Gibbard-Satterthwaite theorem

Published independently by philosopher Allan Gibbard in 1973 and economist Mark Satterthwaite in 1975.

the Gibbard-Satterthwaite theorem

1. A theorem dealing with deterministic ordinal electoral systems that choose a single winner, stating that for every voting rule, one of the following three things must hold: (i) the rule is dictatorial, i.e. there exists a distinguished voter who can choose the winner; or (ii) the rule limits the possible outcomes to two alternatives only; or (iii) the rule is susceptible to tactical voting: in certain conditions some voter's sincere ballot may not defend their opinion best.

• Gibbard's theorem