Simon Rey

AAMAS Conference 2026 Conference Paper

Proportionality Press: Illustrating Proportionality through Comment Selection

• Jan Maly
• Oliviero Nardi
• Simon Rey

We present the Proprotionality Press, a website developed to illustrate how proportional multi-winner voting rules can be applied to real-world online discussions. The Proportionality Press presents different articles that have been commented on by users. For each article several ways of selecting the comments are presented, illustrating the effect of different voting rules.

JAAMAS Journal 2025 Journal Article

Correction: Epistemic selection of costly alternatives: the case of participatory budgeting

• Simon Rey
• Ulle Endriss

AAMAS Conference 2025 Conference Paper

Epistemic Selection of Costly Alternatives: The Case of Participatory Budgeting (Extended Abstract)

• Simon Rey
• Ulle Endriss

We initiate the study of participatory budgeting using the epistemic approach, where one interprets votes as noisy estimates of some ground truth regarding the objectively best set of projects to fund.

AAMAS Conference 2025 Conference Paper

Pabuviz. org: A Visualisation Platform to Explore Participatory Budgeting Elections

• Markus Utke
• Simon Rey
• Ulle Endriss

We present pabuviz. org, an interactive website that provides indepth visualisation tools to compare the outcomes that would have been returned by a range of different voting rules for participatory budgeting when applied to historical election data. This information can help policy makers with choosing an appropriate voting rule.

AAMAS Conference 2025 Conference Paper

The Game Academy: Learn while playing, and play while learning!

• Simon Rey
• Ulle Endriss

We present game-academy. org, an online platform hosting interactive games that can be used for both teaching and outreach activities to illustrate concepts from game theory and related fields.

JAAMAS Journal 2024 Journal Article

Epistemic selection of costly alternatives: the case of participatory budgeting

• Simon Rey
• Ulle Endriss

Abstract We initiate the study of voting rules for participatory budgeting using the so-called epistemic approach, where one interprets votes as noisy reflections of some ground truth regarding the objectively best set of projects to fund. Using this approach, we first show that both the most studied rules in the literature and the most widely used rule in practice cannot be justified on epistemic grounds: they cannot be interpreted as maximum likelihood estimators, whatever assumptions we make about the accuracy of voters. Focusing then on welfare-maximising rules, we obtain both positive and negative results regarding epistemic guarantees.

IJCAI Conference 2024 Conference Paper

Guide to Numerical Experiments on Elections in Computational Social Choice

• Niclas Boehmer
• Piotr Faliszewski
• Łukasz Janeczko
• Andrzej Kaczmarczyk
• Grzegorz Lisowski
• Grzegorz Pierczyński
• Simon Rey
• Dariusz Stolicki

We analyze how numerical experiments regarding elections were conducted within computational social choice literature (focusing on papers published in the IJCAI, AAAI, and AAMAS conferences). We analyze the sizes of the studied elections and the methods of generating preference data, thereby making previously hidden standards and practices explicit. In particular, we survey a number of statistical cultures for generating elections and their commonly used parameters.

AAMAS Conference 2023 Conference Paper

Bounded Approval Ballots: Balancing Expressiveness and Simplicity for Multiwinner Elections

• Dorothea Baumeister
• Linus Boes
• Christian Laußmann
• Simon Rey

Approval ballots have been celebrated for many voting scenarios [16], in particular because of the low cognitive burden they put on the voters. This however, comes at the cost of expressiveness that can be problematic when voters have sophisticated preferences. We consider voters who, in addition to usual approval, may wish to express incompatibilities, dependencies, and/or substitution effects between the alternatives. We introduce, and evaluate a new type of ballot—bounded approval ballots—which captures these effects while being almost as easy as regular approval ballots to cast.

AAMAS Conference 2023 Conference Paper

Fairness in Participatory Budgeting via Equality of Resources

• Jan Maly
• Simon Rey
• Ulle Endriss
• Martin Lackner

We introduce a family of normative principles to assess fairness in the context of participatory budgeting. These principles are based on the fundamental idea that budget allocations should be fair in terms of the resources invested into meeting the wishes of individual voters. This is in contrast to earlier proposals that are based on specific assumptions regarding the satisfaction of voters with a given budget allocation. We analyse these new principles in axiomatic, algorithmic, and experimental terms.

AAMAS Conference 2022 Conference Paper

Designing Mechanisms for Participatory Budgeting

• Simon Rey

When seeking for suitable mechanisms for participatory budgeting (PB), one has to decide on which criteria to assess them. In this paper, I present several appealing criteria for PB mechanisms. I briefly introduce each of them and discuss their impact on the design of PB mechanisms.

IJCAI Conference 2022 Conference Paper

Let’s Agree to Agree: Targeting Consensus for Incomplete Preferences through Majority Dynamics

• Sirin Botan
• Simon Rey
• Zoi Terzopoulou

We study settings in which agents with incomplete preferences need to make a collective decision. We focus on a process of majority dynamics where issues are addressed one at a time and undecided agents follow the opinion of the majority. We assess the effects of this process on various consensus notions—such as the Condorcet winner—and show that in the worst case, myopic adherence to the majority damages existing consensus; yet, simulation experiments indicate that the damage is often mild. We also examine scenarios where the chair of the decision process can control the existence (or the identity) of consensus, by determining the order in which the issues are discussed.

EUMAS Conference 2022 Conference Paper

Participatory Budgeting with Multiple Resources

• Nima Motamed
• Arie Soeteman
• Simon Rey
• Ulle Endriss

Abstract We put forward a formal model of participatory budgeting where projects can incur costs with respect to several different resources, such as money, energy, or emission allowances. We generalise several well-known mechanisms from the usual single-resource setting to this multi-resource setting and analyse their algorithmic efficiency, the extent to which they are immune to strategic manipulation, and the degree of proportional representation they can guarantee. We also prove a general impossibility theorem establishing the incompatibility of proportionality and strategyproofness for this model.

IJCAI Conference 2021 Conference Paper

Fairness in Long-Term Participatory Budgeting

• Martin Lackner
• Jan Maly
• Simon Rey

Participatory Budgeting (PB) processes are usually designed to span several years, with referenda for new budget allocations taking place regularly. This paper presents a first formal framework for long-term PB, based on a sequence of budgeting problems as main input. We introduce a theory of fairness for this setting, focusing on three main concepts that apply to types (groups) of voters: (i) achieving equal welfare for all types, (ii) minimizing inequality of welfare (as measured by the Gini coefficient), and (iii) achieving equal welfare in the long run. We investigate under which conditions these criteria can be satisfied, and analyze the computational complexity of verifying whether they hold.

AAMAS Conference 2021 Conference Paper

Fairness in Long-Term Participatory Budgeting

• Martin Lackner
• Jan Maly
• Simon Rey

Participatory Budgeting processes are usually designed to span several years, with referenda for new budget allocations taking place regularly. This paper presents the first formalization of long-term PB. We introduce a theory of fairness for this setting, investigate under which conditions our fairness criteria can be satisfied, and analyze the computational complexity of verifying them.

IJCAI Conference 2021 Conference Paper

Shortlisting Rules and Incentives in an End-to-End Model for Participatory Budgeting

• Simon Rey
• Ulle Endriss
• Ronald de Haan

We introduce an end-to-end model for participatory budgeting grounded in social choice theory. Our model accounts for the interplay between the two stages commonly encountered in real-life partici- patory budgeting. In the first stage participants pro- pose projects to be shortlisted, while in the second stage they vote on which of the shortlisted projects should be funded. Prior work of a formal nature has focused on analysing the second stage only. We in- troduce several shortlisting rules for the first stage and analyse them in both normative and algorith- mic terms. Our main focus is on the incentives of participants to engage in strategic behaviour during the first stage, in which they need to reason about how their proposals will impact the range of strate- gies available to everyone in the second stage.

IJCAI Conference 2020 Conference Paper

Almost Group Envy-free Allocation of Indivisible Goods and Chores

• Haris Aziz
• Simon Rey

We consider a multi-agent resource allocation setting in which an agent's utility may decrease or increase when an item is allocated. We take the group envy-freeness concept that is well-established in the literature and present stronger and relaxed versions that are especially suitable for the allocation of indivisible items. Of particular interest is a concept called group envy-freeness up to one item (GEF1). We then present a clear taxonomy of the fairness concepts. We study which fairness concepts guarantee the existence of a fair allocation under which preference domain. For two natural classes of additive utilities, we design polynomial-time algorithms to compute a GEF1 allocation. We also prove that checking whether a given allocation satisfies GEF1 is coNP-complete when there are either only goods, only chores or both.

KR Conference 2020 Conference Paper

Designing Participatory Budgeting Mechanisms Grounded in Judgment Aggregation

• Simon Rey
• Ulle Endriss
• Ronald de Haan

We introduce a new approach for designing rules for participatory budgeting, the problem of deciding on the use of public funds based directly on the views expressed by the citizens concerned. The core idea is to embed instances of the participatory budgeting problem into judgment aggregation, a powerful general-purpose framework for modelling collective decision making. Taking advantage of the possibilities offered by judgment aggregation, we enrich the familiar setting of participatory budgeting with additional constraints, namely dependencies between projects and quotas regarding different types of projects. We analyse the rules obtained both in algorithmic and in axiomatic terms.

AAMAS Conference 2019 Conference Paper

Credulous Acceptability, Poison Games and Modal Logic

• Davide Grossi
• Simon Rey

The Poison Game is a two-player game in which players alternatively move a token on a graph’s nodes and such that one player can influence which edges the other player is able to traverse. It operationalizes the notion of existence of credulously acceptable arguments in an argumentation framework or, equivalently, the existence of non-trivial semi-kernels. We develop a modal logic (poison modal logic, PML) tailored to represent winning positions in such a game, thereby identifying the precise modal reasoning that underlies the notion of credulous acceptability in argumentation. We study model-theoretic and decidability properties of PML, and position it with respect to recently studied logics at the cross-road of modal logic, argumentation, and graph games.

AAMAS Conference 2019 Conference Paper

Efficiency, Sequenceability and Deal-Optimality in Fair Division of Indivisible Goods

• Aurélie Beynier
• Sylvain Bouveret
• Michel Lemaître
• Nicolas Maudet
• Simon Rey
• Parham Shams

In fair division of indivisible goods, using sequences of sincere choices (or picking sequences) is a natural way to allocate the objects. The idea is as follows: at each stage, a designated agent picks one object among those that remain. Another intuitive way to obtain an allocation is to give objects to agents in the first place, and to let agents exchange them as long as such “deals” are beneficial. This paper investigates these notions, when agents have additive preferences over objects, and unveils surprising connections between them, and with other efficiency and fairness notions. In particular, we show that an allocation is sequenceable if and only if it is optimal for a certain type of deals, namely cycle deals involving a single object. Furthermore, any Pareto-optimal allocation is sequenceable, but not the converse. Regarding fairness, we show that an allocation can be envy-free and non-sequenceable, but that every competitive equilibrium with equal incomes is sequenceable. To complete the picture, we show how some domain restrictions may affect the relations between these notions. Finally, we experimentally explore the links between the scales of efficiency and fairness.