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Öğe Choosing a Committee Under Majority Voting(Springer-Verlag Berlin, 2019) Aslan, Fatma; Dindar, Hayrullah; Laine, JeanWe consider the elections of a seat-posted committee, and investigate the propensity of seat-wise majority voting to choose a committee that fulfills the majority will with respect to preferences over committees. Voters have seat-wise preferences and preferences over committees are derived from seat-wise preferences by means of a neutral preference extension. Neutrality means that the names of candidates do not play any role. The majority committee paradox refers to a situation where a Condorcet winner exists for each seat, and a Condorcet winner committee also exists but does not coincide with the combination of seat-wise Condorcet winners. The majority committee weak paradox refers to a situation where the combination of seat-wise Condorcet winners is not a Condorcet winner among committees. We characterize the domains of preference extensions immune to each of the paradoxes.Öğe Competitive equilibria in Shapley-Scarf markets with couples(Elsevier Science Sa, 2020) Aslan, Fatma; Laine, JeanWe investigate the existence and properties of competitive equilibrium in Shapley-Scarf markets involving an exogenous partition of individuals into couples. The presence of couples generates preference interdependencies which cause existence problems. For both cases of transferable and nontransferable income among partners, we establish properties for preferences that are sufficient for the existence of an equilibrium. Moreover, we show that these properties define a maximal preference domain. (C) 2020 Elsevier B.V. All rights reserved.Öğe Compromise in combinatorial vote(Springer, 2022) Dindar, Hayrullah; Laine, JeanWe 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 = [I/2]). We assume that coordinate-wise rankings and rankings of social out- comes are related in a neutral way, and we investigate the existence of neutral twolevel 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.Öğe Condorcet choice and the Ostrogorski paradox(Springer, 2009) Laffond, Gilbert; Laine, JeanThe 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.Öğe The Core of Shapley-Scarf markets with couples(Elsevier Science Sa, 2011) Dogan, Onur; Laffond, Gilbert; Laine, JeanWe 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.Öğe Does Choosing Committees from Approval Balloting Fulfill the Electorate's Will?(Springer-Verlag Berlin, 2010) Laffond, Gilbert; Laine, Jean[Abstract Not Available]Öğe Hyper-stable collective rankings(Elsevier Science Bv, 2015) Laine, JeanWe introduce a new consistency property for social welfare functions (SWF), called hyper-stability. An SWF is hyper-stable if at any profile over finitely many alternatives where a weak order R is chosen, there exists a profile of linear orders over linear orders, called hyper-profile, at which only linearizations of R are ranked first by the SWF. Profiles induce hyper-profiles according to some minimal compatibility conditions. We provide sufficient conditions for hyper-stability, and we investigate hyper-stability for several Condorcet SWFs. An important conclusion is that there are non-dictatorial hyper-stable SWFs. (C) 2015 Elsevier B.V. All rights reserved.Öğe Hyper-stable social welfare functions(Elsevier, 2016-01) Laine, Jean; Özkeş, Ali İhsan; Sanver, RemziWe define a new consistency condition for neutral social welfare functions, called hyper-stability. A social welfare function (SWF) selects a weak order from a profile of linear orders over any finite set of alternatives. Each profile induces a profile of hyper-preferences, defined as linear orders over linear orders, in accordance with the betweenness criterion: the hyper-preference of some order P ranks order Q above order Q’ if the set of alternative pairs P and Q agree on contains the one P and Q’ agree on. A special sub-class of hyper-preferences satisfying betweenness is defined by using the Kemeny distance criterion. A neutral SWF is hyper-stable (resp. Kemeny-stable) if given any profile leading to the weak order R, at least one linear extension of R is ranked first when the SWF is applied to any hyper-preference profile induced by means of the betweenness (resp. Kemeny) criterion. We show that no scoring rule is hyper-stable, unless we restrict attention to the case of three alternatives. Moreover, no unanimous scoring rule is Kemeny-stable, while the transitive closure of the majority relation is hyper-stable. © 2015, Springer-Verlag Berlin Heidelberg.Öğe Manipulation of single-winner large elections by vote pairing(Elsevier Science Sa, 2017) Dindar, Hayrullah; Laine, JeanManipulation of indirect elections by vote pairing occurs when a group of voters in different electoral bodies secures a jointly preferred winner by performing pairwise exchanges of votes. We show that in elections involving a large enough number of districts, each with a large enough size, no reasonable constitution is immune to vote-pairing. (C) 2017 Elsevier B.V. All rights reserved.Öğe Metrizable preferences over preferences(Springer, 2020) Laffond, Gilbert; Laine, Jean; Sanver, M. RemziA 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.Öğe Pareto efficiency in multiple referendum(Springer, 2012) Cuhadaroglu, Tugce; Laine, JeanWe consider situations of multiple referendum: finitely many yes-or-no issues have to be socially assessed from a set of approval ballots, where voters approve as many issues as they want. Each approval ballot is extended to a complete preorder over the set of outcomes by means of a preference extension. We characterize, under a mild richness condition, the largest domain of top-consistent and separable preference extensions for which issue-wise majority voting is Pareto efficient, i.e., always yields out a Pareto-optimal outcome. Top-consistency means that voters' ballots are their unique most preferred outcome. It appears that the size of this domain becomes negligible relative to the size of the full domain as the number of issues increases.Öğe Referendum Paradox for Party-List Proportional Representation(Springer, 2021) Dindar, Hayrullah; Laffond, Gilbert; Laine, JeanWe 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.Öğe Searching for a Compromise in Multiple Referendum(Springer, 2012) Laffond, Gilbert; Laine, JeanWe 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.Öğe The strong referendum paradox(Springer, 2017) Dindar, Hayrullah; Laffond, Gilbert; Laine, JeanWe 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.Öğe Vote swapping in irresolute two-tier voting procedures(Springer, 2023) Dindar, Hayrullah; Laine, JeanWe investigate a specific type of group manipulation in two-tier elections, which involves pairs of voters agreeing to exchange their votes. Two-tier elections are modeled as a two-stage choice procedure. In the first stage, voters are distributed into districts, and district preferences result from aggregating voters' preferences district-wise through some aggregation rule. Final outcomes are obtained in the second stage by applying a social choice function that outputs one or several alternatives from the profile of district preferences. Combining an aggregation rule and a social choice function defines a constitution. Voter preferences, defined as linear orders, are extended to complete binary relations by means of some extension rule. A constitution is swap-proof w.r.t. a given extension rule if one cannot find pairs of voters who, by exchanging their preferences get better off (w.r.t. their extended preference over sets). We consider four specific extension rules: Nehring, Kelly, Fishburn, and Gardenfors. We establish sufficient conditions for the swap-proofness of a constitution w.r.t. each extension rule. Special attention is paid to majority constitutions, where both the aggregation rule and the social choice function are based on simple majority voting. We show that swap-proofness for majority constitutions pertains to a specific weakening of group strategy-proofness. Moreover, we characterize swap-proof majority constitutions w.r.t. each extension rule. Finally, we show that no constitution based on scoring methods is swap-proof.Öğe Vote Swapping in Representative Democracy(Springer-Verlag Berlin, 2015) Dindar, Hayrullah; Laffond, Gilbert; Laine, JeanWe 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.Öğe When are committees of Condorcet winners Condorcet winning committees?(Springer Heidelberg, 2022) Aslan, Fatma; Dindar, Hayrullah; Laine, JeanWe consider seat-posted (or designated-seat) committee elections, where disjoint sets of candidates compete for each seat. We assume that each voter has a collection of seat-wise strict rankings of candidates, which are extended to a strict ranking of committees by means of a preference extension. We investigate conditions upon preference extensions for which seat-wise Condorcet candidates, whenever all exist, form the Condorcet winner among committees. We characterize the domain of neutral preference extensions for which the committee of seat-wise winners is the Condorcet winning committee, first assuming the latter exists (Theorem 1) and then relaxing this assumption (Theorem 2). Neutrality means that preference extensions are not sensitive to the names of candidates. Moreover, we show that these two characterizations can be stated regardless of which preference level is considered as a premise.Öğe When are committees of Condorcet winners Condorcet winning committees? (Sept, 10.1007/s10058-021- 00260-9, 2021)(Springer Heidelberg, 2023) Aslan, Fatma; Dindar, Hayrullah; Laine, Jean[Abstract Not Available]
