Non-compulsory support criterion
The Non-compulsory support criterion was devised in 2005 by Thomas Smith. A voting method satisfies the Non-compulsory Support criterion if it provides a means for the voter to exclude one or more candidates from supportive ranking or rating, and by doing so, not causing harm to any supported candidates or help to non-supported candidates. Methods that satisfy this criterion include Approval voting, Range voting, Bucklin voting, and Borda count that is voter truncated from the low rankings. Methods that do not satisfy this criterion include Condorcet methods, plurality voting, standard Borda count, modified Borda count (voter truncated from the high rankings) and Single Transferable Vote methods (such as Instant-runoff voting). For example, Approval and Range voting allow the voter to rate any candidate with a zero, which effectively does not give those candidates any advantage and does not harm the chances of the positively rated candidates. Voter-truncated Borda count from the low rankings allows the voter to not rank any candidate, thus giving those candidates no points in the tallies, but gives the supported candidates the same high-ranking point values as a standard Borda count. Plurality voting does not satisfy ncsc because only one candidate can be selected for a ballot slot, so there is no means to exclude even one candidate without not supporting that candidate. Instant-runoff voting fails ncsc as if a candidate is not ranked, this can harm candidates who are supported by the voter by denying supported candidates transferred votes from the unranked candidate, thereby decreasing the winning chances of those candidates.
The Non-compulsory support criterion provides some incentive for the voter to sincerely disapprove, rate as 0, or not rank a candidate, because those that voters support will never be harmed by the non-support of a candidate, nor will those candidates who are non-supported ever be helped by non-support of a candidate.