A new property of collective choice rules that we refer to as superset robustness is
introduced, and we employ it in several characterization results. The axiom requires that if all
individual preference orderings expand weakly (in the sense of set inclusion), then the
corresponding social preference relation must also expand weakly. In other words, if a given
profile is changed by adding instances of weak preference to some individual relations, then
the social weak preference relation for the expanded profile must contain the social weak …