Strategy-Proof Budgeting via a VCG-Like Mechanism

We present a strategy-proof public goods budgeting mechanism where agents determine both the total volume of expenses and specific allocation. It is constructed as a modification of VCG to a non-typical environment, where we do not assume quasi-linear utilities or direct revelation. We further show that under plausible assumptions it satisfies strategyproofness in strictly dominant strategies, and consequently implements the social optimum as a Coalition-Proof Nash Equilibrium. A primary (albeit not an exclusive) motivation of our model is Participatory Budgeting, where members of a community collectively decide the spending policy of public tax dollars. In that scenario, charging individual payments from voters as the VCG method instructs would be undesirable, thus our second main result provides that, under further specifications relevant in that context, these payments will vanish in large populations, and can further be constructed as non-positive in some cases.