Gibbard Satterthwaite Theorem

The evil twin of ArrowsTheorem: it states that no electoral system can eliminate tactical voting. Well, except dictatorship.

http://www.shef.ac.uk/~puremath/theorems/gibbard.html

"What the Gibbard-Satterthwaite Theorem says is that if there are more than two candidates there is only one system that eliminates tactical voting: the dictatorial system. ... In any other system, with more than two candidates, a situation can arise in which at least one voter gains from voting tactically."

See also:

• 'Arrow's Theorem and the Gibbard-Satterthwaite Theorem: A Unified Approach' http://faculty-web.at.northwestern.edu/economics/chung/mr/Reny.pdf (BrokenLink 2005/11/12)
• 'Existence of a Coalitionally Strategyproof Social Choice Function: A Constructive Proof' - but only if your electorate is countably infinite! http://ideas.uqam.ca/ideas/data/Papers/wpawuwppe9604002.html

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CategoryVoting