Anchor-proofness in Voting with Ranked Ballots

2026-10-07

AI Summary

This study addresses the vulnerability of ranked voting systems to anchoring bias induced by candidate presentation order, which compromises electoral fairness. Drawing on social choice theory and formal proofs, it systematically analyzes the robustness of ranked ballots to presentation order, revealing an inherent trade-off between resistance to anchoring and information utilization. The primary contribution is a rigorous proof that, under ranked voting mechanisms, tops-only rules constitute the unique solution capable of resisting anchoring bias. Furthermore, this work demonstrates that no non-constant rule exhibits such robustness under approval voting. By establishing these theoretical foundations, the study provides a uniquely optimal criterion for the design of manipulation-resistant voting mechanisms.

Abstract

When voters evaluate alternatives sequentially, an anchoring bias makes their ballots depend on the order in which alternatives are presented. For approval ballots, no non-constant voting rule is robust to this dependence. We show that ranked ballots change the picture sharply: a rank-based rule is robust to the presentation order if and only if it is tops-only. Robustness to anchoring is thus available, but only at the price of ignoring everything a voter reports beyond her favourite alternative.

Problem

• anchoring bias
• ranked ballots
• voting rules
• robustness
• presentation order

Innovation

• anchoring bias
• ranked ballots
• tops-only
• voting rules
• robustness