Later-no-harm criterion

Statement of Criterion

Adding a preference to a ballot must not decrease the probability of election of any candidate ranked above the new preference.

Complying Methods

Later-no-harm (usually LNH, but sometimes LNHa or LNHarm to avoid confusion with Later-no-help) is satisfied by Instant Runoff Voting, Minmax(pairwise opposition), and Douglas Woodall's Descending Solid Coalitions method. It is trivially satisfied by First-Preference Plurality and Random Ballot, since those methods do not usually regard lower preferences. Virtually every other method fails this criterion.

Commentary

Later-no-harm guarantees that the method will not use a voter's lower preferences to elect a candidate who that voter likes less than the candidate that would have been elected if this voter had kept his lower preferences a secret.

As a result, voters may feel free to vote their complete ranking of the candidates, which in turn may give the election method more complete information to use to find a winner.

Later-no-harm is incompatible with the Condorcet criterion.