Informed majority coalition criterion

From electowiki

The informed majority coalition criterion (InfMC) or conditional majority determination criterion is a voting method criterion that is a weaker form of the majority criterion. It can be stated as follows:

A group comprising a majority of voters is always able to cast their votes so as to elect any candidate they wish, provided that the votes of the remaining minority are known to them and don't change.

It was independently defined by James Green-Armytage[1] and Durand et al.[2] Any voting method passing the majority criterion automatically passes the informed majority coalition criterion, since the majority can all vote their preferred candidate first to make that candidate win.

Range voting and the Borda count also pass the informed majority coalition criterion, even though they fail the majority criterion. Anti-plurality voting fails.

Relationship to Condorcification

The informed majority coalition criterion is important to Condorcification. Specifically, Condorcifying any method that passes InfMC will never increase that method's susceptibility to strategic voting; there is no set of preferences where the base method was originally strategy-resistant, while the new method is not. In most cases, Condorcification outright reduces a method's manipulability.[2] The argument behind this is simple: a majority under an InfMC system can always elect a Condorcet winner, if they exist, by voting strategically. This makes Condorcification a kind of automatic strategy.

However, such a modification can change what kind of strategy voters need to use in order to manipulate a method, in elections where it is manipulable.

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  1. Green-Armytage, James; Tideman, T. Nicolaus; Cosman, Rafael (2015-08-11). "Statistical evaluation of voting rules" (PDF). Social Choice and Welfare. Springer Science and Business Media LLC. 46 (1): 183–212. doi:10.1007/s00355-015-0909-0. ISSN 0176-1714.
  2. a b Durand, François; Mathieu, Fabien; Noirie, Ludovic (2016-08-29). "Can a Condorcet Rule Have a Low Coalitional Manipulability?". Archive ouverte HAL. Retrieved 2023-06-26.