Dear all,

The next talk in the Computational Social Choice Seminar at the ILLC will be given by Dominik Peters (Oxford) this Wednesday at 16:00. Dominik will be speaking about participatory budgeting. The abstract is included below. I hope to see you at the seminar.

As always, for more information on the COMSOC Seminar, please consult https://staff.science.uva.nl/u.endriss/seminar/.

All the best, Ulle

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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

Abstract: 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.)