Speaker:     Shaun White
Affiliation: The University of Auckland
Title:       Strategic voting: overshooting and undershooting, and safe and unsafe strategic votes
Date:        Tuesday, 11 Sep 2012
Time:        4:00 pm
Location:    Room 5115, Owen Glenn Building

There are many situations in which mis-coordinated strategic voting can leave strategic voters worse off than they would have been had they not tried to manipulate.  We develop a framework for analysing the simplest of such scenarios, in which a set of strategic voters all have the same sincere preferences and all cast the same strategic vote, while all other voters vote sincerely.  We say a voter has `an incentive to vote strategically’ when they can manipulate the choice mechanism by voting strategically in unison with certain other voters who share their sincere preferences.  We classify mis-coordinations as instances of strategic overshooting (when the choice mechanism is anonymous, overshooting occurs when too many vote strategically) or strategic undershooting (too few vote strategically). If casting a strategic vote cannot inadvertently lead to overshooting or undershooting, we call it safe. We extend the Gibbard-Satterthwaite Theorem by proving that every onto and non-dictatorial social choice rule can be individually manipulated by a voter casting a safe strategic vote. All this is joint work with Arkadii Slinko.