Social welfare, Fairness and incentives in Divisible Participatory Budgeting

Abstract: Participatory Budgeting (PB) is Social Choice framework in which a set of agents (e.g. residents of certain community) collectively decide on the spending of public funds via some voting mechanism (or 'rule'). In divisible PB, any distribution of the budget across the set of given alternatives is feasible.  I will present two types of divisible PB mechanisms, each designed to pursue different goals. The first of which is the Continuous Thiele's Rules (CTR) class of PB mechanisms, that offer a continuum of trading points between three major perceptions of distributive justice. Namely, utilitarianism, egalitarianism and proportional fairness.  Next, the US-VCG mechanism targets Incentive Compatibility. This mechanism is our variation on the well-known VCG mechanism, tailored for a divisible PB setting. As this setting is quite different than the classic VCG's 'natural environment', such adaptation imposes both technical and conceptual challenges. Moreover, we employ the mechanism in a model where the budget is directly financed through individual 'tax' payments that are also determined as part of the collective decision, and thereby affixing the overall volume of public budget.