Possible and Necessary Winners in Single-Winner and Multi-Winner Approval Voting
Dura-Georg Granic (Cologne)

This paper reports on the results of a series of experimental
(laboratory) elections. We assess and compare coordination failure
rates of Approval Voting (AV), the Borda Count (BC), and Plurality
Voting (PV) in a repeated divided majority setting. We also vary the
underlying information structure from complete-information to
participants only observing election histories. By offering voters
additional opportunities to express their preferences, the multi-votes
methods AV and BC facilitate coordination. They clearly outperform PV,
coordination failures arise less frequently and coordination is more
efficient. The observed superiority is even stronger with incomplete
information. Individual voting behavior reveals a systematic reduction
of strategic misrepresentations of preferences with less available
information, however, suggest that strategic considerations play an
important role in general. Finally, we observe a strong connection
between an electoral system and the party system it encourages. In
accordance with Duverger's Law, PV elections often lead to a high
concentration of the votes on two alternatives.