Choosing collectively optimal sets of alternatives based on the condorcet criterion

Abstract

In elections, an alternative is said to be a Condorcet winner if it is preferred to any other alternative by a majority of voters. While this is a very attractive solution concept, many elections do not have a Condorcet winner. In this paper, we propose a setvalued relaxation of this concept, which we call a Condorcet winning set: such sets consist of alternatives that collectively dominate any other alternative. We also consider a more general version of this concept, where instead of domination by a majority of voters we require domination by a given fraction Θ of voters; we refer to this concept as Θ-winning set. We explore social choice-theoretic and algorithmic aspects of these solution concepts, both theoretically and empirically.