We propose a solution to the conflict between fairness and efficiency in one-sided matching markets. A matching is essentially stable if any priority-based claim initiates a chain of reassignments that results in the initial claimant losing the object. We show that an essentially stable and Pareto efficient matching always exists and that Kesten's (2010) EADA mechanism always selects one while other common Pareto efficient mechanisms do not. Additionally, we show that there exists a student-pessimal essentially stable matching and that the Rural Hospital Theorem extends to essential stability. Finally, we analyze the incentive properties of essentially stable mechanisms.
Policymakers frequently use reserve categories to combine competing objectives in allocating scarce resources based on priority. For example, schools may prioritize students from underprivileged backgrounds for some of their seats while allocating the rest of them based solely on academic merit. The order in which different categories are processed has been shown to have an important, yet subtle impact on allocative outcomes-and has led to unintended consequences in practice. I introduce a new, more transparent way of processing reserves, which handles all categories simultaneously. I provide an axiomatic characterization of my solution, showing that it satisfies basic desiderata as well as category neutrality: if an agent qualifies for n categories, she takes 1/n units from each of them. A practical advantage of this approach is that the relative importance of categories is entirely captured by their quotas.
Previous literature has shown that private information is a transaction cost that prevents efficient reallocation in two-sided setups with bilateral trade or homogeneous goods. We derive conditions under which the impossibility of efficient trade extends to rich environments in which buyers and sellers have multi-dimensional private types, accommodating many-to-many trades and heterogeneous objects. If agents can be decomposed into unit constituents, the allocation problem can be represented as an assignment game and impossibility obtains through a generalization of Shapley's (1962) result that buyers and sellers are complements. We introduce a general family of payoff functions that ensures decomposability and thus impossibility.
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