Hedonic Games Research Articles

On the stability of coalition structures

We resolve a seeming conflict between a non-existence result on solutions to coalition formation in hedonic games [Barberà, S., Gerber, A., 2007. A note on the impossibility of a satisfactory concept of stability for coalition formation games. Economics Letters 95, 85–90] and the universal existence of stable coalition structures in TU games under the χ-value [Casajus, A., 2008. Outside options, component efficiency, and stability, Games and Economic Behavior (forthcoming). doi: 10.1016/j.geb.2007.04.003].

A note on the paradox of smaller coalitions

We consider hedonic coalition formation games that are induced by a simple TU-game and a cooperative solution. For such models, Shenoy’s (Int J Game Theory 8:133–164, 1979) absence of the paradox of smaller coalitions provides a sufficient condition for core existence. We present three different versions of his condition in order to compare it to the top coalition property of Banerjee et al. (Social Choice Welfare 18:135–153, 2001) that guarantees nonemptiness of the core in more general models. As it turns out, the top coalition property implies a condition in which Shenoy’s paradox is not present for at least one minimal winning coalition. Conversely, if for each non-null player Shenoy’s paradox is not present for at least one minimal winning coalition containing that player, then the induced hedonic game satisfies the top coalition property.

Stable governments and the semistrict core

We consider the class of proper monotonic simple games and study coalition formation when an exogenous weight vector and a solution concept are combined to guide the distribution of power within winning coalitions. These distributions induce players' preferences over coalitions in a hedonic game. We formalize the notion of semistrict core stability, which is stronger than the standard core concept but weaker than the strict core notion and derive two characterization results for the semistrict core, depending on conditions we impose on the solution concept. A bounded power condition, which connects exogenous weights and the solution, turns out to be crucial. It generalizes a condition termed “absence of the paradox of smaller coalitions” that was previously used to derive core existence results.

The core-partition of a hedonic game

In a hedonic game the preference relation of each player is defined on the set of coalitions that contain the player. The paper provides a necessary and sufficient condition for core-partition existence in a hedonic game. The condition is based on a new concept of balancedness, called pivotal balancedness, involving subsets of players in any given family of coalitions. Pivotal balancedness highlights the existence of key players in each coalition that are relevant for the existence of a core-partition.

Gamson’s law and hedonic games

This note adds one celebrated coalition formation game due to Gamson (Am Soc Rev 26:373–382 1961a, Am Soc Rev 26:565–573, 1961b) in the list of applications of the theory of hedonic games explored by Banerjee et al. (Soc Choice Welf 18:135–153, 2001) and Bogomolnaia and Jackson (Games Econ Behav 38: 204–230, 2002). We apply their results to study the original Gamson game and offer extensions both to a multi-dimensional characteristics space and to an infinite number of players.

Stability of jurisdiction structures under the equal share and median rules

We consider a finite society with of individuals distributed along the real line. The individuals form jurisdictions to consume public projects, equally share their costs and, in addition, bear a transportation cost to the location of the project. We examine a core and Nash notions of stable jurisdiction structures and show that in hedonic games both solution sets could be empty. We demonstrate that in a quasi-hedonic set-up there is a Nash stable partition, but, in general, there are no core stable partitions. We then examine a subclass of societies that admits the existence of both types of stable partitions.

On top responsiveness and strict core stability

Alcalde and Revilla [Journal of Mathematical Economics 40 (2004) 869–887] introduce a top responsiveness condition on players’ preferences in hedonic games and show that it guarantees the existence of a core stable partition. In the present note we strengthen this observation by proving that under top responsiveness even the strict core is non-empty.

On Myopic Stability Concepts for Hedonic Games

We present a unified look at myopic stability concepts for hedonic games, and discuss the status of the existence problems of stable coalition structures. In particular, we show that contractual strictly core stable coalition structures always exist, and present a sufficient condition for the existence of contractually Nash stable coalition structures on the class of separable games.

Stable Coalition Structures in Simple Games with Veto Control

In this paper we study hedonic coalition formation games in which players' preferences over coalitions are induced by a semi-value of a monotonic simple game with veto control. We consider partitions of the player set in which the winning coalition contains the union of all minimal winning coalitions, and show that each of these partitions belongs to the strict core of the hedonic game. Exactly such coalition structures constitute the strict core when the simple game is symmetric. Provided that the veto player set is not a winning coalition in a symmetric simple game, then the partition containing the grand coalition is the unique strictly core stable coalition structure.

Government versus Opposition: Who Should be Who in the 16th German Bundestag?

We model the process of coalition formation in the 16th German Bundestag as a hedonic coalition formation game. In order to induce players' preferences in the game we apply the Shapley value of the simple game describing all winning coalitions in the Bundestag. Using different stability notions for hedonic games we prove that the ``most'' stable government is formed by the Union Parties together with the Social Democratic Party.

Coalition Formation in Simple Games: The Semistrict Core

We consider the class of proper monotonic simple games and study coalition formation when an exogenous share vector and a solution concept are combined to guide the distribution of coalitional worth. Using a multiplicative composite solution, we induce players' preferences over coalitions in a hedonic game, and present conditions under which the semistrict core of the game is nonempty.

Simple Priorities and Core Stability in Hedonic Games

In this paper we study hedonic games where each player views every other player either as a friend or as an enemy. Two simple priority criteria for comparison of coalitions are suggested, and the corresponding preference restrictions based on appreciation of friends and aversion to enemies are considered. It turns out that the first domain restriction guarantees non-emptiness of the strong core and the second domain restriction ensures non-emptiness of the weak core of the corresponding hedonic games. Moreover, an element of the strong core under friends appreciation can be found in polynomial time, while finding an element of the weak core under enemies aversion is NP-hard. We examine also the relationship between our domain restrictions and some sufficient conditions for non-emptiness of the core already known in the literature.

Simple Priorities and Core Stability in Hedonic Games

In this paper we study hedonic games where each player views every other player either as a friend or as an enemy. Two simple priority criteria for comparison of coalitions are suggested, and the corresponding preference restrictions based on appreciation of friends and aversion to enemies are considered. We characterize internally stable coalitions on the proposed domains and show how these characterizations can be used for generating a strict core element in the first case and a core element in the second case. Moreover, we prove that an element of the strict core under friends appreciation can be found in polynomial time, while finding an element of the core under enemies aversion is NP-hard.

A Taxonomy of Myopic Stability Concepts for Hedonic Games

We present a taxonomy of myopic stability concepts for hedonic games in terms of deviations, and discuss the status of the existence problems of stable coalition structures. In particular, we show that contractual strictly core stable coalition structures always exist, and provide sufficient conditions for the existence of contractually Nash stable and weak individually stable coalition structures on the class of separable games.

Coalition Formation in Simple Games: The Semistrict Core

We consider the class of proper monotonic simple games and study coalition formation when an exogenous share vector and a solution concept are combined to guide the distribution of coalitional worth. Using a multiplicative composite solution, we induce players' preferences over coalitions in a hedonic game, and present conditions under which the semistrict core of the game is nonempty.

Stability of Jurisdiction Structures Under the Equal Share and Median Rules

In this paper we consider a model with multiple jurisdictions where each formed jurisdiction selects a public project from the given uni-dimensional set, equally shares its cost among its members and places the project at the location of its median resident. We examine a cooperative concept of core stability and a non-cooperative notion of Nash stability of jurisdiction structures. We first consider hedonic games, where each jurisdiction chooses the mean of the extreme points of its median set, and show that both stability notions may lead to an empty set of stable jurisdiction structures. We demonstrate that in a quasi-hedonic set-up, where jurisdictions are allowed to pick any selection from the median set, there is a Nash stable partition, while a core stable partition may still fail to exist. We also consider the equidistant societies and show the existence of both types of stable partitions in this case. Finally, we examine stratifcation properties of stable partitions.

NP-completeness in hedonic games

In this paper, we show that there are natural limitations in attaining stability in cooperative environments. These limitations are related to the fact that the time required to obtain a stable partition of the players can grow exponentially, even under very simple assumptions on the preferences over partners. These difficulties arise even when players assess potential coalitions based solely on their size. For this purpose, we demonstrate that the core, the Nash stable set and the individually stable set have corresponding NP-complete decision problems for a variety of situations and, hence, that only exponential-time (slow) algorithms are known for solving them.

Enemies and Friends in Hedonic Games: Individual Deviations, Stability and Manipulation

We consider hedonic games with separable preferences, and explore the existence of stable coalition structures if only individual deviations are allowed.For two natural subdomains of separable preferences, namely preference domains based on (1) aversion to enemies and (2) appreciation of friends, we show that an individually stable coalition structure always exist, and a Nash stable coalition structure exists when mutuality is imposed.Moreover, we show that on the domain of separable preferences a contractual individually stable coalition structure can be obtained in polynomial time.Finally, we prove that, on each of the two subdomains, the corresponding algorithm that we use for finding Nash stable and individually stable coalition structures turns out to be strategy-proof.

Farsighted stability in hedonic games

We investigate how rational individuals partition themselves into different coalitions in “hedonic games” (see Banerjee et al. 2001 and Bogomolnaia and Jackson 2002), where individuals' preferences depend solely on the composition of the coalition they belong to. Given that the four solution concepts studied in the literature (core, Nash stability, individual stability and contractual individual stability) may exhibit myopia on the part of the players, we amend these notions by endowing players with foresight in that they look many steps ahead and consider only credible outcomes. We study the properties of the farsighted stability solutions; in particular, we show that when preferences are strict, coalition structures in the core are farsighted stable and a similar result also holds for Nash stability but not for individual stability and contractual individual stability.

On coalition formation: durable coalition structures

We define a solution to the problem of coalition formation that applies to purely hedonic games. Coalition structures satisfying our requirements are called durable, and we interpret them as much more likely to last than those coalition structures not satisfying the requirements, which we call transient. Durability results from a combination of foresight and extreme risk aversion on the part of agents, when considering to join others to disrupt an existing structure in search of higher gains. Agents’ calculations are also constrained to satisfy a strong consistency requirement, which is reflected in the recursive structure of our definition. We prove that durable coalition structures always exist, and we provide examples of hedonic games where our solutions apply nicely.