Strategyproof Mechanisms for Additively Separable Hedonic Games and Fractional Hedonic Games

Additively separable hedonic games and fractional hedonic games have received considerable attention. They are coalition forming games of selfish agents based on their mutual preferences. Most of the work in the literature characterizes the existence and structure of stable outcomes (i.e., partitions in coalitions), assuming that preferences are given. However, there is little discussion on this assumption. In fact, agents receive different utilities if they belong to different partitions, and thus it is natural for them to declare their preferences strategically in order to maximize their benefit. In this paper we consider strategyproof mechanisms for additively separable hedonic games and fractional hedonic games, that is, partitioning methods without payments such that utility maximizing agents have no incentive to lie about their true preferences. We focus on social welfare maximization and provide several lower and upper bounds on the performance achievable by strategyproof mechanisms for general and specific additive functions. In most of the cases we provide tight or asymptotically tight results. All our mechanisms are simple and can be computed in polynomial time. Moreover, all the lower bounds are unconditional, that is, they do not rely on any computational or complexity assumptions

On the macroeconomics of uncertainty and incomplete markets

Presidential address for the Twelfth World Congress of the International Economic Association, summarising semi-formally the author's recent work and concerns. Uncertainty and incomplete markets breed demand volatility as well as price and wage rigidities. The conjunction of these leads to multiple, volatile supply-constrained equilibria, typically reflecting coordination failures and apt to display persistence - as documented by three supporting theorems. Specific implications are linked to the conclusions that we should take coordination failures seriously, try to obviate demand volatility and try to by-pass price and wage rigidities