Here we host all talks in this series, that already happened in the 2022 year.
To see the abstract for any talk, please click anywhere inside the announcement for that talk.
9PM GMT (4PM Boston, 6PM Rio de Janeiro, 9PM London, 10PM Paris, 12AM Moscow, 6AM Wednesday in Seoul, 10AM Wednesday in Auckland)
Karl-Dieter Crisman (Gordon College) "Voting on Cyclic Orders, Group Theory, and Ballots"
Host: Simona Fabrizi
Abstract. A "cyclic order" may be thought of informally as a way to seat people around a table for a game of chance, but also as a way to schedule a finite number of events in a coherent, recurring fashion. Given a set of agents/events such as A, B, and C, we can formalize this by defining a cyclic order as a permutation on this finite set, under the equivalence relation where A>B>C is identified with both B>C>A and C>A>B.
Just as with other choices involving sets with some structure, we might want to aggregate preferences of a set of voters (possibly different from the agents) on the set of possible ways to make this choice. However, given the large number of possibilities as the cardinality of the set of agents increases, we may not wish to use the usual machinery of full ranked linear orders on all these options. This raises the question of what sort of ballots may be appropriate; a single cyclic order, a set of them, or some other ballot type?
Similarly, there is a natural action of the group of permutations on the set of agents. A reasonable requirement for a choice procedure would be to respect this symmetry (the equivalent of neutrality in normal voting theory), or even take advantage of it. This talk will use representation theory of the symmetric group to analyze natural types of possible ballots for voting on cyclic orders, and what that tells us about all possible points-based voting methods for them.
(Joint work with Abraham Holleran, Micah Martin, and Josephine Noonan)
2PM GMT (9AM New York, 11AM Rio de Janeiro, 2PM London, 3PM Paris, 5PM Moscow, 11PM Seoul)
Zoi Terzopoulou (LAMSADE, Université Paris Dauphine) "Approval-Compatible Voting Rules"
Host: Jobst Heitzig
Abstract. Voting rules that rely on approval ballots are often contrasted with those that are based on preference rankings. We further explore the compatibility between these two kinds of voting methods, by comparing the winners of two alternative elections: One where voters cast approval ballots and the winners are decided using approval voting, and one where voters report preference rankings and some classical voting rule determines the winners. Assuming that the reported rankings are consistent with the approval ballots, what can we say about the alternative winners of the two elections? If the employed voting rule satisfies a notion of positive approval compatibility, then every approval winner could be a ranking winner, and a negative version of this notion would imply that every approval loser could be a ranking loser. Although negative compatibility is a very weak notion, we find that positive compatibility divides usual voting rules into two classes: On the one hand, several positional scoring rules (but except the Borda rule and plurality) violate it, while the Borda rule, plurality, and Condorcet-consistent rules satisfy it.
This is joint work with Jérôme Lang and William Zwicker.
9PM GMT (4PM Boston, 6PM Rio de Janeiro, 9PM London, 10PM Paris, 12AM Moscow, 6AM Wednesday in Seoul, 10AM Wednesday in Auckland)
Aram Grigoryan (Duke University) "Priority-Based Assignment with Reserves and Quotas" joint with Atila Abdulkadiroğlu
Host: Simona Fabrizi
Abstract. In real-life assignment problems there are oftentimes diversity and distributional objectives. For example, public schools aim for a diverse student body in terms of socioeconomic background, and selective colleges try to admit more applicants from underrepresented groups. First, we study a single school's problem of choosing a set of applicants to be assigned to the school. We provide an axiomatic characterization of a general class of choice rules where distributional objectives are met through type-specific reserves and quotas. The choice rules in the class differ by the order in which applicants are considered for units reserved for different types. We show that a particular rule, where all applicants are first considered for units reserved for their own types and which we call the regular reserves-and-quotas rule, uniquely minimizes priority violations in this class. Next, we study a general setup with multiple schools. We show that when all schools use the regular reserves-and-quotas rule, the Deferred Acceptance mechanism minimizes priority violations in a large class of mechanisms that satisfy the distributional constraints.
9 PM GMT (4PM Boston, 6PM Rio de Janeiro, 9PM London, 10PM Paris, 12AM Moscow, 6AM Wednesday in Seoul & Tokyo, 10AM Wednesday in Auckland)
Andrew McLennan (University of Queensland) "The Strategy of Single Transferable Vote" joint with Shino Takayama and Terence Yeo
Host: Simona Fabrizi
Abstract: In an STV election each voter's ballot is a rank ordering of the candidates. Initially each candidate is allocated those ballots that list her as the favorite. At each stage the remaining candidate with the fewest ballots is eliminated, and each of the ballots she had is reallocated to the ballot's favorite among the remaining candidates, until one candidate has a majority of the ballots. We study the quantitative manipulability of STV in comparison with plurality, antiplurality, and two stage runoff, and we study the relative importance of manipulation at the stages of three and four remaining candidates. We find that STV is less manipulable than the other systems. For STV the dominant mode of manipulation is at the round of three remaining candidates, with the manipulator pushing a weak candidate into the round of two, then benefitting when they lose.
9 AM GMT (6AM Rio de Janeiro, 9AM London, 10AM Bonn, 12PM Moscow, 6PM Seoul & Tokyo, 10PM Auckland)
Gregorio Curello (University of Bonn) ''Agenda-Manipulation in Ranking"
Host: Danilo Coelho
Abstract. We study the susceptibility of committee governance (e.g. by boards of directors), modelled as the collective determination of a ranking of a set of alternatives, to manipulation of the order in which pairs of alternatives are voted on -- agenda-manipulation. We exhibit an agenda strategy called insertion sort that allows a self-interested committee chair with no knowledge of how votes will be cast to do as well as if she had complete knowledge. Strategies with this 'regret-freeness' property are characterised by their efficiency, and by their avoidance of two intuitive errors. What distinguishes regret-free strategies from each other is how they prioritise among alternatives; insertion sort prioritises lexicographically.
This is joint work with Ludvig Sinander.
2PM GMT (10AM New York, 11AM Rio de Janeiro, 2PM London, 3PM St. Gallen, 5PM Moscow, 11PM Seoul)
Paolo Piacquadio (University of St.Gallen) “Fairness and Paretian Social Welfare Functions”
Host: Danilo Coelho
Abstract. We enrich the toolbox of welfare analysis by accommodating fairness concerns in Paretian social welfare functions. Fairness requires treating individuals equally, unless there are good reasons for treating them differently. Our criteria rank allocations in light of these justifiable inequalities and trade-off unjustified inequalities with efficiency. To illustrate our results, we investigate the implications of various fairness views in the context of labor-income taxation. Standard welfare criteria—such as utilitarianism—assume individuals do not deserve their income opportunities; in contrast, our criteria allow any degree of deservingness. We also simulate the optimal non-linear income tax for the US economy. The US tax system is among the least redistributive ones we can justify, requiring a large degree of deservingness and little concern for progressivity.
(Joint with Kristoffer Berg, Oxford).
1PM GMT (9AM New York, 10AM Rio de Janeiro, 2PM London, 3PM Paris, 4PM Istanbul, 10PM Seoul, 1AM Wednesday in Auckland)
Antonin Macé (CNRS and Paris School of Economics) "A Model of Repeated Collective Decisions"
Host: Marcus Pivato
Abstract. A committee makes repeated decisions at a qualified majority rule, under complete information on current preferences. We show that the optimal equilibrium of this repeated game coincides on path with a cutoff decision rule: at each stage, an efficient proposal is collectively accepted if and only if the pivotal individual’s utility exceeds some fixed negative cutoff. In contrast with the stage-game equilibrium, where individuals vote sincerely, the optimal equilibrium involves a form of implicit logroll, individuals sometimes voting against their preference to achieve the efficient decision. As a result, both the expected level of utility and the degree of consensus may be significantly higher than at the stage-game equilibrium. We characterize the set of optimal voting rules in the limit case where the number of individuals grows large and show that it always contains either the simple majority rule or the unanimity rule. We then introduce Abreu et al. (1993)’s renegotiation-proofness requirement, and characterize the optimal renegotiation-proof equilibrium. In the limit case, simple majority is always optimal but the set of optimal voting rules expands as the discount factor increases, eventually including all majority thresholds if individuals are patient enough. The model provides a rationale for the use of unanimity rule, while explaining the prevalence of consensus in committees using a lower majority threshold.
Joint with Rafael Treibich.
9AM GMT (5AM New York, 6AM Rio de Janeiro, 10AM London, 11AM Hamburg, 12PM Istanbul, 6PM Seoul, 9PM Auckland)
Anke Gerber (Universität Hamburg) “(Not) Addressing Issues in Electoral Campaigns”
Host: Danilo Coelho
Abstract. Two candidates competing for election may raise some issues for debate during the electoral campaign, while avoiding others. We present a model in which the decision to introduce an issue, or to reply to the opponent's position on one that she raised, may result in further additions to the list of topics that end up being discussed. Candidates' strategic decisions are driven by their appraisal of the probability that the resulting campaign leads to their election. Our analysis appeals to a protocol-free equilibrium concept, and predicts the list of topics that will be touched upon by each candidate, and the order in which they might be addressed. We show that important phenomena observed during campaigns, like the convergence of the parties to address the same issues, or else their diverging choice on which ones to treat, or the relevance of issue ownership can be explained within our stark basic model.
(Joint work with Salvador Barberà)
9AM GMT (5AM New York, 6AM Rio de Janeiro, 10AM London, 11AM Paris, 12PM Istanbul, 6PM Seoul, 9PM Auckland)
Jérôme Lang (LAMSADE, Université Paris Dauphine) "Runoff, yes; plurality, no"
Host: Marcus Pivato
Abstract. Plurality with runoff is a widely used voting rule, consisting of two rounds, the second round being a duel between two finalists selected at the first round using plurality scores. The talk will also consist of two rounds.
Round I: plurality with runoff, no!
Using examples from French presidential elections (especially the ongoing one, which will be over at the time of the talk), I will comment on the numerous drawbacks of the rule, and its only advantage, namely the existence of a runoff and of a time gap between the two rounds.
Round II: approval with runoff
We claim that it is possible to retain the idea of a runoff and avoid the catastrophic drawbacks of its coupling with plurality, by having voters cast approval-type ballots at the first round. With approval-type ballots, there are various ways to select the finalists. We leverage known approval-based committee rules and study the obtained runoff rules from an axiomatic point of view. Then we analyse the outcome of these rules on single-peaked profiles, and on real data.
(Joint work with Théo Delemazure, Jean-François Laslier and Remzi Sanver.)
9PM GMT (4PM Nashville, 6PM Rio de Janeiro, 10PM London, 11PM Paris, 12AM Istanbul, 6AM (Wednesday) Seoul, 9AM (Wednesday) Auckland)
John Weymark (Vanderbilt University) "Precedent-Based Judgment Aggregation in the U.S. Supreme Court"
Host: Marcus Pivato
Abstract. When a case is before the U.S. Supreme Court, a precedent may apply. In cases in which a precedent is being considered, the Court needs to answer three questions: (1) Is the precedent good law? (2) Does the precedent apply to this case? (3) Should the Court uphold the precedent? In the event that the Court answers yes to the first two questions and no to the last, there is what David Cohen (Boston University Law Review, 2010) calls a precedent-based voting paradox. Cohen has identified eleven instances of this paradox in U.S. Supreme Court decisions prior to 2010. We review Cohen's paradox and relate it to the doctrinal paradox that has played a foundational role in the judgment aggregation literature. We also identify what is arguably one more instance of a precedent-based voting paradox in the period since Cohen's article was published.
(Joint work with Sarah E. Friedman)
2PM GMT (10AM New York, 11AM Rio de Janeiro, 3PM London, 4PM Paris, 5PM Istanbul, 11PM Seoul)
Agnieszka Rusinowska (CNRS, PSE and Centre d'Économie de la Sorbonne) "On the design of public debate in social networks"
Host: Marcus Pivato
Abstract. We propose a model of the joint evolution of opinions and social relationships in a setting where social influence decays over time. The dynamics are based on bounded confidence: social connections between individuals with distant opinions are severed while new connections are formed between individuals with similar opinions. Our model naturally gives rise to strong diversity, i.e., the persistence of heterogeneous opinions in connected societies, a phenomenon that most existing models fail to capture. The intensity of social interactions is the key parameter that governs the dynamics. First, it determines the asymptotic distribution of opinions. In particular, increasing the intensity of social interactions brings society closer to consensus. Second, it determines the risk of polarization, which is shown to increase with the intensity of social interactions. Our results allow us to frame the problem of the design of public debates in a formal setting. We hence characterize the optimal strategy for a social planner who controls the intensity of the public debate and thus faces a trade-off between the pursuit of social consensus and the risk of polarization. We also consider applications to political campaigning and show that both minority and majority candidates can have incentives to lead society towards polarization.
(Joint work with Michel Grabisch and Antoine Mandel)
9AM GMT (5AM New York, 6AM Rio de Janeiro, 10AM London, 11AM Hamburg, 12PM Istanbul, 6PM Seoul, 9PM Auckland)
Jorge Alcalde-Unzu (Public University of Navarre) "Impartial Social Rankings: Some Impossibilities"
Host: Danilo Coelho
Abstract. We model a situation where a set of agents must rank themselves based on their opinions. Each agent submits a message and a function determines the social ranking. We are interested in impartial social ranking functions, that is, those where the message of an agent cannot change any social binary comparison of herself with respect to someone else. We obtain impossibility results by additionally considering some classical properties of symmetry across agents.
(Joint work with Dolors Berga and Riste Gjorgjiev).
9AM GMT (5AM New York, 6AM Rio de Janeiro, 10AM London, 11AM Hamburg, 12PM Istanbul, 6PM Seoul, 9PM Auckland)
Mustafa Oğuz Afacan (Sabanci University) "Arbiter Assignment" joint with Nejat Anbarci and Ozgur Kibris
Host: Simona Fabrizi
Abstract. In dispute resolution, arbitrator assignments are decentralized and they incorporate parties’ preferences, in total contrast to referee assignments in sports. We suggest that there can be gains (i) in dispute resolution from centralizing the allocation by bundling the newly arriving cases, and (ii) in sports from incorporating teams’ preferences. To that end, we introduce a class of Arbiter Assignment Problems where a set of matches (e.g., disputes or games), each made up of two agents, are to be assigned arbiters (e.g., arbitrators or referees). In this new set of problems, the question of how agents in a match should compromise becomes critical. To evaluate the value of an arbiter for a match, we introduce the (Rawlsian) notion of depth, defined as the arbiter’s worst position in the two agents’ rankings. Depth optimal assignments minimize depth over matches, and they are Pareto optimal. We first introduce and analyze depth optimal (and fair) mechanisms. We then propose and study strategy-proof mechanisms.
2PM GMT (10AM New York, 11AM Rio de Janeiro, 3PM London, 4PM Paris, 5PM Istanbul, 11PM Seoul)
Jean-François Laslier (CNRS and Paris School of Economics) "Universalization and altruism"
Host: Marcus Pivato
Abstract. The 𝜅-universalization of a symmetric game is the game in which each player considers that any other player chooses with probability 𝜅 the same strategy as she. To any normal form game, we associate the symmetric two-stage game in which, in a first stage, the roles to be played in the base game are randomly assigned. We show that any pure strategy equilibrium of the 𝜅-universalization of this extended game is an equilibrium of the base game played by altruistic players (``ex ante Homo Moralis equilibrium is altruistic''), and that the converse is false. The paper presents the implications of this remark for the philosophical nature of ethical behavior (Kantianism behind the veil of ignorance implies but is stronger than altruism) and for its evolutionary foundations.
2PM GMT (10AM New York, 11AM Rio de Janeiro, 3PM London, 4PM Paris, 5PM Istanbul, 11PM Seoul)
Rida Laraki (CNRS and LAMSADE, Université Paris-Dauphine) "Level-Strategypoof Belief Aggregation and Application to Majority Judgment under Uncertainty"
Host: Marcus Pivato
Abstract. In the problem of aggregating experts’ probabilistic predictions or opinions over an ordered set of outcomes, we introduce the axiom of level-strategyproofness (Level-SP) and argue that it is natural in several real-life applications and robust as a notion. It implies truthfulness in a rich domain of single-peaked preferences over the space of cumulative distributions. This contrasts with the existing literature, where we usually assume single-peaked preferences over the space of probability distributions. Our main results are:
Explicit characterizations of all Level-SP methods with and without the addition of other axioms (certainty preservation, plausibility preservation, proportionality, weighted voters);
Comparisons and axiomatic characterizations of two new and practical Level-SP methods: the proportional-cumulative and the middlemost-cumulative;
A use of the proportional-cumulative to construct a new voting method (MJU) that extends majority judgment (MJ) method for electing and ranking alternatives.
In MJ, a voter attributes a grade on a scale of merits such as Λ = { Great; Good; Average; Poor; Terrible} to each candidate/alternative. In MJU, voters can express their uncertainties/doubts about the merits (e.g. a voter can submit, for each candidate, a probability distribution over Λ). We show that MJU inherits most of the salient properties of MJ (e.g. it avoids Arrow and Condorcet paradoxes and it resists to some natural strategic manipulations).
(Joint work with Estelle Varloot, University of Liverpool)
2PM GMT (9AM Austin, 11AM Rio de Janeiro, 3PM London, 4PM Paris, 5PM Istanbul, 7:30PM New Delhi, 11PM Tokyo/Seoul)
Dean Spears (University of Texas, Austin, and Research Institute for Compassionate Economics) "Separable Social Welfare Evaluation for Multi-Species Populations"
Host: Marcus Pivato
Abstract. If non-human animals experience wellbeing and suffering, such welfare consequences arguably should be included in a social welfare evaluation. Yet economic evaluations almost universally ignore non-human animals, in part because axiomatic social choice theory has failed to propose and characterize multi-species social welfare functions. Here we propose axioms and functional forms to fill this gap. We provide a range of alternative representations, characterizing a broad range of possibilities for multi-species social welfare. Among these, we identify a new characterization of additively-separable generalized (multi-species) total utilitarianism. The multi-species setting permits a novel, weak species-level separability axiom with important consequences. We provide examples to illustrate that non-separability across species is implausible in a multi-species setting, in part because good lives for different species are at very different welfare levels.
Joint work with Stéphane Zuber (Paris School of Economics) and Mark B. Budolfson (Rutgers University)
8AM GMT (5AM Rio de Janeiro, 9AM London, 10AM Paris, 11AM Istanbul, 1:30PM New Delhi, 5PM Tokyo/Seoul, 7PM Sidney, 9PM Auckland)
Simona Fabrizi (University of Auckland) "Unanimity under Ambiguity"
Host: Marcus Pivato
Abstract. This paper considers a binary decision to be made by a committee — canonically, a jury — through a voting procedure. Each juror must vote on whether a defendant is guilty or not guilty. The voting rule aggregates the votes to determine whether the defendant is convicted or acquitted. We focus on the unanimity rule (convict if, and only if, all vote guilty), and we consider jurors who share ambiguous prior beliefs as in Ellis (2016). Our contribution is twofold. First, we identify all symmetric equilibria of these voting games. Second, we show that ambiguity may drastically undermine McLennan’s (1998) results on decision quality: unlike in the absence of ambiguity, the ex-ante optimal symmetric strategy profile need not be an equilibrium; indeed, there are games for which it is possible to reduce both Type I and Type II error starting from any (non-trivial) equilibrium. Finally, we explain the significance of these results for our on-going experimental work on voting behaviour.
Joint work with Steffen Lippert (University of Auckland), Addison Pan (Xi'an Jiaotong-Liverpool University) and Matthew Ryan (Auckland University of Technology).
2PM GMT (10AM New York, 11AM Rio de Janeiro, 2PM Edinburgh, 3PM Paris, 5PM Istanbul, 7:30PM New Delhi, 11PM Seoul/Tokyo)
Murdoch James Gabbay (Heriot-Watt University) "The semitopology of heterogeneous consensus"
Host: Jobst Heitzig
Abstract. Arriving at consensus is an old problem, to which blockchain systems add some particularities:
performance is at a premium,
the environment may be highly adversarial --- and perhaps most significantly,
the system may be permissionless (any agent can join the network at any time) and heterogeneous (different parts of the network may have different computational power, connectivity, latency, and so forth).
We can make sense of this using the language of topology: a point is a participant, and an open set is a local quorum. A quorum that reaches agreement may progress locally. This has good performance: the system doesn't need to synchronise or reach a global majority to progress. It is also resilient: local adversarial behaviour may corrupt a local quorum, but the rest of the space may continue to operate.
Such a system is up and running in the Stellar payments network, and can be viewed here: https://stellarbeat.io/
The intersection of two quorums is not necessarily a quorum, so in fact we get a new notion of semitopology, which is like a topology but without the condition that intersections of open sets are open. Semitopologies, like topologies, have rich mathematical structure, with explanatory power which includes the emergence of a kind of "Dictator set" --- a subset of privileged participants that determine behaviour which is mathematically guaranteed to exist for any network.
What seems to be emerging is a kind of topological voting theory, with its own version of Arrow's theorem and various other properties that we are still trying to make sense of. Applications are to Stellar as currently implemented, and we see further applications in the design of blockchain governance, and specifically for DAOs (distributed autonomous organisations).
I hope that bringing this material to the attention of a social choice audience may lead to a fruitful exchange of ideas and insights, which might inform the new generation of permissionless systems emerging in the blockchain space and help them to be fair and humane, as well as efficient and resilient.
2PM GMT (9AM New York, 11AM Rio de Janeiro, 2PM London, 3PM Zürich, 5PM Istanbul, 7:30PM New Delhi, 11PM Tokyo/Seoul)
Carlos Alos-Ferrer (Zürich Center for Neuroeconomics, University of Zürich) "Voting for Compromises"
Host: Marcus Pivato
Abstract. In democratic societies, different groups often favor different, conflicting alternatives. Sometimes, political compromises based on social conventions as equity or economic arguments as efficiency are available, but they fail to gather the necessary support. We study different voting methods in three experiments (total N = 5, 820), including small, medium-sized, and large electorates, and find that currently-used methods (Plurality Voting and Rank-Order systems) can lead to widespread selection of egoistic options. In contrast, alternative, more nuanced methods (Approval Voting and Borda Count) favor equity and efficiency, avoiding extreme outcomes. Those two methods differ in their support of equity vs. efficiency when the latter benefits a majority. Our evidence suggests that targeted changes in the electoral system could favor socially-desirable compromises and increase social stability.
9AM GMT (6AM Rio de Janeiro, 9AM London, 10AM Barcelona, 12PM (noon) Istanbul, 2:30PM New Delhi, 6PM Tokyo/Seoul, 8PM Sydney, 10PM Auckland)
David Perez Castrillo (Universitat Autònoma de Barcelona and Barcelona School of Economics) "Constrained-optimal tradewise-stable outcomes in the one-sided assignment game: A solution concept weaker than the core"
Host: Danilo Coelho
Abstract. In the one-sided assignment game, any two agents can form a trade; they can decide to form a partnership and agree on how to share the surplus created. Contrary to the two-sided assignment game, stable outcomes often fail to exist in the one-sided assignment game. Hence the core, which coincides with the set of stable payoffs, may be empty. We introduce the idea of tradewise stable (t-stable) outcomes: they are individually rational outcomes where all trades are stable; that is, no matched agent can form a blocking pair with any other agent, neither matched nor unmatched. We propose the set of constrained-optimal (optimal) t-stable outcomes, the set of the maximal elements of the set of t-stable outcomes, as a natural solution concept for this game. We prove that this set is non-empty, it coincides with the set of stable outcomes when the core is non-empty, and it satisfies similar properties to the set of stable outcomes even when the core is empty. We propose a partnership formation process that starts with the outcome where every player stands alone, goes through steps where the set of active players expands, always forming t-stable outcomes, and ends in an (in any) optimal t-stable outcome. Finally, we also use the new concept to establish conditions under which the core is non-empty.
(Joint work with Marilda Sotomayor)
9AM GMT (6AM Rio de Janeiro, 9AM London, 10AM Barcelona, 12PM (noon) Istanbul, 2:30PM New Delhi, 6PM Tokyo/Seoul, 8PM Sydney, 10PM Auckland)
Danilo Coelho (IPEA) "Compromise rules to select groups of fixed size"
Host: Simona Fabrizi
Abstract. We proposed two mechanisms for two parties to jointly select a group of fixed size. We show that these mechanisms have good properties if the parties' preferences over sets are leximin or leximax extensions of the parties' preferences over candidates then these mechanisms may induce compromise and Pareto efficient outcomes.
(Joint work with Salvador Barberà)