INTERNATIONAL GROUP OF COORDINATORS:

Youngsub Chun  (ychun@snu.ac.kr)
Gleb Koshevoy (koshevoyga@gmail.com)
Clemens Puppe (clemens.puppe@kit.edu)
Arkadii Slinko (a.slinko@auckland.ac.nz)
Bill Zwicker (zwickerw@union.edu)

[To join, please contact one of the coordinators to get a meeting ID and password]

 

LIST OF TALKS

Date and time:  25 August 8AM GMT (i.e., e.g., 10am in Paris, 8pm in AKL)
Contributor: Salvador Barberà
Title: Pairwise justified changes in collective choices

Abstract: Consider the following principle regarding the performance of collective choice rules. ì If a rule selects alternative x in situation 1, and alternative y in situation 2, there must be an alternative z, and some member of society whose appreciation of z relative to x has increased when going from situation 1 to situation 2.î This principle requires a minimal justiÖcation for the fall of x in the consideration of society: someone must have decreased its appreciation relative to some other possible alternative. We study the consequences of imposing this requirement of pairwise justifiability on a large class of collective choice rules that includes social choice and social welfare functions as particular cases. When preference profiles are unrestricted, it implies dictatorship, and both Arrowís and the Gibbard-Satterthwaite theorems become corollaries of our general result. On appropriately restricted domains, pairwise justifiability, along with anonymity and neutrality, characterize Condorcet consistent rules, thus providing a foundation for the choice of the alternatives that win by majority over all others in pairwise comparisons, when they exist. This is a joint paper with Dolors Berga, Bernardo Moreno, and Antonio Nicoló.

Date and time:  18 August 8AM GMT
Contributor: Constantine Sorokin
Title: A New Approach to Contests with Complete and Incomplete Information

Abstract: We propose an alternative strategy of modelling contests by introducing a new class of truncated polynomial probability-of-win functions. Our approach permits finding a closed form solution and obtaining valuable comparative static results not only for the complete information case, (both for mixed and pure strategies), but also for the case of incomplete (private) information. Particularly, we are able to address an important question of information design in contests. In addition, this approach also allows to explore the case when a further increase in maximal effort exerted in a contest bestows a positive externality on other contenders with lower efforts, increasing their marginal probability of winning. We argue that this situation usually arises in R&D competition and patent races and cannot be modelled by standard Tullock-style contest models. This is a joint work with Alexander Matros, North Carolina and Lancester.

Date and time:  11 August 8AM GMT
Contributor: Marcus Pivato
Host: Simona Fabrizi
Title: Population ethics in an infinite universe [paper and slides]

Abstract: Population ethics studies the tradeoff between the total number of people who will ever live, and their welfare.  But widely accepted theories in modern cosmology say that spacetime is probably infinite. In this case, its population is also probably infinite, so the quantity/quality tradeoff of population ethics is no longer meaningful.  Instead, we face the problem of how to evaluate the social welfare of an infinite population dispersed through time and space.  I propose spatiotemporal Cesàro average utility as a way to make this evaluation, and axiomatically characterize it.
Date and time:  4 August 8AM GMT
Contributor: Michele Gori
Host: Remzi Sanver
Title: Manipulation of social choice functions under incomplete information

Abstract: The failure of strategy-proofness requires the presence of an individual who completely knows the others’ preferences. However, as observed by some authors, an individual might decide to misrepresent her preferences on the basis of a smaller amount of information. It is then possible to consider weak versions of strategy-proofness which take into account the amount of information individuals can get. We analyse one of those weak versions of strategy proofness, called PC-strategy-proofness. A social choice function is PC-strategy-proof if it cannot be manipulated by an individual whose information about the preferences of the other members of the society is limited to the knowledge, for every pair of alternatives, of the number of individuals preferring the first alternative to the second one. We show that there are Pareto optimal, PC-strategy-proof and non-dictatorial social choice functions. We also prove that, when at least three alternatives are considered, no Pareto optimal, PC-strategy-proof and anonymous social choice function exists. Generalizing the definition of PC-strategy-proofness, we further propose a special family of weak versions of strategy-proofness which allows to define a new index for measuring the degree of manipulability of social choice functions. Some simple remarks on that index are discussed.
Date and time:  28 July 8AM GMT
Contributor: Alexander Karpov
Host: Clemens Puppe
Title: A new scheme for constructing large Condorcet domains

Abstract: We present a new method of constructing Condorcet domains from pairs of Condorcet domains of smaller sizes (concatenation+shuffle scheme). The concatenation+shuffle scheme provides maximal, connected, copious, peak-pit domains whenever the original domains had these properties. It provides peak-pit maximal Condorcet domains that are larger than those obtained by the Fishburn’s alternating scheme for all n≥13 (it was previously known for n≥40). The paper provides a new lower bound  2.1685^n  for the size of the largest peak-pit Condorcet domain with n alternatives, and a new lower bound  2.2021^n for the size of the largest Condorcet domain without any restrictions.
Date and time:  21 July 9AM GMT (i.e., e.g., 11am in Paris, 9pm in AKL)
Contributor: Danilo Coelho
Title: Compromising on Compromise Rules

Abstract: We propose three mechanisms to reach a compromise between two opposite parties that must choose one out of a set of candidates and operate under full information. All three mechanisms weakly implement the Unanimity Compromise Set. They all rely on the use of some rule of k names, whereby one of the parties proposes a shortlist of k candidates, from which the opposite party selects the one to appoint. The decision regarding which particular rule in the class will be used involves determining who will be the first mover and the size of k. The chosen rule results endogenously from the strategic interaction between the parties, rather than being imposed a priori by any exogenous convention.

Date and time:  7 July 8AM GMT
Contributor: Hervé Moulin
Title: Fair Division with Money

Abstract: Agents share indivisible objects (desirable or not) and use cash transfers to achieve fairness.  Utilities are linear in money but otherwise arbitrary. We look for n-person division rules preserving the informational simplicity of Divide and Choose or the Texas Shoot Out between two agents, treating agents symmetrically, and offering high individual welfare Guarantees. A single round of bidding for the whole manna is one such method but it does not capture the potential efficiency gains from debundling the objects as in Divide and Choose. Our Bid and Choose rules fix a price vector p for the objects; in each of the n-1 rounds of bidding the winner must also pay for the remaining objects he picks. These rules are simpler than Kuhn’s n-person generalisation of Divide and Choose, and they typically offer better Guarantees. They help agents with subadditive utilities, to the detriment of those with superadditive utilities. The talk is based on joint research with Anna Bogomolnaia.

Date and time:  30 June 8AM GMT
Contributor: Piotr Skowron
Title: Proportionality and the Limits of Welfarism

Abstract: We study two influential voting rules proposed in the 1890s by Phragmen and Thiele, which elect a committee or parliament of k candidates which proportionally represents the voters. Voters provide their preferences by approving an arbitrary number of candidates. Previous work has proposed proportionality axioms satisfied by Thiele but not Phragmen. By proposing two new proportionality axioms (laminar proportionality and priceability) satisfied by Phragmen but not Thiele, we show that the two rules achieve two distinct forms of proportional representation. Phragmen’s rule ensures that all voters have a similar amount of influence on the choice of the committee, and Thiele’s rule ensures a fair utility distribution. (Thiele’s rule is a welfarist voting rule that maximises a function of voters’ utilities). We show that no welfarist rule can satisfy our new axiom, and we prove that no such rule can satisfy the core. Conversely, some welfarist fairness properties cannot be guaranteed by Phragmen-type rules. This formalises the difference between the two types of proportionality. We then introduce an attractive committee rule which satisfies a property intermediate between the core and extended justified representation (EJR). It satisfies laminar proportionality, priceability, and is computable in polynomial time.  The talk is based on the recent paper: Dominik Peters and Piotr Skowron. Proportionality and the Limits of Welfarism. EC-2020.

Date and time:  23 June 8AM GMT
Contributor: Nimrod Talmon
Title: Participatory Budgeting with Cumulative Votes

Slides of the talk

Recording of the talk

Abstract: In participatory budgeting we are given a set of projects—each project having a cost, an integer specifying the available budget, and a set of voters who express their preferences over the projects. The goal is to select—based on voter preferences—a subset of projects whose total cost does not exceed the budget. We propose several aggregation methods based on cumulative votes, i.e., for the setting where each voter is given one coin and specifies how this coin should be split among the projects. We compare our aggregation methods based on (1) axiomatic properties and (2) computer simulations. We identify one method, Minimal Transfers over Costs, that demonstrates particularly desirable behaviour — in particular, it significantly improves on existing methods and satisfy a strong notion of proportionality — and thus is promising to be used in practice. This is a joint paper with Piotr Skowron and Arkadii Slinko.

Date and time:  16 June 8AM GMT
Contributor: Gleb Koshevoy
Title: Condorcet super-domains

Abstract: We consider Condorcet domains (CD) formed by a rhombus tiling on a zonogone Z(n; 2) as voting designs and consider a problem of aggregation of voting designs using the majority rule. A Condorcet super-domain is a collection of CDs obtained from rhombus tilings with the property that if voting designs (serving as ballots) belong to this collection, then the simple majority rule does not yield cycles. I will discuss methods of constructing Condorcet super-domains and related problems. The talk is based on joint paper with Vladimir Danilov and Aleksandre Karzanov (arxiv 2004.08183 math.CO).

Date and time:  9 June 8AM GMT
Contributor: Simona Fabrizi
Title: Unanimous Jury Voting with an Ambiguous Likelihood

Recording of the talk

Abstract: We study collective decision-making in a voting game under the unanimity rule, with an ambiguous likelihood and ambiguity-averse voters who are MaxMin Expected Utility maximizers. We characterize the symmetric voting equilibria of this game, demonstrating that ambiguity helps reduce Type I errors: under ambiguity, voters are less likely to vote strategically against their information. Information aggregation improves as a result, and may even be restored to a fully informative equilibrium. We report evidence from a laboratory experiment supporting these predictions. This is joint work with Steffen Lippert, Addison Pan, and Matthew Ryan.

Date and time: 2 June 11AM GMT
Contributors: Ayumi Igarashi and William S. Zwicker
Title: Fair division of graphs and of tangled cakes

Slides of the presentation: Part I and Part II

Abstract: Recent work by Bilò et al [2019] concerns allocating graph vertices (treated as indivisible objects) so that each share forms a connected subgraph, and so that no agent x envies another’s share “up to one outer good.” They obtain positive results that apply to arbitrarily many agents, but these are limited to Hamiltonian (aka traceable) graphs.  What of the non-Hamiltonian case? We show that among topological classes of graphs, any non-Hamiltonian class has an upper bound on the number of agents for which fair shares are guaranteed.  On the other hand, for the case of exactly 3 agents, positive results exist for some infinite, non-Hamiltonian graph classes.  Our results – positive and negative – are obtained via transfer from related theorems in continuous fair division, but we must go beyond the standard model, which employs the unit interval [0,1] as the continuously divisible “cake.”  Instead, we use several copies of [0,1] glued at their endpoints, to form the letter Y, or the figure 8, or the outline of a kiss . . . a “tangle.”

Date and time: 26 May 8AM GMT
Contributors: M. Remzi Sanver and Shin Sato
Title: Evaluationwise strategy-proof social choice correspondences

Recording of the talk

Abstract: We consider manipulation of social choice correspondences in a preference-approval environment where voters not only rank the alternatives but also evaluate them as acceptable or unacceptable. A social choice correspondence is evaluationwise strategy-proof iff no voter can misrepresent his preference and obtain an outcome which he finds more acceptable than the one that would occur if he had told the truth. As outcomes are irresolute sets of alternatives, our analysis needs to extend the notion of acceptability of alternatives over sets. Under a plausible extension, we show the existence of efficient and evaluationwise strategy-proof social choice correspondences that satisfy one of anonymity and neutrality. However, if anonymity and neutrality are jointly imposed, then an impossibility occurs when the number of voters is a multiple of 4. On the other hand, when there are three alternatives and the number of voters is not a multiple of 4, we show the existence of social choice correspondences which are efficient, evaluationwise strategy-proof, anonymous and neutral.

Date and time: 19 May, 8AM GMT
Contributor: Arkadii Slinko
Title: Generalisation and Properties of the Danilov-Karzanov-Koshevoy Construction for Peak-Pit Condorcet Domains

Recording of the talk

Abstract: Danilov, Karzanov and Koshevoy (2012) geometrically introduced an interesting operation of composition on Condorcet domains and using it they disproved a long-standing problem of Fishburn about the maximal size of connected Condorcet domains. We give an algebraic definition of this operation and investigate its properties. We give a precise formula for the cardinality of composition of two Condorcet domains and improve the Danilov, Karzanov and Koshevoy result showing that Fishburn’s alternating scheme does not always produce a largest connected Condorcet domain. I will outline some new exciting developments in the search of largest Condorcet domains.

Speaker: Dodge Cahan (Faculty of Arts, University of Alberta)

Title of presentation: “Spatial competition on 2-dimensional markets and networks when consumers don’t always go to the closest firm” joint work with Arkadii Slinko

Date, Time and Venue: Wednesday, 29 April 2020, 11:00-12:00 – online (via Zoom – see calendar invite)

Abstract: We investigate the strategic behavior of firms in a Hotelling spatial setting. The innovation is to combine two important features that are ubiquitous in real markets: (i) the location space is two-dimensional, often with physical restrictions on where firms can locate; (ii) consumers with some probability shop at firms other than the nearest. We characterise convergent Nash equilibria (CNE), in which all firms cluster at one point, for several alternative markets. In the benchmark case of a square convex market, we provide a new direct geometric proof of a result by Cox (1987) that CNE can arise in a sufficiently central part of the market. The convexity of the square space is of restricted realism, however, and we proceed to investigate networks, which more faithfully represent a stylised city’s streets. In the case of a grid, we characterise CNE, which exhibit several new phenomena. CNE in more central locations tend to be easier to support, echoing the unrestricted square case. However, CNE on the interior of edges differ substantially from CNE at nodes and follow quite surprising patterns. Our results also highlight the role of positive masses of indifferent consumers, which arise naturally in a network setting. In most previous models, in contrast, such masses cannot exist or are assumed away as unrealistic.

Everyone Welcome!

Speaker: Arkadii Slinko (Department of Mathematics, UOA)

Title of presentation: “Generalisation of the Danilov-Karzanov-Koshevoy construction for Peak-Pit Condorcet domains”

Date, Time and Venue: Wednesday, 4 March 2020, 14:30-15:30, 260-5115 [Business School Building, Level 5]

Abstract: Danilov, Karzanov and Koshevoy (2012) geometrically introduced an interesting operation of composition on Condorcet domains and using it they disproved a long-standing problem of Fishburn about the maximal size of connected Condorcet domains. We give an algebraic definition of this operation and investigate its properties. We give a precise formula for the cardinality of composition of two Condorcet domains and improve the Danilov, Karzanov and Koshevoy result showing that Fishburn’s alternating scheme does not always produce a largest connected Condorcet domain.

Everyone Welcome!

 

27th November 2019, 2-4pm, room 303-G014 (Science building)

To attend, please RSVP here

2pm Welcome by the Theme Leader

Steven Galbraith (UoA, Mathematics)

2:15pm – 3.45pm CMSS Members – Lightning Talks

  • Simona Fabrizi (UoA, Economics): Voting under ambiguity
  • Matthew Ryan (AUT, Economics): Mathematical psychology and the algebra of choice
  • Arkadii Slinko (UoA, Mathematics): Condorcet Domains
  • Patrick Girard (UoA, Philosophy): Facing the inevitable: avoiding, tolerating, or embracing inconsistency
  • Steffen Lippert (UoA, Economics): Transfers and climate cooperation
  • James Tremewan (UoA, Economics): An overview of my ongoing research using experimental methods in social science
  • Thomas Pfeiffer (New Zealand Institute for Advanced Studies (NZIA), Massey University): Predict to decide: Decision markets in theory, experiments and real-world implementations

CMSS Affiliated Postgraduate Students – More lightning talks

  • Michael Gordon (PhD Candidate, Massey University): Large-scale forecasting in the DARPA SCORE project
  • Wenlong (Stan) Wang (PhD Candidate, Massey University): Towards implementing a decentralized decision market application
  • Yen Ling Tan (Honours student, UoA): Consumer decision-making under uncertainty on digital platforms
3:45pm –

4pm

Discussion on future activities of the CMSS and/or Knowledge Sciences Theme

Chair: Steven Galbraith (UoA, Mathematics)

4pm Drinks and discussions

 

 

 

Speaker: Yi-Hsuan Lin (Academia Sinica, Taipei)

Paper to be presented: “Stochastic Choice and Rational Inattention”

Date, Time and Venue: Wednesday, 23 October 2019, 15:00-16:00, 260-5115 [Business School Building, Level 5]

Abstract: We consider a decision maker who first chooses an information structure, and then chooses an action after receiving a signal. The cost of information may be either material or cognitive and is unobserved. Thus, cost must be inferred from observable behavior. We assume that the choice of action is observed, but the choice of information is not. Due to the unobservability of the acquired private information, the choice of action appears random from an outside analyst’s point of view. We show that, given only stochastic choice from menus of actions, an analyst can identify the agent’s taste (risk attitude), prior belief, and information cost function. Identification of the cost function from behavior stands in contrast with the large literature on applications of the rational inattention model where the functional form of the cost function is assumed known by the analyst (Sims 2003). In addition, we discuss the behavioral implications of our model which are weaker than some key properties of random expected utility models. In particular, the property of Monotonicity (the addition of a new action cannot increase the probabilities of choice of the existing actions) is violated in our model because of the endogeneity and hence menu-dependence of private information. However, two axioms that jointly weaken Monotonicity are satisfied. Finally, we provide necessary and sufficient conditions for stochastic choice to be rationalized by our model.

Everyone Welcome!

Speaker: Arkadii Slinko (University of Auckland)

Paper to be presented: “Secret Sharing and Prisonner’s Dilemma” joint with Yvo Desmedt (University of Texas at Dallas)

Date, Time and Venue: Wednesday, 21 August 2019, 15:00-16:00, 260-5115 [Business School Building, Level 5]

Abstract: The study of Rational Secret Sharing initiated by Halpern and Teague (2004) regards the reconstruction of the secret in secret sharing as a game. It was shown that participants (parties) may refuse to reveal their shares and so the reconstruction may fail. Moreover, a refusal to reveal the share may be a dominant strategy of a party. In this paper we consider secret sharing as a sub-action or subgame of a larger action/game where the secret opens a possibility of consumption of a certain common good. We claim that utilities of participants will be dependent on the nature of this common good. In particular, Halpern and Teague (2014)’s scenario corresponds to a rivalrous and excludable common good. We consider the case when this common good is non-rivalrous and non-excludable and many natural Nash equilibria. We list several applications of secret sharing to demonstrate our claim and give corresponding scenarios. In such circumstances the secret sharing scheme facilitates a power sharing agreement in the society. We also state that non-reconstruction may be beneficial for this society and give several examples.

Everyone Welcome!

Speaker: David Kelsey (University of Exeter)

Paper to be presented: “Tort Liability and Unawareness” joint with Surajeet Chakravarty (Exeter University, UK) and Joshua Teitelbaum (Georgetown University, USA)

Date, Time and Venue: Wednesday, 14 August 2019, 15:00-16:00, 260-5115 [Business School Building, Level 5]

Abstract: Unawareness is a form of bounded rationality where a person fails to conceive all feasible acts or consequences or to perceive as feasible all conceivable act-consequence links. We study the implications of unawareness for tort law, where relevant examples include the discovery of a new product or technology (new act), of a new disease or injury (new consequence), or that a product can cause an injury (new link). We argue that negligence has an important advantage over strict liability in a world with unawareness-negligence, through the stipulation of due care standards, spreads awareness about the updated probability of harm.

Everyone Welcome!

Speaker: Michael Gordon (PhD Candidate, Massey University)

Paper to be presented: “Can we predict outcomes of systematic replication studies? Decision Markets Part 2 – Real World Applications”

Date, Time and Venue: Monday, 27 May 2019, 14:00-15:00, 260-6115 [Business School Building, Level 6]

Abstract: The credibility of scientific findings is of fundamental importance to enhance future research. One potential approach of collecting information about this credibility is to elicit beliefs about the reproducibility of scientific claims among scientists. Four studies have recently used surveys and prediction markets to estimate beliefs about replication in systematic large scale replication projects, but the sample size in each study has been small. Here we pool data for the four studies (n = 104) to assess the performance of surveys and prediction markets. This dataset and subsequent analysis allow us to comment on the real world success of prediction markets in terms of eliciting and aggregating information.

Everyone Welcome!

Speaker: Thomas Pfeiffer (NZIAS, Massey University)

Paper to be presented: “Predict to decide: An Introduction to decision markets”

Date, Time and Venue: Monday, 20 May 2019, 14:00-15:00, 260-6115 [Business School Building, Level 6]

Abstract: Prediction markets are popular tools to “crowd-source” distributed information and aggregate it into forecasts. Such forecasts can be very valuable for decision makers. Commercial companies, for instance, can benefit from accurate forecasts regarding the future demand for their products. Many decision-making problems, however, require conditional forecasts. To decide, for instance, between alternative marketing campaigns, a company needs to understand how each of the alternatives will affect sales. Finding mechanisms that properly incentivise participants to provide their information for such conditional forecasts is non-trivial, but can be done through so-called decision markets. I will kick off a mini-series on decision markets by describing how they work and giving an overview of our research activities on this topic.

Everyone Welcome!