Fair Voting Outcomes with Impact and Novelty Compromises? Unraveling Biases in Electing Participatory Budgeting Winners

TL;DR

This paper investigates biases in participatory budgeting voting outcomes, introducing new impact and novelty metrics, analyzing 345 real-world cases, and revealing how different algorithms influence societal impact and representation.

Contribution

It develops a new axiomatic framework for impact and novelty assessment in voting, supported by large-scale bias analysis and novel metrics.

Findings

Equal shares voting reduces infrastructural impact but increases welfare, education, and culture impact.

Biases favor over-represented projects, affecting fairness and societal benefits.

Impact loss can be mitigated through improved campaign design and project ideation.

Abstract

Participatory budgeting, as a paradigm for democratic innovations, engages citizens in the distribution of a public budget to projects, which they propose and vote for implementation. So far, voting algorithms have been proposed and studied in social choice literature to elect projects that are popular, while others prioritize on a proportional representation of voters' preferences, for instance, the rule of equal shares. However, the anticipated impact and novelty in the broader society by the winning projects, as selected by different algorithms, remains totally under-explored, lacking both a universal theory of impact for voting and a rigorous unifying framework for impact and novelty assessments. This paper tackles this grand challenge towards new axiomatic foundations for designing effective and fair voting methods. This is via new and striking insights derived from a large-scale analysis of biases over 345 real-world voting outcomes, characterized for the first time by a novel portfolio of impact and novelty metrics. We find strong causal evidence that equal shares comes with impact loss in several infrastructural projects of different cost levels that have been so far over-represented. However, it also comes with a novel, yet over-represented, impact gain in welfare, education and culture. We discuss broader implications of these results and how impact loss can be mitigated at the stage of campaign design and project ideation.