Frédéric Deroïan

Chercheur ,  CNRS

Économétrie, finance et méthodes mathématiques
Deroïan
Statut
Directeur de recherche
Domaine(s) de recherche
Théorie des jeux et réseaux sociaux
Thèse
2000, Aix-Marseille Université
Contact
frederic.deroian[at]univ-amu.fr
+33(0)4 13 55 25 43
Adresse

AMU - AMSE
5-9 Boulevard Maurice Bourdet, CS 50498
​13205 Marseille Cedex 1

Site
https://sites.google.com/view/fredericderoian/
Résumé We study how altruism networks affect the demand for formal insurance. Agents with CARA utilities are connected through a network of altruistic relationships. Incomes are subject to a common shock and to a large individual shock, generating heterogeneous damages. Agents can buy formal insurance to cover the common shock, up to a coverage cap. We find that ex-post altruistic transfers induce interdependence in ex-ante formal insurance decisions. We characterize the Nash equilibria of the insurance game and show that agents act as if they are trying to maximize the expected utility of a representative agent with average damages. Altruism thus tends to increase demand of low-damage agents and to decrease demand of high-damage agents. Its aggregate impact depends on the interplay between demand homogenization, the zero lower bound and the coverage cap. We find that aggregate demand is higher with altruism than without altruism at low prices and lower at high prices. Nash equilibria are constrained Pareto efficient.
Mots clés Altruism networks, Informal transfers, Formal insurance
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Résumé We consider agents organized in an undirected network of local complementarities. A principal with a fixed budget offers costly bilateral contracts in order to increase the sum of agents' effort. We study contracts rewarding effort exceeding the effort made in the absence of the principal. First, targeting a subgroup of the whole society becomes optimal under substantial contracting costs, which significantly increases the computational complexity of the principal's problem. In particular, under sufficiently low intensity of complementarities, a correspondence is established between optimal targeting and an NP-hard problem. Second, for any intensities of complementarities, the optimal unit returns offered to all targeted agents are positive for all contracting costs and in general heterogeneous, even though networks are undirected. Yet, heterogeneity never leads to negative returns, which implies that, with these linear payment schemes, coordination is never an issue for the principal.
Mots clés Networked synergies, Optimal targeting, Linear scheme
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Résumé A monopoly sells a network good to a large population of consumers. We explore how the monopoly's profit and the consumer surplus vary with the arrival of public information about the network structure. The analysis reveals that, under homogeneous preferences for the good, degree assortativity ensures that information arrival increases both profit and consumer surplus. In contrast, heterogeneous preferences for the good can create a tension between consumer surplus and profit.
Mots clés Monopoly, Network effects, Network information, Bonacich centrality, Degree assortativity, Assortative mixing
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Résumé A principal targets agents organized in a network of local complementarities, in order to increase the sum of agents' effort. We consider bilateral public contracts à la Segal (1999). The paper shows that the synergies between contracting and non-contracting agents deeply impact optimal contracts: they can lead the principal to contract with a subset of the agents, and to refrain from contracting with central agents.
Mots clés Network, Synergies, Aggregate effort, Optimal group targeting
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Résumé We consider a network game with local complementarities. A policymaker, aiming at minimizing or maximizing aggregate effort, contracts with a single agent on the network to trade effort change against transfer. The policymaker has to find the best agent and the optimal contract to offer. Our study shows that for all utilities with linear best-responses, it only takes two statistics about the position of each agent on the network to identify the key player: the Bonacich centrality and the self-loop centrality. We also characterize key players under linear quadratic utilities for various contractual arrangements.
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Résumé We address the problem of a planner looking for the efficient network when agents play a network game with local complementarities and links are costly. We show that for general network cost functions, efficient networks belong to the class of Nested-Split Graphs. Next, we refine our results and find that, depending on the specification of the network cost function, complete networks, core-periphery networks, dominant group architectures, quasi-star and quasi-complete networks can be efficient.
Mots clés Strategic complementarity, Network games, Nested split graphs
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Résumé We consider a society in which each agent has one unit of a resource to allocate between two activities. Agents are organized in a social network, and each activity generates complementarities between neighbors. We find multiplicity of equilibrium for high intensity of interaction, and we characterize equilibria in terms of specialization and polarization. Overall, results reveal the crucial role played by network geometry. The results also suggest that the structure of the social network should be taken into account for the design of a public policy in favor of a specific activity.
Mots clés Economie quantitative
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Résumé We study network games under strategic complementarities. Agents are embedded in a fixed network. They choose a positive, continuous action and interact with their network neighbors. Interactions are positive and actions are bounded from above. We first derive new sufficient conditions for uniqueness, covering all concave as well as some non-concave best responses. We then study the relationship between position and action and identify situations where a more central agent always plays a higher action in equilibrium. We finally analyze comparative statics. We show that a shock may not propagate throughout the entire network and uncover a general pattern of decreasing interdependence.
Mots clés Uniqueness, Strategic complementarities, Network games, Interdependence, Centrality
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Résumé This paper explores the effect of moral hazard on both risk-taking and informal risk-sharing incentives. Two agents invest in their own project, each choosing a level of risk and effort, and share risk through transfers. This can correspond to farmers in developing countries, who share risk and decide individually upon the adoption of a risky technology. The paper mainly shows that the impact of moral hazard on risk crucially depends on the observability of investment risk, whereas the impact on transfers is much more utility dependent.
Mots clés Economie quantitative
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Résumé In this article we construct a network of roads connecting large Indian cities and we evaluate this network’s overall performance. We consider a model where the production efforts of connected cities are strategic complements, and we relate the equilibrium effort profile to a well known centrality measure, the Katz-Bonacich centrality. We then make use of this result to compute the level of efforts of different cities in the current network and identify which city contributes most to overall efforts, which existing road is the most influential and which new road should be constructed in priority. Our results shed light on the importance of relatively small cities on aggregate efforts. Our exercise illustrates how network details might generate unexpected effects. JEL: C72, D85
Mots clés Strategic complementarity, Network policy, Indian National Roads Network, Aggregate efforts maximization, Aggregate efforts maximiza
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Résumé We consider a model of interdependent efforts, with linear interaction and lower bound on effort. Our setting encompasses asymmetric interaction and heterogeneous agents’ characteristics. We examine the impact of a rise of cross-effects on aggregate efforts. We show that the sign of the comparative static effects is related to a condition of balancedness of the interaction. Moreover, we point out that asymmetry and heterogeneous characteristics are sources of non-monotonic variation of aggregate efforts.
Mots clés Strategic interaction, Social network, Heterogeneous char, Asymmetric interaction, Aggregate efforts
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Résumé We study the formation of a directed communication network in which agents distribute a fixed amount of resource over links. Indirect benefits transit through the path maximizing the product of link strength. In this environment, the wheel architecture is shown to be both the unique efficient and the unique Nash architecture.
Mots clés Directed Communication Network, Endogenous Link Strength, Efficiency, Nash Stability
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Résumé Firms raise cost-reducing alliances before competing with each other, but cannot fully appropriate the shared knowledge. When spillovers disseminate through the network of alliances, link formation enables firms to capture more spillovers, but by doing so they become intermediary in the spreading of spillovers to other firms. This leads to the emergence of asymmetric networks.
Mots clés Oligopoly, R&amp, D alliances, Spillovers&#039, dissemination, Network stability
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Résumé We consider a Cournot oligopoly setting in which consumers have an intrinsic preference for variety, while unit production costs of firms increase with the number of goods they produce. This environment exhibits a general under-provision of variety with respect to social welfare.
Mots clés Standardization, Preference for Variety, Oligopoly
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Résumé We study rival firms' incentives in quality-improving Research and Development (R&D) networks. The analysis stresses the role of free riding associated to collaboration and three major consequences emerge: R&D efforts decrease with the number of partners, networks of alliances are over-connected as compared to the social optimum and the profitmaximizing number of alliances is possibly non monotonic (decreasing then increasing) with respect to inverse measure of product differentiation.
Mots clés Vertically and horizontally Differentiated Oligopoly, Product innovation, R&amp, D Alliance
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Mots clés Communication network, Network formation, Perfect foresight
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Mots clés Differentiated Oligopoly, Firms&#039, s Heterogeneity, Cost-Reducing Alliances, Stability
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Résumé Nous explorons les phénomènes de comportements collectifs dans un modèle déterministe formalisant une population d'agents en interaction. Nous relions les phénomènes de diversité ou d'unanimité aux conditions structurelles du réseau d'interaction. /// We explore collective behaviors in a deterministic model of interacting agents. We relate unanimity or diversity to the structural conditions of the interaction network.
Mots clés Réseaux Sociaux, Structure, Unanimité, Diversité
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Résumé Le rôle des réseaux sociaux dans la diffusion de l'innovation demeure une question stratégique. Dans des travaux antérieurs, nous avons introduit un apprentissage relationnel, de type hebbien, qui conduit à un état critique, dans lequel certains agents acquièrent des positions, purement structurelles, de leaders d'opinion. Dans cet article, nous montrons que l'auto-organisation d'un réseau d'influence, par l'effet d'un apprentissage social, ne constitue pas un phénomène monotone, aussi bien du point de vue des caractéristiques structurelles du réseau que de celui de ses performances en diffusion. Ceci nécessite, pour être analysé, de recourir à la notion d'intermédiarité qui est inhérente au concept de réseau. Une analyse relative au rôle des "liens faibles" dans les différents régimes de diffusion devrait alors permettre d'offrir un éclairage nouveau sur cette dynamique d'évolution.
Mots clés Apperntissage social, Diffusion d&#039, innovation, Intermédiarité
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Résumé I examine the formation of a specific communication network, a variant of the two-way flow model, in which agents have farsighted strategies. I show that the likelihood to form efficient networks tends to zero for sufficiently large network sizes.
Mots clés Network formation, Farsighted Strategy, Efficiency
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Résumé Une littérature récente étudie des situations économiques où des agents sont amenés à former des relations avec d'autres dans un but stratégique. Un apport essentiel de ces contributions est l'existence de dilemmes entre réseaux stables au sens de l'intérêt stratégique de chacun et réseaux efficaces, au sens de la maximisation d'un critère global quelconque (comme la somme des paiements individuels). Cet article expose les dilemmes généraux entre réseaux stratégiques stables et efficaces tels qu'ils existent dans cette littérature, ainsi que les possibilités de résolution qui ont pu suivre
Mots clés Réseaux, Stratégies, Stabilité, Efficacité
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Résumé Some innovations need delay to diffuse, others often fail. The formation of social networks is a possible explanation. Considering a population of potential adopters of a technology, we set-up a model composed of interacting agents. Interaction is conceived as influence effects and the network of interpersonal influences is learning step-by-step. The gradual formation of the social network leads, after a period of latency, to a collective evaluation of the innovation.
Mots clés Interaction, Slow Diffusion, Social network, Learning, Bifurcation
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Résumé En nous situant dans le cadre des modèles interactionnistes de diffusion de l'innovation, nous mettons en évidence le rôle joué par la notion de cumul d'influence dans un contexte de réseaux sociaux. Nous introduisons d'abord un effet de cumul dans le modèle épidémique classique se traduisant par un démarrage plus lent et une phase d'accélération plus marquée. Nous montrons ensuite que la topologie du réseau peut intervenir aussi bien dans la détermination de l'équilibre atteint que dans celle de la vitesse de diffusion. L'issue de la diffusion est ainsi marquée d'une forme d'incertitude qui confère un rôle essentiel à la structure du réseau et à la distribution des adopteurs initiaux.
Mots clés Cumul d&#039, influence, Réseaux Sociaux, Diffusion d&#039, innovation
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Mots clés Innovation, Social network, Lock-out
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Résumé The aim of the article is to relate the formation of influence networks to the coexistence of technologies in the long run. In the spirit of Plouraboue et al. (1998), we postulate that potential adopters of a technology are situated in a social network. In our model, initial relations are partly negative and all the expected utilities are revised in parallel. In the case of an exogenous network, opinions can fluctuate endlessly. When agents reallocate their relationships, this reinforces trust in agents whose opinion is close to theirs. As a result of this process, the network stabilizes in the long run, generating diversity in expected utilities.
Mots clés Social networks, Diffusion of innovations, Coexistence of technologies
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Résumé This paper introduces demotivation in the context of social comparison in networks. Social comparison is modeled as a status effect rewarding or penalizing agents according to their relative performance with respect to local peers. A demotivated agent faces both a reduced marginal return to effort and a psychological cost. In the absence of demotivation, social comparison leads to higher effort levels but reduces equilibrium welfare. Introducing demotivation leads to two main findings. First, it generates a network game of strategic substitutes. Second, despite the individual psychological costs incurred by demotivated agents, it can enhance overall welfare—by alleviating social pressure to exert effort and by generating positive externalities for peers.
Mots clés Social Comparison, Demotivation, Networks, Strategic Substitutes, Equilibrium Welfare
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Résumé This paper develops a dynamic model of addiction on networks, where individuals’ consumption is shaped by peer influence. We analyze the longrun effects of social interactions by characterizing steady-state consumption as a function of both network position and forward-looking behavior. We also examine the welfare implications of network structure and evaluate the effectiveness of various public policies aimed at reducing the demand for addictive goods. In particular, we study a key-player policy—modeled as a targeted rehabilitation program—that leverages the network’s interpersonal influences to maximize impact.
Mots clés Addiction, Peer Network, Rational addiction, Time-inconsistency
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Résumé A set of agents is aware of the existence of an economic opportunity, and compete for the associated prize. We study incentives to communicate about the existence of this economic opportunity to uninformed agents when the winner of the prize shares it with others, through some exogenous sharing rule. Communicating about the opportunity has two conflicting effects: it increases competition, but it can also increase the likelihood of receiving a large share of the prize. We find that, for any sharing rule, there is a minimum equilibrium, which Pareto dominates all other equilibria. We also find that under bilaterally symmetric sharing, more sharing generates more communication. We then discuss these results along several extensions.
Mots clés Investment, Communication, Sharing Network, Rival Opportunity
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Résumé We study how altruism networks affect the adoption of formal insurance. Agents have private CARA utilities and are embedded in a network of altruistic relationships. Incomes are subject to both a common shock and a large idiosyncratic shock. Agents can adopt formal insurance to cover the common shock. We show that ex-post altruistic transfers induce interdependence in ex-ante adoption decisions. We characterize the Nash equilibria of the insurance adoption game. We show that adoption decisions are substitutes and that the number of adopters is unique in equilibrium. The demand for formal insurance is lower with altruism than without at low prices, but higher at high prices. Remarkably, individual incentives are aligned with social welfare. We extend our analysis to CRRA utilities and to a fixed utility cost of adoption.
Mots clés Formal insurance, Informal transfers, Altruism networks
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Résumé We analyze risk-taking regulation when financial institutions are linked through shareholdings. We model regulation as an upper bound on institutions' default probability, and pin down the corresponding limits on risk-taking as a function of the shareholding network. We show that these limits depend on an original centrality measure that relies on the cross-shareholding network twice: (i) through a risk-sharing effect coming from complementarities in risk-taking and (ii) through a resource effect that creates heterogeneity among institutions. When risk is large, we find that the risk-sharing effect relies on a simple centrality measure: the ratio between Bonacich and self-loop centralities. More generally, we show that an increase in cross-shareholding increases optimal risk-taking through the risk-sharing effect, but that resource effect can be detrimental to some banks. We show how optimal risk-taking levels can be implemented through cash or capital requirements, and analyze complementary interventions through key-player analyses. We finally illustrate our model using real-world financial data and discuss extensions toward including debt-network, correlated investment portfolios and endogenous networks.
Mots clés Financial Network, Risk-Taking, Prudential Regulation
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Résumé We consider agents organized in an undirected network of local complementarities. A principal with a limited budget offers costly bilateral contracts in order to increase the sum of agents' effort. We study excess-effort linear payment schemes, i.e. contracts rewarding effort in excess to the effort made in absence of principal. The analysis provides the following main insights. First, for all contracting costs, the optimal unit returns offered to every targeted agent are positive and generically heterogeneous. This heterogeneity is due to the presence of outsiders, who create asymmetric interaction between contracting agents. Second, when contracting costs are low, it is optimal to contract with everyone and optimal unit returns are identical for all agents. Third, when contracting costs are sufficiently high, it becomes optimal to target a subset of agents, and optimal targeting can lead to NP-hard problems. In particular, when the intensity of complementarities is sufficiently low, a correspondence is established between optimal targeting and the densest k subgraph problem. Overall, the optimal targeting problem involves a trade-off between centrality and budget spending-central agents are influential, but are also more budget-consuming. These considerations can lead the principal to not target central agents.
Mots clés Networked synergies, Aggregate effort, Optimal group targeting, Linear contract
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Résumé A principal targets agents organized in a network of local complementarities, in order to increase the sum of agents' effort. We consider bilateral public contracts à la Segal (1999). The paper shows that the synergies between contracting and non-contracting agents deeply impact optimal contracts: they can lead the principal to contract with a subset of the agents, and to refrain from contracting with central agents.
Mots clés Multi-agent contracting, Network, Synergies, Aggregate effort, Optimal group targeting
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Résumé We consider a network game with local complementarities. A policymaker, aiming at minimizing or maximizing aggregate effort, contracts with a single agent on the network to trade effort change against transfer. The policymaker has to find the best agent and the optimal contract to offer. Our study shows that for all utilities with linear best-responses, it only takes two statistics about the position of each agent on the network to identify the key player: the Bonacich centrality and a weighted measure of the number of closed walks originating from the agent. We also characterize key players under linear quadratic utilities for various contractual arrangements.
Mots clés Key player, Network, Linear interaction, Incentives, Contract, Limited budget
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Résumé We study the value of network information in a context of monopoly pricing in the presence of local network externalities. We compare a setting in which all players, i.e. the monopoly and consumers, know the network structure and consumers' private preferences with a setting in which players only know the joint distribution of preferences, in-degrees and out-degrees. We give conditions under which network information increases profit or/and consumer surplus. The analysis reveals the crucial role played by four properties: degree assortativity, homophily (in preferences), preference-degree assortativity and preference-Bonacich centrality assortativity.
Mots clés Price discrimination, Bonacich centrality, Network information, Degree assortativity, Homophily, Preference-degree assortativity, Preference-Bonacich centrality assortativity, Network effects, Monopoly
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Résumé A principal offers bilateral contracts to a set of agents organized in a network conveying synergies, in a context where agents' efforts are observable and where the principal's objective increases with the sum of efforts. We characterize optimal contracts as a function of agents' positions on the network. The analysis shows that contract enforceability is key to understand optimality. We also examine linear contracting and we analyze the situation where the principal is constrained to contract with a single agent on the network. Last, we extend this setting to network entry.
Mots clés Network, Strategic complementarity, Enforceability, Optimal contracting, Multi-agency
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Résumé We consider agents playing a linear network game with strategic complementarities. We analyse the problem of a policy maker who can change the structure of the network in order to increase the aggregate efforts of the individuals and/or the sum of their utilities, given that the number of links of the network has to remain fixed. We identify some link reallocations that guarantee an improvement of aggregate efforts and/or aggregate utilities. With this comparative statics exercise, we then prove that the networks maximising both aggregate outcomes (efforts and utilities) belong to the class of Nested-Split Graphs.
Mots clés Network, Linear interaction, Bonacich Centralities, Strategic complementarity, Nested split graphs
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Résumé We study network games with linear best-replies and strategic complementarities. We assume that actions are continuous but bounded from above. We show that there is always a unique equilibrium. We find that two key features of these games under small network effects may not hold when network effects are large. Action may not be aligned with network centrality and the interdependence between agents' actions may be broken.
Mots clés Network games, Strategic complementarities, Supermodular Games, Bonacich centrality
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