Voting Systems that Combine Approval and Preference

Steven J. Brams (1) , Remzi Sanver (2)

1 Department of Politics
2 Department of Economics

Information on the rankings and information on the approval of candidates in an election, though related, are fundamentally different—one cannot be derived from the other. Both kinds of information are important in the determination of social choices. We propose a way of combining them in two hybrid voting systems, preference approval voting (PAV) and fallback voting (FV), that satisfy several desirable properties, including monotonicity. Both systems may give different winners from standard ranking and nonranking voting systems. PAV, especially, encourages candidates to take coherent majoritarian positions, but it is more information-demanding than FV. PAV and FV are manipulable through voters' contracting or expanding their approval sets, but a 3-candidate dynamic poll model suggests that Condorcet winners, and candidates ranked first or second by the most voters if there is no Condorcet winner, will be favored, though not necessarily in equilibrium.

Mots clés: Approval voting; Ranking systems; Condorcet winner; Manipula- bility; Monotonicity

Domaines: Complexité [cs.CC]; Recherche opérationnelle [math.OC]

Steven J. Brams, Remzi Sanver. Voting Systems that Combine Approval and Preference. 2006. ⟨hal-00119047⟩