Fair and Efficient Assignment via 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. We offer two characterizations of the prominent lottery assignment mechanism called the probabilistic serial. We show that it is the only mechanism satisfying non-wastefulness and ordinal fairness. Our second result shows that a direct ordinal mechanism satisfies ordinal efficiency, envy-freeness, and upper invariance if and only if it is the probabilistic serial.