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27 March 2019, Computational Social Choice Seminar, Dominik Peters

Speaker: Dominik Peters (Oxford)
Title: Truthful Aggregation of Budget Proposals
Date: Wednesday 27 March 2019
Time: 16:00
Location: Room F3.20, Science Park 107, Amsterdam


We consider a participatory budgeting problem in which each voter submits a proposal for how to divide a single divisible resource (such as money or time) among several possible alternatives (such as public projects or activities) and these proposals must be aggregated into a single consensus division. Under ℓ_1 preferences---for which a voter's disutility is given by the ℓ_1 distance between the consensus division and the division he or she most prefers---the social welfare-maximizing mechanism, which minimizes the average ℓ_1 distance between the outcome and each voter's proposal, is incentive compatible [Goel et al. 2016]. However, it fails to satisfy a natural fairness notion of proportionality, placing too much weight on majority preferences. Leveraging a connection between market prices and the generalized median rules of Moulin [1980], we introduce the independent markets mechanism, which is both incentive compatible and proportional. We unify the social welfare-maximizing mechanism and the independent markets mechanism by defining a broad class of moving phantom mechanisms that includes both. We show that every moving phantom mechanism is incentive compatible. Finally, we characterize the social welfare-maximizing mechanism as the unique Pareto-optimal mechanism in this class, suggesting an inherent tradeoff between Pareto optimality and proportionality. (This is joint work with Rupert Freeman, David Pennock, and Jenn Wortman Vaughan.)

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