Speaker: Dominik Peters
Affiliation: University of Toronto
Date and Time: Friday, October 15, 11am - 12 noon.

Title: Proportional Participatory Budgeting

Abstract: We study voting rules for participatory budgeting, where a group of voters collectively decides which projects should be funded using a common budget. Many city governments around the world now use participatory budgeting to allow their residents to influence how the city spends its money. Most cities handle such elections using the most intuitive voting rule: for each project, count how many votes it got, and then greedily fill a knapsack with projects that received the highest number of votes. We argue that this is a bad voting rule, because it can ignore the preferences of substantial minorities. Instead, we advocate the use of voting rules that are proportional: every voter should have roughly equal influence on the final budget, and thus different interests should be represented in proportion to the number of their supporters. We formalize the notion of proportionality as an axiom and design a simple and attractive voting rule that satisfies our formal axiom, and that can be evaluated in polynomial time. We also prove that a large class of voting rules based on optimization cannot achieve proportional outcomes: this is surprising because in many other contexts, researchers have found specific objective functions that lead to fair outcomes.

Based on joint work with Grzegorz Pierczyńskiand Piotr Skowron (https://arxiv.org/abs/2008.13276 )

Bio: Dominik is a postdoc at U of T working with Nisarg Shah on designing fair algorithms and voting rules. He also works on resource allocation and on using SAT solvers to analyze preference aggregation problems. Previously, he was a postdoc at Harvard University and Carnegie Mellon University, and a PhD student at the University of Oxford.