Monotonicity criterion

In voting systems, a voting system is monotonic if it satisfies the following condition:

If an alternative X loses, and the ballots are changed only by placing X in lower positions, without changing the relative position of other candidates, then X must still lose.

It is considered a good thing if a voting system is monotonic. If a voting system is not monotonic, it can encourage tactical voting, as there will be situations in which a voter will be encouraged to bury their favorite (i.e. rank their favorite option lower than their sincere preference).

The Borda count is monotonic, while Coombs' method and Instant-runoff voting are not. Approval voting is monotonic, using a slightly different definition, because it is not a preferential system: you can never help a candidate by not voting for them.

Some parts of this article are derived from text at http://condorcet.org/emr/criteria.shtml

Compare Monotonicity.