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    Condorcet choice and the Ostrogorski paradox
    (Springer, 2009) Laffond, Gilbert; Laine, Jean
    The Ostrogorski paradox refers to the fact that, facing finitely many dichotomous issues, choosing issue-wise according to the majority rule may lead to a majority defeated overall outcome. This paper investigates the possibility for a similar paradox to occur under alternative specifications of the collective preference relation. The generalized Ostrogorski paradox occurs when the issue-wise majority rule leads to an outcome which is not maximal according to some binary relation phi defined over pairs of alternatives. We focus on three possible definitions of phi, whose sets of maximal elements are respectively the Uncovered Set, the Top-Cycle, and the Pareto Set. We prove that a generalized paradox may prevail for the Uncovered Set. Moreover, it may be avoided for the same issue-wise majority margins as for the Ostrogorski paradox. However, the issue-wise majority rule always selects a Pareto-optimal alternative in the Top-Cycle.
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    Does Choosing Committees from Approval Balloting Fulfill the Electorate's Will?
    (Springer-Verlag Berlin, 2010) Laffond, Gilbert; Laine, Jean
    [Abstract Not Available]
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    Metrizable preferences over preferences
    (Springer, 2020) Laffond, Gilbert; Laine, Jean; Sanver, M. Remzi
    A hyper-preference is a weak order over all linear orders defined over a finite set A of alternatives. An extension rule associates with each linear order p over A a hyper-preference. The well-known Kemeny extension rule ranks all linear orders over A according to their Kemeny distance to p. More generally, an extension rule is metrizable iff it extends p to a hyper-preference consistent with a distance criterion. We characterize the class of metrizable extension rules by means of two properties, namely self-consistency and acyclicity across orders. Moreover, we provide a characterization of neutral and metrizable extension rules, based on a simpler formulation of acyclicity across orders. Furthermore, we establish the logical incompatibility between neutrality, metrizability and strictness. However, we show that these three conditions are pairwise logically compatible.
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    Referendum Paradox for Party-List Proportional Representation
    (Springer, 2021) Dindar, Hayrullah; Laffond, Gilbert; Laine, Jean
    We consider two-tiers elections based on closed party-list proportional representation (PLPR), where party platforms involving multiple dichotomous issues are endogenously determined by their supporters' preferences (via issue-wise simple majority voting). Assuming that voters compare platforms according to the criterion of the Hamming distance and provided a high enough number of voters and issues, we show that the outcome of PLPR may be Pareto dominated by the multiple referendum outcome defined as the issue-wise majority will in the whole electorate. We refer to this situation as the PLPR paradox. We characterize the set of party platforms for which the PLPR paradox is possible. We also investigate several restrictions upon voting situations that may be sufficient for avoiding the paradox.
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    Searching for a Compromise in Multiple Referendum
    (Springer, 2012) Laffond, Gilbert; Laine, Jean
    We consider a multiple referendum setting where voters cast approval ballots, in which they either approve or disapprove of each of finitely many dichotomous issues. A program is a set of socially approved issues. Assuming that individual preferences over programs are derived from ballots by means of the Hamming distance criterion, we consider two alternative notions of compromise. The majoritarian compromise is the set of all programs supported by the largest majority of voters at the minimum utility loss. A program is an approval compromise if it is supported by the highest number of voters at a utility loss at most half of the maximal achievable one. We investigate the conditions under which issue-wise majority voting allows for reaching each type of compromise. Finally, we argue that our results hold for many other preferences that are consistent with the observed ballots.
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    The Core of Shapley-Scarf markets with couples
    (Elsevier Science Sa, 2011) Dogan, Onur; Laffond, Gilbert; Laine, Jean
    We extend the Shapley-Scarf model of markets for indivisible goods without money to the case where couples of agents have joint preferences over the set of allocations. We show that the domain of (weakly) lexicographic preferences is maximal (for inclusion) for the existence of Core allocations. This result also holds in the case where the set of agents is partitioned into non-empty coalitions with any size, as well as for the class of markets studied in Konishi et al. (2001), where individuals exchange several types of purely indivisible goods. (C) 2010 Elsevier B.V. All rights reserved.
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    The strong referendum paradox
    (Springer, 2017) Dindar, Hayrullah; Laffond, Gilbert; Laine, Jean
    We study a model of indirect elections where voters having weak orders as preferences over finitely many alternatives are distributed across a given set of districts. In each district preferences are aggregated into a district preference, and a voting rule selects one or several alternatives from the profile of district preferences. The referendum paradox holds at some profile and some distribution of voters across districts if the outcome of indirect elections does not coincide with the one of direct elections. We prove that whenever an indirect election procedure is separable, it is exposed to the referendum paradox if and only if it is exposed to a stronger version of the referendum paradox, where direct and indirect elections give different outcomes for any distribution of the voters across districts. We prove that many indirect elections based on a tournament solution are separable, whereas some based on a scoring rule are not. Finally, we show that all indirect elections based on a scoring rule are exposed to the strong referendum paradox.
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    Vote Swapping in Representative Democracy
    (Springer-Verlag Berlin, 2015) Dindar, Hayrullah; Laffond, Gilbert; Laine, Jean
    We investigate group manipulation by vote exchange in two-tiers elections, where voters are first distributed into districts, each with one delegate. Delegates' preferences result from aggregating voters' preferences district-wise by means of some aggregation rule. Final outcomes are sets of alternatives obtained by applying a social choice function to delegate profiles. An aggregation rule together with a social choice function define a constitution. Voters' preferences over alternatives are extended to partial orders over sets by means of either the Kelly or the Fishburn extension rule. A constitution is Kelly (resp. Fishburn) swapping-proof if no group of voters can get by exchanging their preferences a jointly preferred outcome according to the Kelly (resp. Fishburn) extension. We establish sufficient conditions for swapping-proofness. We characterize Kelly and Fishburn swapping-proofness for Condorcet constitutions, where both the aggregation rule and the social choice function are based on simple majority voting. JEL Class D71, C70.