Jörgen Weibull

This note establishes that if the face spanned by a strategy block in a finite normal-form game is asymptotically stable under any evolutionary selection dynamic in a wide class, then the block is a game block as defined by Ritzberger et al. (2025). That is, the block respects Nash equilibrium components and the index sum of the components with support in the block equals +1. The result extends Theorem 1 of Demichelis and Ritzberger (2003) from equilibrium components to faces of strategy blocks.

In the winter of 2021, the Swedish Nobel Foundation organized a Nobel symposium 'One Hundred Years of Game Theory' to commemorate the publication of famous mathematician Emile Borel's 'La théorie du jeu et les équations intégrales à noyau symétrique'. The symposium gathered roughly forty of the world's most prominent scholars ranging from mathematical foundations to applications in economics, political science, computer science, biology, sociology, and other fields. One Hundred Years of Game Theory brings together their writings to summarize and put in perspective the main achievements of game theory in the last one hundred years. They address past achievements, taking stock of what has been accomplished and contemplating potential future developments and challenges. Offering cross-disciplinary discussions between eminent researchers including five Nobel laureates, one Fields medalist and two Gödel prize winners, the contributors provide a fascinating landscape of game theory and its wide range of applications.

A new solution concept for finite games is presented and analyzed. It is defined in terms of outcomes—probability distributions over the plays of the game. Solid outcomes are robust to the representation of the game, whether in normal or extensive form, and are consistent with backward induction. They are also unaffected by the removal or addition of dominated strategies. Solid outcome sets exist in all finite extensive-form games with perfect recall. They have support in minimal “game blocks,” a class of product sets of pure-strategy profiles that are robust set-valued candidates for conventions and social norms in recurrent population play of the game. Algorithms for identifying all solid outcomes are presented, and simulations illustrate the solution concept's significant cutting power and computational efficiency.

Greater knowledge is always an advantage for a rational individual. However, this article shows that for a group of rational individuals greater knowledge can backfire, leading to a worse outcome for all. Surprisingly, this can happen even when new knowledge does not mean the discovery of a new action but simply provides a deeper understanding of the interaction at stake. More specifically, enhanced knowledge about the current state of nature may hinder cooperation among purely self-interested individuals. The paper describes this paradoxical possibility—a ‘knowledge curse’—and analyses the evolutionary process that occurs if, initially, only a few people have access to the greater knowledge. It concludes with a tentative comment on ways to avert this potential knowledge backlash.

Theorem 1 in Alger and Weibull (2016, Games and Economic Behavior) consists of two statements. The first establishes that Homo moralis with the right degree of morality is evolutionarily stable. The second statement is a claim about sufficient conditions for other goal functions to be evolutionarily unstable. However, the proof given for that claim presumes that all relevant sets are non-empty, while the hypothesis of the theorem does not guarantee that. We here prove instability under a stronger hypothesis that guarantees existence, and we also establish a new and closely related result. As a by-product, we also obtain an extension of Theorem 1 in Alger and Weibull (2013, Econometrica).

We analyze equilibrium outcomes under costly screening of candidates from distinct categories. Candidates choose how much effort to invest in skill acquisition, and the screener decides how carefully to screen each candidate. Typically, there are multiple equilibria, and ex ante identical individuals in different categories can receive different equilibrium treatment. The imposition of a quota for an initially disadvantaged group can result in increased screening intensity and skill investment by destabilizing the least active equilibrium. The endogeneity of screening enables powerful comparative statics, which we use to unify different strands in the literature on discrimination.

We analyze a principal-agent model with moral hazard where the principal is risk neutral while the agent is risk averse or risk neutral. The agent is free to choose any probability distribution over outcomes, where some distributions require more effort than others. The agent's effort-cost function is of “Legendre type” and satisfies an axiom of invariance under mergers of outcomes that are equally paid by the principal. We analyze a family of such effort-cost functions. For a canonical subclass of these, and arbitrary outcome spaces, the principal's contract problem allows for closed-form solutions. Optimal contracts then combine debt with a monotonic sharing rule for the surplus above a threshold chosen by the principal. When the agent is risk neutral, the contract boils down to a pure debt contract. For agents with unit degree of relative risk aversion, the surplus is divided in fixed shares.

Nonpharmaceutical interventions (NPIs) such as mask wearing can be effective in mitigating the spread of infectious diseases. Therefore, understanding the behavioral dynamics of NPIs is critical for characterizing the dynamics of disease spread. Nevertheless, standard infection models tend to focus only on disease states, overlooking the dynamics of "beneficial contagions," e.g., compliance with NPIs. In this work, we investigate the concurrent spread of disease and mask-wearing behavior over multiplex networks. Our proposed framework captures both the competing and complementary relationships between the dueling contagion processes. Further, the model accounts for various behavioral mechanisms that influence mask wearing, such as peer pressure and fear of infection. Our results reveal that under the coupled disease-behavior dynamics, the attack rate of a disease-as a function of transition probability-exhibits a critical transition. Specifically, as the transmission probability exceeds a critical threshold, the attack rate decreases abruptly due to sustained mask-wearing responses. We empirically explore the causes of the critical transition and demonstrate the robustness of the observed phenomena. Our results highlight that without proper enforcement of NPIs, reductions in the disease transmission probability via other interventions may not be sufficient to reduce the final epidemic size

We analyze a principal-agent model with moral hazard where the principal is risk neutral and the agent is risk averse or risk neutral. The agents' actions determine the probability distribution over outcomes, where some probability distributions require more effort than others. We analyze a family of effort disutility functions with arguably desirable properties. For a canonical class of such functions and arbitrary outcome spaces, the model is explicitly solvable. Optimal contracts combine debt with a monotonic sharing rule for the surplus created. When the agent is risk neutral, the contract is a pure debt contract. For agents with unit degree of relative risk aversion, the surplus is divided in fixed shares