M. Utku Ünver

A Kidney Exchange Clearinghouse in New England

In 2003 there were 8,665 transplants of deceased donor kidneys for the approximately 60,000 patients waiting for such transplants in the United States. While waiting, 3,436 patients died. There were also 6,464 kidney transplants from living donors (Scientific Registry of Transplant Recipients web site). Live donation is an option for kidneys, since healthy people have two and can remain healthy with one. While it is illegal to buy or sell organs, there have started to be kidney exchanges involving two donor–patient pairs such that each (living) donor cannot give a kidney to the intended recipient because of blood type or immunological incompatibility, but each patient can receive a kidney from the other donor. So far these have been rare: as of December 2004, only five exchanges had been performed in the 14 transplant centers in New England. One reason there have been so few kidney exchanges is that there have not been databases of incompatible patient–donor pairs. Incompatible donors were simply sent home. (Databases are now being assembled not only in New England, but also in Ohio and Baltimore.) Lainie Friedman Ross et al. (1997) discussed the possibility of exchange between incompatible patient–donor pairs. Not only have a few such two-way exchanges been performed, but two three-way exchanges (in which the donor kidney from one pair is transplanted into the patient in a second pair, whose donor kidney goes to a third pair, whose donor kidney goes to the first pair) have been performed at Johns Hopkins. There have also been a number of “list exchanges” in which an incompatible patient– donor pair makes a donation to someone on the waiting list for a cadaver kidney, in return for the patient in the pair receiving high priority for a cadaver kidney when one becomes available.

Matching, Allocation, and Exchange of Discrete Resources

A survey of the literature on matching market design regarding house allocation, kidney exchange, and school choice problems.

Credible Group-Stability in Many-to-Many Matching Problems

It is known that in two-sided many-to-many matching problems, pairwise stable matchings may not be immune to group deviations, unlike in many- to-one matching problems (Blair 1988). In this paper, we show that pairwise stability is equivalent to credible group stability when one side has responsive preferences and the other side has categorywise- responsive preferences. A credibly group-stable matching is immune to any “executable” group deviations with an appropriate definition of executability. Under the same preference restriction, we also show the equivalence between the set of pairwise-stable matchings and the set of matchings generated by coalition-proof Nash equilibria of an appropriately defined strategic-form game.

Incentive Compatible Allocation and Exchange of Discrete Resources

Allocation and exchange of discrete resources such as kidneys, school seats, and many other resources for which agents have single-unit demand is conducted via direct mechanisms without monetary transfers. Incentive compatibility and efficiency are primary concerns in designing such mechanisms. We show that a mechanism is indi- vidually strategy-proof and always selects the efficient outcome with respect to some Arrovian social welfare function if and only if the mechanism is group strategy-proof and Pareto efficient. We construct the full class of these mechanisms and show that each of them can be implemented by endowing agents with control rights over resources. This new class, which we call trading cycles, contains new mechanisms as well as known mechanisms such as top trading cycles, serial dictatorships, and hierarchical exchange. We illustrate how one can use our construction to show what can and what cannot be achieved in a variety of allocation and exchange problems, and we provide an example in which the new trading-cycles mechanisms strictly Lorenz dominate all previously known mechanisms.

House Allocation with Existing Tenants: A Characterization

We analyze mechanisms that are used to allocate dormitory rooms to students at college campuses. Students consist of newcoming freshmen, who do not currently occupy any rooms, and more senior students each of whom occupies a room from the previous year. In addition to the rooms already occupied by the existing tenants, there are vacated rooms by the graduating class. Students have strict preferences over dormitory rooms. Each student shall be assigned a dormitory room in an environment where monetary transfers are not allowed. An existing tenant can move to another room as a result of the assignment. We show that you request my house-I get your turn mechanisms are the only mechanisms that are Pareto-efficient, individually rational, strategy-proof, weakly neutral, and consistent.

Games of Capacity Manipulation in Hospital-intern Markets

In this paper, we analyze capacity manipulation games in hospital-intern markets inspired by the real-life entry-level labor markets for young physicians who seek residencies at hospitals. In a hospital-intern market, the matching is determined by a centralized clearinghouse using the preferences revealed by interns and hospitals and the number of vacant positions revealed by hospitals. We consider a model in which preferences of hospitals and interns are common knowledge. Hospitals play a capacity-reporting game. We analyze the equilibria of the game-form under the two most widely used matching rules: hospital-optimal and intern-optimal stable rules. We show that (i) there may not be a pure strategy equilibrium in general; and (ii) when a pure strategy equilibrium exists, every hospital weakly prefers this equilibrium outcome to the outcome of any larger capacity profile. Finally, we present conditions on preferences to guarantee the existence of pure strategy equilibria.

A Theory of School-Choice Lotteries

A new centralized mechanism was introduced in New York City and Boston to assign students to public schools in district school-choice programs. This mechanism was advocated for its superior fairness property, besides others, over the mechanisms it replaced. In this paper, we introduce a new framework for investigating school-choice matching problems and two ex-ante notions of fairness in lottery design, strong ex-ante stability and ex-ante stability. This frame- work generalizes known one-to-many two-sided and one-sided matching models. We first show that the new NYC/Boston mechanism fails to satisfy these fairness properties. We then propose two new mechanisms, the fractional deferred-acceptance mechanism, which is ordinally Pareto dominant within the class of strongly ex-ante stable mechanisms, and the fractional deferred- acceptance and trading mechanism, which satisfies equal treatment of equals and constrained ordinal Pareto efficiency within the class of ex-ante stable mechanisms.

Two axiomatic approaches to the probabilistic serial mechanism

This paper studies the problem of assigning a set of indivisible objects to a set of agents when monetary transfers are not allowed and agents reveal only ordinal preferences, but random assignments are possible. We offer two characterizations of the probabilistic serial mechanism, which assigns lotteries over objects. We show that it is the only mechanism satisfying non-wastefulness and ordinal fairness and the only mechanism satisfying sd-efficiency, sd-envy-freeness, and weak invariance or weak truncation robustness (where “sd” stands for first-order stochastic dominance).

Random Paths to Pairwise Stability in Many-to-Many Matching Problems: A Study on Market Equilibration

This paper considers a decentralized process in many-to-many matching problems. We show that if agents on one side of the market have substitutable preferences and those on the other side have responsive preferences, then, from an arbitrary matching, there exists a finite path of matchings such that each matching on the path is formed by satisfying a blocking individual or a blocking pair for the previous matching, and the final matching is pairwise-stable. This implies that an associated stochastic process converges to a pairwise-stable matching in finite time with probability one, if each blocking individual or pair is satisfied with a positive probability at each period along the process.

Unraveling Yields Inefficient Matchings: Evidence from Post-Season College Football Bowls

Many markets have “unraveled” and experienced transactions at dispersed and apparently inefficiently early times. Often these markets develop institutions to coordinate and delay the timing of transactions. However it has proved difficult to gather data that allows the efficiency gains to be identified and measured. The present paper considers a market for which such data can be gathered. Prior to 1992, college football teams were matched for post-season play, in “bowl” games, up to several weeks before the end of the regular football season. Since 1992, the market has undergone a series of reorganizations that postpone this matching until the end of the regular season. We show that this has promoted more efficient matching of teams, as measured by the resulting television viewership. The chief driver has been the increased ability of later matching to produce “championship” games.