Axiomatic Characterization of Committee Scoring Rules.

Committee scoring rules form a rich class of aggregators of voters' preferences for the purpose of selecting subsets of candidates of a given size. We provide an axiomatic characterization of committee scoring rules in the spirit of celebrated Young's characterization of single-winner scoring rules. We show that committee scoring rules are characterized by the set of four standard axioms: symmetry, consistency, continuity and Pareto dominance. In the course of our proof, we introduce and axiomatically characterize multiwinner decision scoring rules, a class of rules that generalizes the well-known majority relation.