Later-no-harm criterion

Later-no-harm is a property of some ranked-choice voting systems, first described by Douglas Woodall. In later-no-harm systems, increasing the rating or rank of a candidate ranked below the winner of an election cannot cause this higher-ranked candidate to lose.^[1]

For example, say a group of voters ranks Alice 2^nd and Bob 6^th, and Alice wins the election. In the next election, Bob focuses on expanding his appeal with this group of voters, but does not manage to defeat Alice—Bob's rating increases from 6^th-place to 3^rd. Later-no-harm says that this increased support from Alice's voters should not allow Bob to win.^[1]

Later-no-harm is a defining characteristic of first-preference plurality (FPP), instant-runoff voting (IRV), and descending solid coalitions (DSC), three similar systems for comparing candidates based on how many eligible voters consider each uneliminated candidate their favorite. In later-no-harm systems, the results either do not depend on lower preferences at all (as in plurality) or only depend on them if all higher preferences have been eliminated (as in IRV and DSC).^[2][3] This tends to favor candidates with strong (but narrow) support over candidates closer to the center of public opinion, which can lead to a phenomenon known as center-squeeze.^[4][5][6] Cardinal and Condorcet methods, by contrast, tend to select candidates whose ideology is a closer match to that of the median voter.^[4][5][6] This has led many social choice theorists to question whether the property is desirable in the first place or should instead be seen as a negative property.^[6][7][8]

Later-no-harm is sometimes confused with resistance to a kind of strategic voting called truncation or bullet voting.^[9] However, satisfying later-no-harm does not (by itself) provide resistance to such strategies, unless paired with the participation criterion; systems like instant runoff that pass later-no-harm but fail participation still incentivize truncation or bullet voting in some situations.^[7][10]

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