Programme

Poster Session

Posters were presented by Sreoshi Banerjee (Budapest), Tuva Bardal (Warwick), Anton Baychkov (Warwick), Satchit Chatterji (Amsterdam). Ari Conati (Helsinki), Clément Contet (Toulouse), Théo Delemazure (Paris), Federico Fioravanti (Saint-Etienne), Matthieu Hervouin (Paris), Nicolien Janssens (Rotterdam), Hanna Kern (Vienna), Luca Kreisel (Potsdam), Feline Lindeboom (Groningen), Huynh Le Nhat Linh (Saint-Etienne), Grzegorz Lisowski (Kraków), Oliviero Nardi (Vienna), Antoine Prévotat (Saint-Etienne), Ariane Ravier (Paris), Šimon Schierreich (Prague), Felicia Schmidt (Vienna), Marc Serramia (Barcelona), Stanisław Szufa (Paris), Giannis Tyrovolas (Oxford), and Tomasz Wąs (Oxford). 

Efthymios Athanasiou (Athens University of Economics and Business)

Dominant Strategy Implementation with Transferable Utilities

We consider the problem of a constituency, comprising a finite number of agents, that needs to decide among a finite number of alternatives. Agents' preferences are quasi-linear. The model encompasses both private and public goods and thus it conforms to several known applications such auctions, public good provision, assignment, etc. We provide a constructive characterization of the set of Strategy-proof and Responsive mechanisms. Subsequently, we discuss the design possibilities afforded by Strategy-proofness and juxtapose them to related implementation criteria such as Group Strategy-proofness and Obvious Strategy-proofness. We conclude with an application to binary public decisions under the obligation of Voluntary Participation. 

Markus Brill (University of Warwick)

Multiwinner Approval Voting with Possibly Unavailable Candidates

Selecting a committee that meets diversity and proportionality criteria is a challenging endeavor that has been studied extensively in recent years. This task becomes even more challenging when some of the selected candidates decline the invitation to join the committee. Since the unavailability of one candidate may impact the rest of the selection, inviting all candidates at the same time may lead to a suboptimal committee. Instead, invitations should be sequential and conditional on which candidates invited so far accepted the invitation: the solution to the committee selection problem is a query policy. If invitation queries are binding, they should be safe: one should not query a candidate without being sure that whatever the set of available candidates possible at that stage, her inclusion will not jeopardize committee optimality.

Assuming approval-based inputs, we characterize the set of rules for which a safe query exists at every stage. In particular, we show that—under a mild technical assumption—a necessary and sufficient condition for a rule to admit a safe query policy is sequentiality. This allows us to show, for example, that Phragmén's sequential rule and the Method of Equal Shares admit a safe query policy, and that Proportional Approval Voting (PAV) and other non-sequential Thiele rules do not. The definition of sequentiality might be of independent interest. In order to parallelize the invitation process, we investigate the computation of safe parallel queries, and show that it is often hard. We also study the existence of safe parallel queries with respect to proportionality axioms such as extended justified representation.

Joint work with Hayrullah Dindar, Jonas Israel, Jérôme Lang, Jannik Peters, and Ulrike Schmidt-Kraepelin.

Caterina Calsamiglia (Barcelona Supercomputing Center)

Can Single-Sex Schooling Shape the Gender Differences in Response to Pressure?  (Evidence from South Korea)

A growing body of research explores gender differences in performance, particularly in response to high-stakes situations, as a factor contributing to labor market disparities. Prior studies (e.g., Azmat, 2016; Attali, 2011) suggest that females generally outperform males in exams, but their advantage diminishes in high-stakes settings, often interpreted as females "choking" and males "excelling" under pressure.

Using data from the Korean Youth Panel Survey, we analyze students' performance in both a zero-stakes mock exam and the high-stakes university entrance exam. Contrary to prior findings, we observe that Korean males exhibit a larger decline in performance under pressure than females. Exploiting the random assignment of students within districts, we find that this pattern is reversed for females attending all-girls schools—these students perform relatively better than males when stakes rise. This suggests that the educational environment plays a crucial role in shaping responses to pressure. The institutional features of the Korean system allow us to rule out selection effects, reinforcing the idea that gender differences in reaction to stakes are malleable and influenced by schooling context.

Oihane Gallo (University of Barcelona)

Anonymity and Strategy-Proofness on a Domain of Single-Peaked and Single-Dipped Preferences

We analyze the problem of locating a public facility on a line in a society where agents have single-peaked or single-dipped preferences. We consider the domain analyzed by Alcalde-Unzu et al. (2024), where the type of preference of each agent is public information, but the location of her peak/dip as well as the rest of the preference are unknown. We characterize all strategy-proof and type anonymous rules on this domain. Building on existing results, we provide a two-step characterization: first, the median between the peaks and a collection of fixed locations is computed (Moulin, 1980), resulting in either a single alternative or a pair of contiguous alternatives. If the outcome of the median is a pair, we apply a "double-quota majority method" in the second step to choose between the two alternatives in the pair (Moulin, 1983). We also show the additional restrictions imposed by type-anonymity on the strategy-proof rules characterized by Alcalde Unzu et al. (2024). Finally, we show the equivalence of the two characterizations.

Ronald de Haan (ILLC, University of Amsterdam)

Some Ideas for Using Computationally Hard Rules in Social Choice

Virtually all areas of computational social choice feature rules that are computationally hard to use (in the worst case), but satisfy other normatively desirable properties. Think of voting rules like Kemeny's, finding envy-free and Pareto efficient solutions in fair allocation, or computing the (strict) core for hedonic games, to name a few examples. Engineering progress on SAT solving (and related) algorithms increasingly allows us to compute solutions for these social choice settings within reasonable time limits in many cases. However, it is not clear whether using these rules without running time guarantees adheres to important social choice principles such as fairness and strategyproofness. For example, would it be fair to simply choose the solution with least envy that the algorithm found in a given time frame? In this talk, we will outline some directions that will have to be investigated to see whether the use of computationally hard rules has any chance of being successful (and in what settings).

Guido Schäfer (CWI and ILLC, University of Amsterdam)

Willy Wonka Mechanisms on a Budget

One of the main challenges in mechanism design is to carefully engineer incentives ensuring truthfulness while maintaining strong social welfare approximation guarantees. But these objectives are often in conflict, making it impossible to design effective mechanisms. An important class of mechanism design problems that belong to this category are budget-feasible mechanisms, introduced by Singer (2010). Here, the designer needs to procure services of maximum value from a set of agents while being on a budget. It is known that no deterministic (or, randomized) budget-feasible mechanism satisfying dominant-strategy incentive compatibility (DSIC) can surpass an approximation ratio of 1 + √2 (respectively, 2). However, as empirical studies suggest, factors like limited information and bounded rationality question the idealized assumption that the agents behave perfectly rational. Motivated by this, Troyan and Morill (2022) introduced non-obvious manipulability (NOM) as a more lenient incentive compatibility notion, which only guards against the most "obvious" misreports. 

In this talk, we investigate whether resorting to NOM enables us to derive improved mechanisms in budget-feasible domains. We derive a budget-feasible Willy Wonka mechanism satisfying NOM that achieves a tight approximation guarantee of 2 for the general class of monotone subadditive valuation functions. Our result thus establishes a clear separation between the achievable guarantees for DSIC (perfectly rational agents) and NOM (imperfectly rational agents). We also consider more complex feasibility constraints and show that, basically, the same design template can be used. Finally, we show that we can obtain a randomized budget-feasible mechanism satisfying universal BNOM and achieving an optimal approximation ratio of 1 + ε for any ε > 0. 

Joint work with Bart de Keijzer, Artem Tsikiridis, and Carmine Ventre.

Maija Setälä (University of Turku)

Democratic Deliberation and Voting: A ‘Realist’ Perspective

In democratic theory, deliberation and voting are no longer seen as alternative 'models' of democracy, but rather as complementary functions that are necessary for making collective decisions in democratic systems. In reality, however, the relationship between deliberation and voting is not quite as straightforward. The paper proposes a more 'realist' perspective and points out some problems that may emerge when combining deliberation and voting. For example, anticipation of decision-making by voting may undermine motivation to deliberate, and voting may disrupt deliberative reasoning. The paper also discusses different proposals to address these issues by the design of voting procedures; for example, proposals pertaining to voting rules and the publicity of ballots.

Karine Van Der Straeten (Toulouse School of Economics, Institute for Advanced Study in Toulouse & CNRS)

On the Veil-of-Ignorance Principle: Welfare-Optimal Information Disclosure in Voting

We consider a two-alternative (reform vs status quo) voting environment. A benevolent utilitarian social planner controls both the information pertaining to a payoff-relevant state of the world and the voting rule. We characterize the information disclosure policy and the voting rule that are jointly socially optimal. We show that full transparency is strictly suboptimal. Instead, the optimal policy discloses just the "anonymized" information about the value of the alternatives, effectively keeping voters behind a partial veil of ignorance. The optimal voting rule is a qualified majority rule that becomes more lenient as the "anonymized" value of the reform increases.

Joint work with Takuro Yamashita.

Markus Utke (TU Eindhoven)

Truthful Budget Aggregation: Beyond Moving-Phantom Mechanisms

We study a budget-aggregation setting in which a number of voters report their ideal distribution of a budget over a set of alternatives, and a mechanism aggregates these reports into an allocation. Ideally, such mechanisms are truthful, i.e., voters should not be incentivized to misreport their preferences. For the case of two alternatives, the set of mechanisms that are truthful and additionally meet a range of basic desiderata (anonymity, neutrality, and continuity) exactly coincides with the so-called moving-phantom mechanisms, but whether this space is richer for more alternatives was repeatedly stated as an open question. We answer this question in the affirmative by presenting a class of truthful mechanisms that are not moving-phantoms but satisfy the three properties. Since moving-phantom mechanisms can only provide limited fairness guarantees (measured as the worst-case distance to a fair share solution), one motivation for broadening the class of truthful mechanisms is the hope for improved fairness guarantees. We dispel this hope by showing that lower bounds holding for the class of moving-phantom mechanisms extend to all truthful, anonymous, neutral, and continuous mechanisms. 

Adam Zylbersztejn (University of Lyon 2 and GATE Lyon/Saint-Etienne)

Strategizing under Vote and Rule Uncertainty

Individuals participate daily in collective decision-making. Sometimes they face a clear and explicit voting rule (such as during political elections), but often they are also asked to express their preferences in situations where the voting rule is either unspecified or unknown (such as choosing a time slot for a work meeting or voting in an online contest). Understanding how personal preferences are translated into voting behavior is far from trivial. One key factor is the information individuals have about the votes of their peers, while another is the information they have about the applied voting rule. Although the first aspect has received substantial attention in the literature, the second remains largely unexplored. This paper presents an experiment that aims to address this gap. 

Joint work with Antoine Prévotat and Zoi Terzopoulou.