Manipulability of voting by sincere truncation of preferences

A weak form of strategic voting, called 'sincere truncation,' occurs when a voter with a strict preference ranking does not rank all his or her choices on the ballot. A voting procedure is said to be manipulable by sincere truncation if one or more voters can obtain a preferred outcome through sincere truncation. Voting procedures that are not manipulable by sincere truncation are shown to be incompatible with the election of Condorcet (majority) candidates when they exist. A relaxation of simple majority rule, called the '7/12 rule,' is also shown to conflict with nonmanipulability when additional conditions are imposed. These results are formally independent of the strategy-proofness theorems for voting and decision schemes established by Gibbard, Satterthwaite, and others. While their analyses are more inclusive in terms of the varieties of decision procedures allowed, they are also less demanding in their requirements for manipulability since voters are permitted to reverse sincere pReferences in their voting. Thus, plurality voting is manipulable in the sense of Gibbard-Satterthwaite (by preference reversals), but it is clearly nonmanipulable by sincere truncation.