Definitions
A voter votes X equal to Y if the voter doesn't vote X over Y, and
doesn't vote Y over X, but votes X over someone, and votes Y over
someone.
A sincere vote is one with no falsified preferences or preferences
left unspecified when the election method allows them to be specified
(in addition to the preferences already specified).
One candidate is preferred over another candidate if, in a one-on-one
competition, more voters prefer the first candidate than prefer the
other candidate.
Statement of Criterion
If a majority prefers one particular candidate to another, then
they should have a way of voting that will ensure that the other cannot
win, without any member of that majority reversing a preference for one
candidate over another or falsely voting two candidates equal.
Complying Methods
Cloneproof Schwartz Sequential Dropping and Majority Choice Approval comply with the Strong Defensive Strategy Criterion, while Approval voting, Cardinal Ratings, Borda count, Plurality voting, and Instant-Runoff Voting do not comply.
Commentary
Compliance with SDSC means that a majority never needs any more than
truncation strategy to defeat a particular candidate, even when
countering offensive order reversal by that candidate's voters.
Offensive order reversal is the only strategy that can create the need
for defensive strategy in Cloneproof Schwartz Sequential Dropping.
Some parts of this article are derived with permission from text at http://electionmethods.org
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