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Publikationen des Lehrstuhls


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2019

Network Formation and Disruption - An Experiment: Are equilibrium networks too complex?

A.E. Endres, S. Recker, B. Mir Djawadi, B. Hoyer, Journal of Economic Behavior and Organization (2019), 157, pp. 708-734

Models on network formation have often been extended to include the potential of network disruption in recent years. Whereas the theoretical research on network formation under the threat of disruption has thus gained prominence, hardly any experimental research exists so far. In this paper, we therefore experimentally study the emergence of networks including the aspect of a known external threat by relating theoretical predictions by Dzuibiński and Goyal (2013) to actual observed behaviour. We deal with the question if subjects in the role of a strategic Designer are able to form safe networks for least costs while facing a strategic Adversary who is going to attack their networks. Varying the costs for protecting nodes, we designed and tested two treatments with different predictions for the equilibrium network and investigated whether one of the least cost equilibrium networks was more likely to be reached. Furthermore, the influence of the subjects’ farsightedness on their decision-making process was elicited and analysed. We find that while subjects are able to build safe networks in both treatments, equilibrium networks are only built in one of the two treatments. In the other treatment, predominantly safe networks are built but they are not for least costs. Additionally, we find that farsightedness –as measured in our experiment– has no influence on whether subjects are able to build safe or least cost equilibrium networks. Two robustness settings with a reduced external threat or more liberties to modify the initial networks qualitatively confirm our results. Overall, in this experiment observed behaviour is only partially in line with the theoretical predictions by Dzuibiński and Goyal (2013).


The Common Enemy Effect under Strategic Network Formation and Disruption

B. Hoyer, H. Haller, Journal of Economic Behavior & Organization (2019), 162, pp. 146-163

Social psychology studies the "common enemy effect", the phenomenon that members of a group work together when they face an opponent, although they otherwise have little in common. An interesting scenario is the formation of an information network where group members individually sponsor costly links. Suppose that ceteris paribus, an outsider appears who aims to disrupt the information flow within the network by deleting some of the links. The question is how the group responds to this common enemy. We address this question for the homogeneous connections model of strategic network formation, with two-way flow of information and without information decay. For sufficiently low linkage costs, the external threat can lead to a more connected network, a positive common enemy effect. For very high but not prohibitively high linkage costs, the equilibrium network can be minimally connected and efficient in the absence of the external threat whereas it is always empty and ineffi cient in the presence of the external threat, a negative common enemy effect. For intermediate linkage costs, both connected networks and the empty network are Nash for certain cost ranges.


Evolutionary equilibrium in contests with stochastic participation: Entry, effort and overdissipation

Y. Gu, B. Hehenkamp, W. Leininger, Journal of Economic Behavior & Organization (2019), pp. 469-485

This paper examines the evolutionary stability of behaviour in contests where players’ participation can be stochastic. We find, for exogenously given participation probabilities, players exert more effort under the concept of a finite-population evolutionarily stable strategy (FPESS) than under Nash equilibrium (NE). We show that there is ex-ante overdissipation under FPESS for sufficiently large participation probabilities, if, and only if, the impact function is convex. With costly endogenous entry, players enter the contest with a higher probability and exert more effort under FPESS than under NE. Importantly, under endogenous entry, overdissipation can occur for all (Tullock) contest success functions, in particular those with concave impact functions.


2018



Determinants of Equilibrium Selection in Network Formation - An Experiment

B. Hoyer, S. Rosenkranz, Games (2018), 9(4)


Preemptive Repression: Deterrence, Backfiring, Iron Fists and Velvet Gloves

K. De Jaegher, B. Hoyer, Journal of Conflict Resolution (2018), 63(2), pp. 502--527

We present a game-theoretic model of the repression–dissent nexus, focusing on preemptive repression. A small group of instigating dissidents triggers a protest if each dissident participates. The dissidents face random checks by security forces, and when an individual dissident is caught while preparing to participate, he or she is prevented from doing so. Each dissident can invest in countermeasures, which make checks ineffective. For large benefits of protest, higher preemptive repression in the form of a higher number of checks has a deterrence effect and makes dissidents less prone to invest in countermeasures, decreasing the probability of protest. For small benefits of protest, higher preemptive repression instead has a backfiring effect. Both myopic and farsighted governments avoid the backfiring effect by setting low levels of preemptive repression (velvet-glove strategy). However, only a farsighted government is able to exploit the deterrence effect by maintaining a high level of preemptive repression (iron-fist strategy).


Price competition and the Bertrand model: The paradox of the German mobile discount market

D. Kaimann, B. Hoyer, Applied Economics Letters (2018), 26(1), pp. 54-57

We investigate the degree of price competition among telecommunication firms. Underlying a Bertrand model of price competition, we empirically model pricing behaviour in an oligopoly. We analyse panel data of individual pricing information of mobile phone contracts offered between 2011 and 2017. We provide empirical evidence that price differences as well as reputational effects serve as a signal to buyers and significantly affect market demand. Additionally, we find that brands lead to an increase in demand and thus are able to generate spillover effects even after price increase.


2017

The effects of competition on medical service provision

J. Brosig-Koch, B. Hehenkamp, J. Kokot, Health Economics (2017), 26(53), pp. 6-20

We explore how competition between physicians affects medical service provision. Previous research has shown that, without competition, physicians deviate from patient‐optimal treatment under payment systems like capitation and fee‐for‐service. Although competition might reduce these distortions, physicians usually interact with each other repeatedly over time and only a fraction of patients switches providers at all. Both patterns might prevent competition to work in the desired direction. To analyze the behavioral effects of competition, we develop a theoretical benchmark that is then tested in a controlled laboratory experiment. Experimental conditions vary physician payment and patient characteristics. Real patients benefit from provision decisions made in the experiment. Our results reveal that, in line with the theoretical prediction, introducing competition can reduce overprovision and underprovision, respectively. The observed effects depend on patient characteristics and the payment system, though. Tacit collusion is observed and particularly pronounced with fee‐for‐service payment, but it appears to be less frequent than in related experimental research on price competition.


2016


Strategic Network Disruption and Defense

B. Hoyer, K. De Jaegher, Journal of Public Economic Theory (2016), 18(5), pp. 802-830

We study a game between a network designer, who uses costly links to connect nodes in a network, and a network disruptor who tries to disrupt the resulting network as much as possible by deleting either nodes or links. For low linking costs networks with all nodes in symmetric positions are a best response of the designer under both link deletion and node deletion. For high linking costs the designer builds a star network under link deletion, but for node deletion excludes some nodes from the network to build a smaller but stronger network. For intermediate linking costs the designer again builds a symmetric network under node deletion but a star‐like network with weak spots under link deletion.


By-product mutualism and the ambiguous effects of harsher environments – A game-theoretic model

K. De Jaegher, B. Hoyer, Journal of Theoretical Biology (2016), 393, pp. 82-97

We construct two-player two-strategy game-theoretic models of by-product mutualism, where our focus lies on the way in which the probability of cooperation among players is affected by the degree of adversity facing the players. In our first model, cooperation consists of the production of a public good, and adversity is linked to the degree of complementarity of the players׳ efforts in producing the public good. In our second model, cooperation consists of the defense of a public, and/or a private good with by-product benefits, and adversity is measured by the number of random attacks (e.g., by a predator) facing the players. In both of these models, our analysis confirms the existence of the so-called boomerang effect, which states that in a harsh environment, the individual player has few incentives to unilaterally defect in a situation of joint cooperation. Focusing on such an effect in isolation leads to the "common-enemy" hypothesis that a larger degree of adversity increases the probability of cooperation. Yet, we also find that a sucker effect may simultaneously exist, which says that in a harsh environment, the individual player has few incentives to unilaterally cooperate in a situation of joint defection. Looked at in isolation, the sucker effect leads to the competing hypothesis that a larger degree of adversity decreases the probability of cooperation. Our analysis predicts circumstances in which the "common enemy" hypothesis prevails, and circumstances in which the competing hypothesis prevails.


2014

Too Much of a Good Thing? Welfare Consequences of Market Transparency

Y. Gu, B. Hehenkamp, Journal of Institutional and Theoretical Economics JITE (2014), 170(2), pp. 225-248

This paper studies welfare consequences of consumer-side market transparency with endogenous entry of firms. Different from most studies, we consider the unique symmetric entry equilibrium, which is in mixed strategies. We identify two effects of market transparency on welfare: a competition effect and a novel market-structure effect. We show, surprisingly, that for almost all demand functions the negative market-structure effect eventually dominates the positive competition effect as the market becomes increasingly transparent. Consumer-side market transparency can therefore be socially excessive even without collusion. The only exception among commonly used demand functions is the set of constant demand functions.


Collective action and the common enemy effect

K. De Jaegher, B. Hoyer, Defence and Peace Economics (2014), 27(5), pp. 644-664

How is collective defence by players affected when they face a threat from an intelligent attacker rather than a natural threat? This paper analyses this question using a game-theoretic model. Facing an intelligent attacker has an effect if players move first and visibly set their defence strategies, thereby exposing any players who do not defend, and if the attacker is, moreover, not able to commit to a random attack. Depending on the parameters of the game, the presence of an intelligent attacker either increases the probability that players jointly defend (where such joint defence either does or does not constitute a utilitarian optimum), or decreases the probability that players jointly defend (even though joint defence is a utilitarian optimum).


2011

Paying for Performance in Hospitals

B. Hehenkamp, O. Kaarboe, Economic Analysis and Policy (2011), 41(1), pp. 49-70

DOI


2010

On the equivalence of Nash and evolutionary equilibrium in finite populations

B. Hehenkamp, A. Possajennikov, T. Guse, Journal of Economic Behavior & Organization (2010), 73(2), pp. 254-258

DOI


Strategic Unemployment

J. Angerhausen, C. Bayer, B. Hehenkamp, Journal of Institutional and Theoretical Economics (JITE) (2010), 166(3), pp. 439-461


Survival at the center—the stability of minimum differentiation

B. Hehenkamp, A. Wambach, Journal of Economic Behavior & Organization (2010)(3), pp. 853--858


2008

Imitators and optimizers in a changing environment

B. Hehenkamp, O.M. Kaarbøe, Journal of Economic Dynamics and Control (2008), 32(5), pp. 1357-1380

DOI


2007

Die Grundlagen der Mechanismus-Design-Theorie

B. Hehenkamp, Wirtschaftsdienst (2007), 87(11), pp. 768-772

DOI


2006

The strategic advantage of interdependent preferences in rent-seeking contests

T. Guse, B. Hehenkamp, Public Choice (2006), 129(3-4), pp. 323-352

DOI


When Should the Talented Receive Weaker Incentives? Peer Pressure in Teams

B. Hehenkamp, O. Kaarboe, FinanzArchiv: Public Finance Analysis (2006), 62(1), pp. 124-148

DOI


2004

Evolutionary equilibrium in Tullock contests: spite and overdissipation

B. Hehenkamp, W. Leininger, A. Possajennikov, European Journal of Political Economy (2004), 20(4), pp. 1045-1057

DOI


2003

Equilibrium Selection in the Two-Population KMR Model

B. Hehenkamp, International Game Theory Review (2003), 05(03), pp. 249-262

DOI


2002

Sluggish Consumers: An Evolutionary Solution to the Bertrand Paradox

B. Hehenkamp, Games and Economic Behavior (2002), 40(1), pp. 44-76

DOI


1999

A note on evolutionary stability of Bertrand equilibrium

B. Hehenkamp, W. Leininger, Journal of Evolutionary Economics (1999), 9(3), pp. 367-371

DOI


Economic natural selection in Bertrand and Cournot settings

B. Hehenkamp, C. Qin, C. Stuart, Journal of Evolutionary Economics (1999), 9(2), pp. 211-224

DOI


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Working Paper


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2019

Bundling in a Distribution Channel with Retail Competition

J.M.J. Heinzel, CIE Working Paper Series, 2019

We analyze the incentives for retail bundling and the welfare effects of retail bundling in a decentralized distribution channel with two retailers and two monopolistic manufacturers. One manufacturer exclusively sells his good to one retailer, whereas the other manufacturer sells his good to both retailers. Thus, one retailer is a monopolist for one product but competes with the other retailer in the second product market. The two-product retailer has the option to bundle his goods or to sell them separately. We find that bundling aggravates the double marginalization problem for the bundling retailer. Nevertheless, when the retailers compete in prices, bundling can be more profitable than separate selling for the retailer as bundling softens the retail competition. The ultimate outcome depends on the manufacturers’ marginal costs. Given retail quantity competition, however, bundling is in no case the retailer’s best strategy. Furthermore, we show that profitable bundling reduces consumer and producer surplus in the equilibrium.


Credence Goods Markets with Fair and Opportunistic Experts

J.M.J. Heinzel, CIE Working Paper Series, 2019

We analyze a credence goods market adapted to a health care market with regulated prices, where physicians are heterogeneous regarding their fairness concerns. The opportunistic physicians only consider monetary incentives while the fair physicians, in addition to a monetary payoff, gain an non-monetary utility from being honest towards patients. We investigate how this heterogeneity affects the physicians’ equilibrium level of overcharging and the patients’ search for second opinions (which determines overall welfare). The impact of the heterogeneity on the fraud level is ambiguous and depends on several factors such as the size of the fairness utility, the share of fair physicians, the search level and the initial fraud level. Introducing heterogeneity does not affect the fraud or the search level when the share of fair physicians is small. However, when social welfare is not at its maximum, social welfare always increases if we introduce a sufficiently large share of fair physicians.


Credence Goods Markets with Heterogeneous Experts

J.M.J. Heinzel, CIE Working Paper Series, 2019

In this paper, we analyze a credence goods model adjusted to the health care market with regulated prices and heterogeneous experts. Experts are physicians and are assumed to differ in their cost of treating a small problem. We investigate the effects of this heterogeneity on the physicians’ level of fraud and on the patients’ search for second opinions. We find that introducing a fraction of more efficient low-cost physicians always increases social welfare, but in some cases only because of the raised physicians’ surplus. When the low-cost physicians’ cost advantage is small, imposing a share of low-cost physicians does not change the equilibrium fraud level. When the cost advantage is large, however, different changes in the fraud level occur depending on the share of generated low-cost physicians, the search rate and the initial level of fraud.


2018

The Influence of Bribery and Relative Reciprocity on a Physician's Prescription Decision - An Experiment

V. Hilleringmann, CIE Working Paper Series, 2018

Focusing on a physician's relationship to a briber and a patient, this experiment analyzes the influence of a bribe on a physician's treatment decision. We conduct a partner treatment, in which briber and physician play together for the whole experiment and a stranger treament, where briber and physician are re-matched every period. With the help of the two treatments, we vary the relative reciprocity between the physician and the two other actors, briber and patient. Additionally we use a follow up questionnaire to measure the behavioral motivation of the participants. We find that reciprocity leads to bribery relationships: In the partner treatment physicians act corruptly more often. Just the variation of the relative reciprocity between the treatments shows differences in the behavior of the subjects. Differences in the participants' preferences deliver no explanation for their behavior in our experiment.


2017

Location Choice and Quality Competition in Mixed Hospital Markets

B. Hehenkamp, O. Kaarboe, Universität Paderborn, 2017


Evolutionary Equilibrium in Stochastic Contests - Entry, Effort, and Overdissipation

Y. Gu, B. Hehenkamp, W. Leininger, Universität Paderborn, 2017


Matching Strategies of Heterogeneous Agents under Incomplete Information in a University Clearinghouse

B. Hoyer, N. Stroh-Maraun, CIE Working Paper Series, Paderborn University, 2017

In actual school choice applications the theoretical underpinnings of the Boston School Choice Mechanism (BM) (complete information and rationality of the agents) are often not given. We analyze the actual behavior of agents in such a matching mechanism, using data from the matching mechanism currently used in a clearinghouse at a faculty of Business Administration and Economics at a German university, where a variant of the BM is used, and supplement this data with data generated in a survey among students who participated in the clearinghouse. We find that under the current mechanism over 70% of students act strategically. Controlling for students' limited information, we find that they do act rationally in their decision to act strategically. While students thus seem to react to the incentives to act strategically under the BM, they do not seem to be able to use this to their own advantage. However, those students acting in line with their beliefs manage a significantly better personal outcome than those who do not. We also run simulations by using a variant of the deferred acceptance algorithm, adapted to our situation, to show that the use of a different algorithm may be to the students' advantage.


2016

Do talented women shy away from competition?

B. Hoyer, T. van Huizen, L. Keijzer, T. Rezai Khavas, S. Rosenkranz, 2016

We study the willingness to compete in a cognitive task among an entire cohort of fresh man business and economics students. Combining data from a lab-in-thefield experiment with university admissions data, we trace the gender gap in competitiveness at different levels of high school performance. Our results confirm that, on average, men choose to compete more often. The gender gap disappears, however, among students with above average high school performance. Female high school top performers are equally competitive as their male counterparts. In fact, the overall gender gap is entirely driven by the group of female high school underperformers who shied away from competition, even when they performed well in our task. Overall, our findings suggest that high school grades are more than just a signal of cognitive abilities, because they seem to influence the receivers selfperception of his or her performance in a competitive environment involved in later on in life.


2012

Network Disruption and the Common Enemy Effect

B. Hoyer, K. De Jaegher, 2012

The phenomenon that groups or people work together when they face an opponent, although they have little in common otherwise, has been termed the "common enemy effect". We study a model of network formation, where players can use links to build a network, knowing that they are facing a common enemy who can disrupt the links within the network, and whose goal it is to minimize the sum of the benefits of the network. We find that introducing a common enemy can lead to the formation of stable and efficient networks as well as fragmented networks and the empty network.


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