Winners . 27 5.2 Amalgamation . 29 5.3 Rejectability .