Bodo Vogt

Incentives engineering for structured P2P systems - a feasibility demonstration using economic experiments

Structured peer-to-peer systems allow to administer large volumes of data. Several peers collaborate to generate a query result. Analyses of unstructured peer-to-peer systems, namely of those for file-sharing, show that peers tend to shirk collaboration. We anticipate similar behavior in structured peer-to-peer systems. Recently, protocols to counter uncooperative behavior in such systems have been proposed. This article examines the behavior of peers under such protocols, using game theory. A first result of this paper is a set of hypotheses, e.g.: Peers answer queries if more than a certain percentage of their queries is answered. In many situations, free-riding does not lead to a break-down of the system. Trust, reciprocity and reputation building via a feedback mechanism are behavioral motives that increase cooperation. As a second step, we have conducted economic experiments with human participants to validate our predictions. Such experiments are important because we do not need to make any assumptions regarding the behavior of peers. It turns out that the predictions remain valid in these experiments.

On the Micro-Foundations of Money: The Capitol Hill Baby-Sitting Co-Op

This paper contributes to the micro-foundation of money in centralized markets with idiosyncratic uncertainty. It shows existence of stationary monetary equilibria and ensures that there is an optimum quantity of money. The rational solution of our model is compared with actual behavior in a laboratory experiment. The experiment gives support to the theoretical approach.

Experimental Evidence for Attractions to Chance

Abstract Divide the decision-makers future into: (i) a pre-outcome period (lasting from the decision until the outcome of that decision is known), and (ii) a sequel postoutcome period (beginning when the outcome becomes known). Anticipated emotions in both periods may influence the decision, in particular, with regard to an outcome that matters to the person, the enjoyable tension from not yet knowing what this outcome will be. In the experiments presented, lottery choice can be explained by this attraction to chance, and cannot be explained by either convex von Neumann-Morgenstern utility, or by rank-dependent risk-loving weights: attraction to chance is a separate motivator.

Coverage dependence of the spin-resolved photoemission from epitaxially grown Ag layers on Pt(111)

Abstract Ag/Pt(111) has been studied by spin-, angle-, and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY for different Ag coverages. The prepared layers were characterized by AES and LEED and turned out to grow epitaxially. An independent layer thickness determination and some additional structure information were obtained from Rutherford backscattering studies in combination with channeling. The spin-resolved photoemission experiments were performed for normal incidence of circularly polarized light and normal emission of the photoelectrons. The spin information allowed a separation of spin-orbit split peaks of the Ag adsorbate layer and partly also a separation of Ag adsorbate peaks from the Pt substrate background intensity. This “method-induced improved resolution” was used to deduce information about the development of the Ag band structure in the Λ-direction from the coverage dependence of spin-resolved photoemission spectra. Dispersion of occupied bands was found to be already almost completely developed for a three-layer system. Information about the development of the band structure in its unoccupied part was obtained from the resonant behavior of peaks at about 6 eV below EF. The resonant behavior is already observed for three Ag layers and increases with layer thickness. We observe a peak in the photoemission spectra contributing to the resonances which is not predicted by existing theoretical models.

The Great Capitol Hill Baby Sitting Co-Op: Anecdote or Evidence for the Optimum Quantity of Money?

This paper studies a centralized market with idiosyncratic uncertainty and money as a medium of exchange from a theoretical as well as an experimental perspective. In our model, prices are fixed and markets are cleared by rationing. We prove the existence of stationary monetary equilibria and of an optimum quantity of money. The rational solution of our model, which is based on the assumption of individual rationality and rational expectations, is compared with actual behavior in a laboratory experiment. The theoretical results are strongly supported by this experiment.

Vasopressin increases human risky cooperative behavior

Significance Most forms of cooperative behavior take place in a mutually beneficial context where cooperation is risky as its success depends on unknown actions of others. In two pharmacological experiments, we show that intranasal administration of arginine vasopressin (AVP), a hormone that regulates mammalian social behaviors such as monogamy and aggression, increases humans’ tendency to engage in mutually beneficial cooperation. Several control tasks ruled out that AVP’s effects were driven by increased willingness to bare risks in the absence of social context, beliefs about the actions of one’s partner, or altruistic concerns. Our findings provide novel causal evidence for a biological factor underlying cooperation and are in accord with previous findings that cooperation is intrinsically rewarding for humans. The history of humankind is an epic of cooperation, which is ubiquitous across societies and increasing in scale. Much human cooperation occurs where it is risky to cooperate for mutual benefit because successful cooperation depends on a sufficient level of cooperation by others. Here we show that arginine vasopressin (AVP), a neuropeptide that mediates complex mammalian social behaviors such as pair bonding, social recognition and aggression causally increases humans’ willingness to engage in risky, mutually beneficial cooperation. In two double-blind experiments, male participants received either AVP or placebo intranasally and made decisions with financial consequences in the “Stag hunt” cooperation game. AVP increases humans’ willingness to cooperate. That increase is not due to an increase in the general willingness to bear risks or to altruistically help others. Using functional brain imaging, we show that, when subjects make the risky Stag choice, AVP down-regulates the BOLD signal in the left dorsolateral prefrontal cortex (dlPFC), a risk-integration region, and increases the left dlPFC functional connectivity with the ventral pallidum, an AVP receptor-rich region previously associated with AVP-mediated social reward processing in mammals. These findings show a previously unidentified causal role for AVP in social approach behavior in humans, as established by animal research.

Indirect partner interaction in peer-to-peer networks: stimulating cooperation by means of structure

Structured P2P systems are a prominent representative of a class of systems where participants communicate via a given structure. Peers in such systems must show cooperative behavior, to avoid performance degradation of the system. Economic literature has proposed various mechanisms to stimulate cooperation. Their effectiveness strongly depends on the interaction scenario. The two important scenarios are the partner scenario, where participants interact repeatedly, and the stranger scenario, where participants tend to interact only once. The use of shared histories is beneficial in the stranger scenario, but it is not necessary in the partner scenario. This paper is based on the observation that the systems investigated here do not match either of these scenarios. Thus, we propose indirect partner interaction as a new interaction scenario, i.e., peers interact indirectly via a sequence of peers. To study peer behavior in this new scenario without any assumptions, we have carried out economic experiments. They give way to the following results: Participants interacting on behalf of strangers show roughly the same degree of cooperative behavior as with the other mechanisms examined, like partner design or punishment. While participants tend to rely on the shared history if no other information is available, they use the network structure as basis for their strategic decisions whenever possible. The presence of a shared history does not lead to an increase of the payoff earned in such a system. We conclude that the settings investigated here do not need shared histories to stimulate cooperation.

Money and Reciprocity

Based on an experimental analysis of a simple monetary economy we argue that a monetary system is more stable than one would expect from individual rationality. We show that positive reciprocity stabilizes the monetary system, provided every participant considers the feedbacks of his choice to the stationary equilibrium. If however the participants do not play stationary strategies and some participants notoriously refuse to accept money then due to negative reciprocity their behavior will eventually induce a break down of the monetary system.

Spin-resolved photoemission from Ag (111): theory and experiment

Using circularly polarized synchrotron radiation between 14 and 24 eV, spin-resolved normal photoemission spectra have been measured from an unreconstructed Ag(111) surface. Corresponding spectra were calculated by means of a fully relativistic one-step theory of photoemission together with the bulk band structures for real and complex potentials, using two different local approximations for the exchange-correlation potential. Experiment and theory employing anX α potential agree well with regard to existence and positions of peaks. Relative peak heights match except for an observed enhancement at photon energies, at which two or more direct interband transitions may “resonate”.

Spin-resolved photoemission from epitaxial Au layers on Pt(111): Coverage dependence of the bandstructure and evidence of surface resonances

Au/Pt(111) has been studied by spin-, angle- and energy-resolved photoemission with normal incident circularly polarized synchrotron radiation of BESSY and normal photoelectron emission for different Au coverages. The prepared layers were characterized by LEED and Augerelectron spectroscopy and turned out to grow up two dimensional and epitaxially. In the photoemission experiments the development of the 3-dimensional bandstructure in the Λ-direction could be observed. For a coverage of 2.6 layers the highest occupied spin-orbit split bands are located at about 0.6 eV lower binding energy than the corresponding bands for a 3D-Au crystal and show dispersion which is, however, weaker than in a 3D-Au crystal. A 5 layer Au adsorbate was found to have already the same dispersion and energetic location as a Au(111)-crystal. For thick gold layers, which behave in photoemission like Au(111)-crystals, we find structures that cannot be due to direct transitions into a free electron like final band. The coverage dependence and spin polarization of these structures show that some of them are due to surface resonances, while the origin of one strong peak could not yet be explained conclusively. In addition we find strong hybridization and two avoided crossings in the occupied part of the bandstructure.