2025 Edition
2025 Edition
Time and place: Thursday, 23 October 2025, noon-to-1pm, room 260-6115
Speaker: Michael Demetrius [PhD Candidate in Economics, UoA]
Title: TBA
Abstracts: TBA
Time and place: Thursday, 16 October 2025, noon-to-1pm, room 260-6115
Speaker: Yuanyuan (Lydia) Li [PhD Candidate in Economics, UoA]
Title: Belief-Based Behavioral Diffusion and the Limits of Incentive Design: Interpretability as a Structural Constraint" (joint with Simona Fabrizi and Steffen Lippert)
Abstracts: This paper develops a dynamic framework for behavior-based belief diffusion in networked environments, where individuals choose whether to support, reject, or ignore content according to a softmax utility based on evolving private beliefs, plat- form incentives, and alignment with institutional goals. Crucially, belief updates occur through localized social learning, but are filtered by a potentially distorted inference process—mapping observed behavior to inferred beliefs—which evolves recursively over time. We show that such endogenous filtering distortion can accumulate, leading to structural irreversibility in belief dynamics and a collapse of effective incentive reach (EIR). Leveraging tools from dynamic programming and control theory, we derive optimal incentive trajectories that internalize both strategic constraints and interpretability feedback. Our theoretical results characterize the structure of viable incentive paths and identify necessary conditions for long-run belief convergence. Simulations demonstrate that only mechanisms which maintain interpretive coherence—via structural compression, belief anchoring, and recursive repair—can sustain robust control. In contrast, exposure-only or delayed strategies result in irreversible collapse due to epistemic misalignment. Our framework establishes interpretability not as a refinement, but as a structural prerequisite for influence and robust mechanism design in decentralized systems.
Time and place: Thursday, 25 September 2025, noon-to-1pm, room 260-6115
Speaker: Davide Papapicco [PhD Candidate in Mathematics, UoA]
Title of Part II: "Hints of Collapse: Early-warnings of Impending Catastrophes”
Abstracts: In the second part of our journey through catastrophic collapses in complex systems we will address the potential existence and detection of early-warning signals (EWS). Albeit lacking a rigorous definition, EWS are usually interpreted as “simple properties that change in characteristic ways prior to a critical transitions”. A mathematical framework to derive and identify EWS has only been formalised in 2011 and since then some progress has been made. We will thus review the latest developments in the literature as well as establishing mathematical justification for EWS to exist and be detected from raw timeseries data. In conclusion we will briefly understand how some tools from statistical physics can be used to infer the probability of tipping events in simplified stochastic models.
Time and place: Thursday, 18 September 2025, noon-to-1pm, room 260-6115
Speaker: Davide Papapicco [PhD Candidate in Mathematics, UoA]
Title of Part I: “Hints of Collapse: The Theory of Tipping Points"
Abstracts: The long-term behaviour of complex systems is typically captured by the analysis of non-linear models and their statistical properties. Recent works by ecologists and climate scientists have highlighted that these asymptotic equilibria may, from time to time, be subject to extreme disruptions due to loss of stability.
In the past decade mathematicians have sought to identify and quantify this emergent phenomena using the language of dynamical systems and bifurcation theory. In the first of this two parts talk we will discover how catastrophic events that occur in nature and society can be described in the developing framework of tipping points under restrictive assumptions. We will follow this in Part 2 by introducing a novel area of research that deals with the detection of signs of early-warning that precede such catastrophes.
Time and place: Thursday, 29 May 2025, noon-to-1pm, room 260-317
Guest Speaker: Peter Gibbard [University of Otago]
Title: “Homogeneous Products and Search Costs Yield Bertrand Demand with Differentiated Products” (joint with Ronald Peeters)
Abstract: In this paper, we present a simple theoretical model of firm pricing with consumer search. A finite number of firms, who offer a homogeneous good, set prices. A continuum of consumers with heterogeneous search costs have information about the distribution of prices; on the basis of this, they search sequentially. Our first result is a simplicity result: in our baseline model, firms face demand functions analogous to those in a differentiated Bertrand model, where demand is related to the number of firms and the search cost parameter. The simplicity of our result allows the model to be used as a ‘workhorse model’, which potentially can be applied to address a range of questions arising in markets characterised by consumer search. We present one such application, using our baseline model to analyse incentives by firms to obfuscate. We also consider a range of extensions to the baseline model.
Time and place: Thursday, 3 April 2025, noon-to-1pm, room 260-6115
Speaker: Michael Demetrius [PhD Candidate in Economics, UoA]
Title: Revisiting "A Formal Theory of Democratic Deliberation" by Hun Chung and John Duggan (2020)
Abstract: I present and critique "A Formal Theory of Democratic Deliberation" by Hun Chung and John Duggan (2020), with the aim to start a discussion about extensions of the model which might improve our understanding of democratic deliberation. Some democratic theorists argue that social-choice-theoretic impossibility results justify a turn away from aggregative democracy (i.e., voting) and towards deliberative democracy. Deliberative democracy is a practice which involves reaching decisions through a process of reasoning. The authors have developed a model of democratic deliberation which considers discussions involving positions (outcomes) and arguments (corresponding to rank-orders over positions). The authors consider three modes of deliberation: myopic discussion, constructive discussion, and debate. The authors derive results pertaining to conclusiveness (does the discussion land on a position that is not revised) and path-dependence. The paper is novel in that it models deliberation as an argumentative process, as opposed to an information sharing process (as in Dietrich and Spiekermann 2024 or Ding and Pivato 2021).
Time and place: Thursday, 27 March 2025, noon-to-1pm, room 260-223 [Case Room, level 2 of Sir OGGB] - Jointly hosted by the CMSS and the Dept of Mathematics
Guest Speaker: Clemens Puppe [Karlsruhe Institute of Technology]
Title: Arrovian Independence and the Aggregation of Choice Functions (joint with Claudio Kretz)
Abstract: We reappraise the Arrow problem by studying the aggregation of choice functions. We do so in the general framework of judgment aggregation, in which choice functions are naturally representable by specifying, for each menu A and each alternative x in A, whether x is choosable from A, or not. Our framework suggests a natural strengthening of Arrow’s independence condition positing that the collective choosability of an alternative from a menu depends on the individual views on that issue, and that issue alone. Our analysis reveals that Arrovian impossibility results crucially hinge on what internal consistency requirements we impose on choice functions. While aggregation of binary choice functions, satisfying both contraction (α) and expansion (γ) consistency, is necessarily dictatorial, possibilities in the form of oligarchic rules emerge for path-independent choice functions (that is, when the expansion property γ is replaced by the Aizerman condition). When giving up (expansion) consistency conditions altogether, quota rules are possible.
Time and place: Thursday, 20 March 2025, noon-to-1pm, room 260-215 [level 2 of Sir OGGB] - Jointly hosted by the CMSS and the Dept of Economics
Guest Speaker: Rabah Amir [University of Iowa]
Title: On the Comparative Statics of Equilibrium Points
Abstract: The paper proposes a methodology for comparing pure-strategy Nash equilibrium actions in strategic games. The basic question is, when do upward shifts in reaction correspondences lead to higher Nash equilibrium actions for all players? While the methodology relies on the properties of supermodular games, the games whose equilibrium actions are to be compared need not themselves be supermodular (as in existing literature). The prototypical setting could be a family of games with parameter-dependent payoffs or just a pair of related games. The latter is crucial when one wishes to use this approach to compare Nash vs Stackelberg equilibria or Pareto optima vs Nash equilibria, of the same game. The results are illustrated by invoking several well-known economic applications.