What is Later-no-harm criterion?

The later-no-harm criterion is a voting system criterion formulated by Douglas Woodall. Woodall defined the criterion as "dding a later preference to a ballot should not harm any candidate already listed." For example, a ranked voting method in which a voter adding a 3 preference could reduce the likelihood their 1 preference being selected, fails later-no-harm.

Voting systems that fail the later-no-harm criterion are vulnerable to the tactical voting strategies called bullet voting and burying, which can deny victory to a sincere Condorcet winner. However, the fact that all cardinal voting methods fail the later-no-harm criterion is essential to their favoring consensus options over majoritarian options.