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Compromise in combinatorial vote

Abstract : We consider collective choice problems where the set of social outcomes is a Cartesian product of finitely many finite sets. Each individual is assigned a two-level preference, defined as a pair involving a vector of strict rankings of elements in each of the sets and a strict ranking of social outcomes. A voting rule is called (resp. weakly) product stable at some two-level preference profile if every (resp. at least one) outcome formed by separate coordinate-wise choices is also an outcome of the rule applied to preferences over social outcomes. We investigate the (weak) product stability for the specific class of compromise solutions involving q-approval rules, where q lies between 1 and the number I of voters. Given a finite set X and a profile of I linear orders over X, a q-approval rule selects elements of X that gathers the largest support above q at the highest rank in the profile. Well-known q-approval rules are the Fallback Bargaining solution (q=I) and the Majoritarian Compromise (q=⌈I2⌉). We assume that coordinate-wise rankings and rankings of social outcomes are related in a neutral way, and we investigate the existence of neutral two-level preference domains that ensure the weak product stability of q-approval rules. We show that no such domain exists unless either q=I or very special cases prevail. Moreover, we characterize the neutral two-level preference domains over which the Fallback Bargaining solution is weakly product stable.
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https://hal-cnam.archives-ouvertes.fr/hal-03576075
Contributor : Jean Lainé Connect in order to contact the contributor
Submitted on : Tuesday, February 15, 2022 - 5:41:46 PM
Last modification on : Friday, August 5, 2022 - 2:54:00 PM

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Hayrullah Dindar, Jean Lainé. Compromise in combinatorial vote. Social Choice and Welfare, Springer Verlag, 2022, ⟨10.1007/s00355-022-01387-6⟩. ⟨hal-03576075⟩

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