Pairwise efficiency and monotonicity imply Pareto efficiency in (probabilistic) object allocation

We consider object allocation problems with capacities where objects have to be assigned to agents.

We show that a probabilistically monotonic lottery rule satisfies ex-post Pareto efficiency if and only if it satisfies ex-post pairwise efficiency and ex-post nonwastefulness.

This result allows us to strengthen various existing characterization results, both for lottery rules and for deterministic rules, by replacing (ex-post) Pareto efficiency with (ex-post) pairwise efficiency and (ex-post) non-wastefulness, e.g., for characterizations of the Random Serial Dictatorship rule (Basteck, 2025), Trading Cycles rules (Pycia and Unver, 2017), and Hierarchical Exchange rules (Papai, 2000).