Minimal consistent enlargements of the immediate acceptance rule and the top trading cycles rule in school choice

 

We consider school choice problems. We are interested in solutions that satisfy consistency. Consider a problem and a recommendation made by the solution for the problem. Suppose some students are removed with their positions in schools. Consider the “reduced” problem consisting of the remaining students and the remaining positions. Consistency states that in the reduced problem, the solution should assign each remaining student to the same school as initially. Neither the immediate acceptance rule (also known as the Boston mechanism) nor the top trading cycles rule is consistent. We show that the efficient solution is the smallest consistent solution containing the immediate acceptance rule. It is also the smallest consistent solution containing the top trading cycles rule.

This paper studies many-to-one matching markets where each student is assigned to a hospital. Each hospital has possibly multiple positions and responsive preferences. We study the game induced by the student-optimal stable matching mechanism. We assume that students play their weakly dominant strategy of truth-telling. Roth and Sotomayor (1990) showed that there can be unstable equilibrium outcomes. We prove that any stable matching can be obtained in some equilibrium. We also show that the exhaustive class of dropping strategies does not necessarily generate the full set of equilibrium outcomes. Finally, we find that the so-called ‘rural hospital theorem' cannot be extended to the set of equilibrium outcomes and that welfare levels are in general unrelated to the set of stable matchings. Two important consequences are that, contrary to one-to-one matching markets, (a) filled positions depend on the particular equilibrium that is reached and (b) welfare levels are not bounded by the student and hospital-optimal stable matchings (with respect to the true preferences).

Consider a problem in which the cost of building an irrigation canal has to be divided among a set of people. Each person has different needs. When the needs of two or more people overlap, there is congestion. In problems without congestion, a unique canal serves all the people and it is enough to finance the cost of the largest need to accommodate all the other needs. In contrast, when congestion is considered, more than one canal might need to be built and each canal has to be financed. In problems without congestion, axioms related with fairness (equal treatment of equals) and group participation constraints (no-subsidy or core constraints) are compatible. With congestion, we show that these two axioms are incompatible. We define weaker axioms of fairness (equal treatment of equals per canal) and group participation constraints (no-subsidy across canals). These axioms in conjunction with a solidarity axiom (congestion monotonicity) and another axiom (independence of at-least-as-large-length) characterize the sequential weighted contribution family. Moreover, when we include a stronger version of congestion monotonicity and other axioms, we characterize subfamilies of these rules.