Designing Strategyproof Election Systems with Score Voting

TL;DR

This paper introduces a new framework for designing strategyproof score voting systems, ensuring truthful voting in participatory budgeting and similar decision-making processes by leveraging the Constrained Change Property.

Contribution

It develops a model and new social choice functions for strategyproof score voting, including an algorithm to optimize score functions for strategyproofness.

Findings

Score voting with core-based score functions is equivalent to knapsack voting.

Total score functions are strategyproof if they satisfy CCP.

An algorithm can find the closest strategyproof score function for any given score voting.

Abstract

We focus on the strategyproofness of voting systems where voters must choose a number of options among several possibilities. These systems include those that are used for Participatory Budgeting, where we organize an election to determine the allocation of a community's budget (city, region, etc.) dedicated to the financing of projects. We present a model for studying voting mechanisms and the Constrained Change Property (CCP), which will be used to design voting mechanisms that are always strategyproof. We also define a new notion of social choice function and use it to design a new class of utilitarian voting mechanisms that we call score voting. We prove that the mechanisms designed with core voting with a neutral score function are equivalent to knapsack voting on the same instance and that any score voting designed with a total score function is strategyproof if and only if its score function satisfies CCP. These results are combined to devise an algorithm that can find the closest total score function that makes any given score voting to be strategyproof.