Programme

Poster Session

The list of poster presenters will get posted here by 20 March 2026.

Sophie Bade (Royal Holloway, University of London)

TBA

Salvador Barberà (Barcelona School of Economics)

A Characterization of Black’s Rule  

In his 1958 classic, The Theory of Committees and Elections, Duncan Black proposes the following lexicographic choice  rule: for any set of feasible alternatives, and any profile of voters’ preference relations, choose the strong Condorcet winner if it exists, and select the set of Borda winners otherwise. We provide a novel axiomatic characterization of this proposal. We do so through the intermediary study of the social welfare functions that underlie the rule’s choices, and the use of axioms that emphasize what is common and what is different in the spirit of the amply debated proposals made by these two 18th-century authors.  

This is joint work with Walter Bossert.

Agustín Bonifacio (GATE, Saint-Etienne School of Economics)

Non-Obvious Manipulability in Division Problems with General Preferences

In problems involving the allocation of a single non-disposable commodity, we study rules defined on a general domain of preferences requiring only that each preference exhibit a unique global maximum. Our focus is on rules that satisfy a relaxed form of strategy-proofness, known as non-obvious manipulability. We show that the combination of efficiency and non-obvious manipulability leads to impossibility results, whereas weakening efficiency to unanimity gives rise to a large family of well-behaved non-obviously manipulable rules.

This is joint work with Pablo Arribillaga.

Théo Delemazure (ILLC, University of Amsterdam)

Friends or Foes? Inferring Latent Candidate Structures from Preference Rankings

While much of the social choice literature focuses on aggregating voter preferences to select a winner or a committee, these preferences also contain rich information about the underlying relationships between candidates. This presentation explores how we can leverage preference data to uncover latent candidate structures, moving beyond the outcome to understand the "topology" of the candidate space. We approach this from two perspectives. First, we investigate candidate similarity to identify candidates who occupy nearly identical positions in the voters' eyes, and that we call "approximate clones'". Second, we focus on the opposite case: identifying pairs of candidates that induce the most conflict or polarization within the electorate. We will use tools from the axiomatic methods as well as experiments to support our analyses.

Lihi Dery (Ariel University)

Interactive and Iterative Peer Assessment: A Social Choice Approach to Eliciting and Aggregating Student Evaluations

Iterative peer grading activities can sustain student engagement during project presentations, but their effectiveness depends on how assessments are collected and aggregated. Numeric grades are easy to elicit, yet students tend to award inflated scores, producing ties and sometimes strategically lowering competitors' grades. Full rankings avoid this inflation but impose heavy cognitive demands on students.

We propose a peer grading model that combines the ease of numeric grading with the discriminatory power of ranking. It integrates (a) an elicitation algorithm that structures how students provide evaluations and (b) a median-based voting protocol that aggregates inputs into a ranked order with fewer ties. A classroom deployment demonstrated that this approach reduced grade inflation and strategic bias while lowering the cognitive and communicative burden on students.

Bettina Klaus (University of Lausanne)

Pairwise Efficiency and Monotonicity Imply Pareto Efficiency in (Probabilistic) Object Allocation

We consider object allocation problems with capacities (see, e.g., Abdulkadiroglu and Sonmez, 1998; Basteck, 2025) where objects have to be assigned to agents. We show that if a lottery rule satisfies ex-post non-wastefulness and probabilistic (Maskin) monotonicity, then ex-post pairwise efficiency is equivalent to ex-post Pareto efficiency. This result allows for a strengthening of various existing characterization results, both for lottery rules and deterministic rules, by replacing (ex-post) Pareto efficiency with (ex-post) pairwise efficiency, e.g., for characterizations of the Random Serial Dictatorship rule (Basteck, 2025), Trading Cycles rules (Pycia and Unver, 2017), and Hierarchical Exchange rules (Papai, 2000).

This is joint work with Tom Demeulemeester.

Patrick Lederer (ILLC, University of Amsterdam)

Proportional Representation in Rank Aggregation

In rank aggregation, the task is to aggregate multiple weighted input rankings into a single output ranking. While numerous methods, so-called social welfare functions (SWFs), have been suggested for this problem, all of the classical SWFs tend to be majoritarian and are thus not acceptable when a proportional ranking is required. Motivated by this observation, we design SWFs that guarantee that every input ranking is proportionally represented  by the output ranking. Specifically, our central fairness condition requires that the number of pairwise comparisons between candidates on which an input ranking and the output ranking agree is at least proportional to the weight of the input ranking. As our main contribution, we present a simple SWF called the Proportional Sequential Borda rule which satisfies this condition. Moreover, we introduce a more involved variant of this rule, the Flow-adjusting Borda rule, which satisfies a stronger fairness condition that applies to arbitrary groups of rankings. Many of our axioms and techniques are inspired by results in approval-based committee voting and participatory budgeting, where the concept of proportional representation has been studied in depth.

Barton Lee (ETH Zürich)

When Growth Leads to Zero-Sum Conflict

Individuals often face a tradeoff between cooperating to expand a collective good (the “pie”) or competing to expand their share of it. We study this tradeoff as a dynamic public goods problem where the pie’s size and its shares can only be gradually changed and contributions to the pie are irreversible. In the unique subgame perfect equilibrium, growth of pie occurs but halts at an inefficiently low level, at which point perpetual conflict over the shares ensues. Growth ultimately leads to a prisoner’s dilemma stage game, but cooperation is unsustainable for any discount factor. We also explore the dynamics of growth and conflict and empirical relevance of our results.

This is joint work with Álvaro Delgado-Vega.

Philippe Solal (GATE, Saint-Etienne School of Economics)

Filters in Abstract Aggregation with Infinite Sequences of Signals

This study proposes a framework for aggregation problems in which agents' signals given in an infinite sequence are aggregated. Notably, no restriction is imposed on the set of signals; hence, this framework is applicable to any type of attributes, including preferences, ballots, pieces of information, or judgments. The result can include the empty set that corresponds to the silence of the aggregation rule. Our main focus is to analyze the process by which the structure of the family of decisive coalitions forms a filter or an ultrafilter. In our framework, a coalition is regarded as decisive if a signal is socially approved as long as every agent in the coalition pick the signal up unanimously. First, we show that the family of decisive coalitions forms a filter under a certain axiom representing a stability condition. Next, in the case where the set of signals is finite, we provide axiomatic characterizations of the class of agreement-based rules associated with filters or ultrafilters: a signal is approved if and only if it is a unanimous agreement of some coalition in a specified filter or ultrafilter. Our axiomatic results demonstrate the robustness and stability of such rules. 

This is joint work with Susumu Cato, Stéphane Gonzalez, and Eric Rémila.

Anaëlle Wilczynski (CentraleSupélec, Université Paris-Saclay)

Explaining the Lack of Fair Matchings

In matching under preferences, ensuring fairness is a key desirable requirement. When assuming strict ordinal preferences, the classical envy-freeness criterion turns out to be too strong. However, even when considering natural relaxations of envy-freeness, a fair matching may fail to exist. In such cases, we explore how to explain that no matching can satisfy the associated fairness criterion. We will focus on local or rank-based relaxations of envy-freeness in matching settings assigning exactly either one item or one partner to each agent. We will provide different SAT encodings of the problem which allow us to extract Minimal Unsatisfiable Subsets (MUSes) when instances are unsatisfiable. Given a MUS, we then build a dynamic graph structure from which a step-by-step explanation is derived. Several criteria are considered to select MUSes, some of them being based on the MUS structure, while others rely on the graphical explanation structure. We provide theoretical bounds on these metrics, showing that they can vary significantly for some instances. Experimental results on synthetic data complement these results and illustrate the impact of the encodings and the relevance of our metrics to select among the many MUSes. 

This is joint work with Aurélie Beynier, Jean-Guy Mailly, Nicolas Maudet, Wassila Ouerdane, and Francesco Sabatino.