Altruists and envious people who meet in contests are symbionts. They do better than a population of narrowly rational individuals. If there are only altruists and envious individuals, a particular mixture of altruists and envious individuals is evolutionarily stable.

Altruists and envious people who meet in contests are symbionts. They do better than a population of narrowly rational individuals. If there are only altruists and envious individuals, a particular mixture of altruists and envious individuals is evolutionarily stable.

We show that there is no consistent Pareto improvement over any stable mechanism. To overcome this impossibility, we introduce the following weak consistency requirement: Whenever a set of students, each of whom is assigned to a school that is under-demanded at the student-optimal stable...