276 research outputs found
Self-organization of punishment in structured populations
Cooperation is crucial for the remarkable evolutionary success of the human
species. Not surprisingly, some individuals are willing to bare additional
costs in order to punish defectors. Current models assume that, once set, the
fine and cost of punishment do not change over time. Here we show that relaxing
this assumption by allowing players to adapt their sanctioning efforts in
dependence on the success of cooperation can explain both, the spontaneous
emergence of punishment, as well as its ability to deter defectors and those
unwilling to punish them with globally negligible investments. By means of
phase diagrams and the analysis of emerging spatial patterns, we demonstrate
that adaptive punishment promotes public cooperation either through the
invigoration of spatial reciprocity, the prevention of the emergence of cyclic
dominance, or through the provision of competitive advantages to those that
sanction antisocial behavior. Presented results indicate that the process of
self-organization significantly elevates the effectiveness of punishment, and
they reveal new mechanisms by means of which this fascinating and widespread
social behavior could have evolved.Comment: 13 pages, 4 figures; accepted for publication in New Journal of
Physic
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
Evolutionary advantages of adaptive rewarding
Our wellbeing depends as much on our personal success, as it does on the
success of our society. The realization of this fact makes cooperation a very
much needed trait. Experiments have shown that rewards can elevate our
readiness to cooperate, but since giving a reward inevitably entails paying a
cost for it, the emergence and stability of such behavior remain elusive. Here
we show that allowing for the act of rewarding to self-organize in dependence
on the success of cooperation creates several evolutionary advantages that
instill new ways through which collaborative efforts are promoted. Ranging from
indirect territorial battle to the spontaneous emergence and destruction of
coexistence, phase diagrams and the underlying spatial patterns reveal
fascinatingly reach social dynamics that explains why this costly behavior has
evolved and persevered. Comparisons with adaptive punishment, however, uncover
an Achilles heel of adaptive rewarding that is due to over-aggression, which in
turn hinders optimal utilization of network reciprocity. This may explain why,
despite of its success, rewarding is not as firmly weaved into our societal
organization as punishment.Comment: 14 pages, 8 figures; accepted for publication in New Journal of
Physic
If players are sparse social dilemmas are too: Importance of percolation for evolution of cooperation
Spatial reciprocity is a well known tour de force of cooperation promotion. A
thorough understanding of the effects of different population densities is
therefore crucial. Here we study the evolution of cooperation in social
dilemmas on different interaction graphs with a certain fraction of vacant
nodes. We find that sparsity may favor the resolution of social dilemmas,
especially if the population density is close to the percolation threshold of
the underlying graph. Regardless of the type of the governing social dilemma as
well as particularities of the interaction graph, we show that under pairwise
imitation the percolation threshold is a universal indicator of how dense the
occupancy ought to be for cooperation to be optimally promoted. We also
demonstrate that myopic updating, due to the lack of efficient spread of
information via imitation, renders the reported mechanism dysfunctional, which
in turn further strengthens its foundations.Comment: 6 two-column pages, 5 figures; accepted for publication in Scientific
Reports [related work available at http://arxiv.org/abs/1205.0541
Quercetin prevents progression of disease in elastase/LPS-exposed mice by negatively regulating MMP expression
Abstract Background Chronic obstructive pulmonary disease (COPD) is characterized by chronic bronchitis, emphysema and irreversible airflow limitation. These changes are thought to be due to oxidative stress and an imbalance of proteases and antiproteases. Quercetin, a plant flavonoid, is a potent antioxidant and anti-inflammatory agent. We hypothesized that quercetin reduces lung inflammation and improves lung function in elastase/lipopolysaccharide (LPS)-exposed mice which show typical features of COPD, including airways inflammation, goblet cell metaplasia, and emphysema. Methods Mice treated with elastase and LPS once a week for 4 weeks were subsequently administered 0.5 mg of quercetin dihydrate or 50% propylene glycol (vehicle) by gavage for 10 days. Lungs were examined for elastance, oxidative stress, inflammation, and matrix metalloproteinase (MMP) activity. Effects of quercetin on MMP transcription and activity were examined in LPS-exposed murine macrophages. Results Quercetin-treated, elastase/LPS-exposed mice showed improved elastic recoil and decreased alveolar chord length compared to vehicle-treated controls. Quercetin-treated mice showed decreased levels of thiobarbituric acid reactive substances, a measure of lipid peroxidation caused by oxidative stress. Quercetin also reduced lung inflammation, goblet cell metaplasia, and mRNA expression of pro-inflammatory cytokines and muc5AC. Quercetin treatment decreased the expression and activity of MMP9 and MMP12 in vivo and in vitro, while increasing expression of the histone deacetylase Sirt-1 and suppressing MMP promoter H4 acetylation. Finally, co-treatment with the Sirt-1 inhibitor sirtinol blocked the effects of quercetin on the lung phenotype. Conclusions Quercetin prevents progression of emphysema in elastase/LPS-treated mice by reducing oxidative stress, lung inflammation and expression of MMP9 and MMP12.http://deepblue.lib.umich.edu/bitstream/2027.42/78260/1/1465-9921-11-131.xmlhttp://deepblue.lib.umich.edu/bitstream/2027.42/78260/2/1465-9921-11-131.pdfPeer Reviewe
Sustainable institutionalized punishment requires elimination of second-order free-riders
Although empirical and theoretical studies affirm that punishment can elevate
collaborative efforts, its emergence and stability remain elusive. By
peer-punishment the sanctioning is something an individual elects to do
depending on the strategies in its neighborhood. The consequences of
unsustainable efforts are therefore local. By pool-punishment, on the other
hand, where resources for sanctioning are committed in advance and at large,
the notion of sustainability has greater significance. In a population with
free-riders, punishers must be strong in numbers to keep the "punishment pool"
from emptying. Failure to do so renders the concept of institutionalized
sanctioning futile. We show that pool-punishment in structured populations is
sustainable, but only if second-order free-riders are sanctioned as well, and
to a such degree that they cannot prevail. A discontinuous phase transition
leads to an outbreak of sustainability when punishers subvert second-order
free-riders in the competition against defectors.Comment: 7 two-column pages, 3 figures; accepted for publication in Scientific
Report
Emergence of responsible sanctions without second order free riders, antisocial punishment or spite
While empirical evidence highlights the importance of punishment for cooperation in collective action, it remains disputed how responsible sanctions targeted predominantly at uncooperative subjects can evolve. Punishment is costly; in order to spread it typically requires local interactions, voluntary participation, or rewards. Moreover, theory and experiments indicate that some subjects abuse sanctioning opportunities by engaging in antisocial punishment (which harms cooperators), spiteful acts (harming everyone) or revenge (as a response to being punished). These arguments have led to the conclusion that punishment is maladaptive. Here, we use evolutionary game theory to show that this conclusion is premature: If interactions are non-anonymous, cooperation and punishment evolve even if initially rare, and sanctions are directed towards non-cooperators only. Thus, our willingness to punish free riders is ultimately a selfish decision rather than an altruistic act; punishment serves as a warning, showing that one is not willing to accept unfair treatments
Moment Closure - A Brief Review
Moment closure methods appear in myriad scientific disciplines in the
modelling of complex systems. The goal is to achieve a closed form of a large,
usually even infinite, set of coupled differential (or difference) equations.
Each equation describes the evolution of one "moment", a suitable
coarse-grained quantity computable from the full state space. If the system is
too large for analytical and/or numerical methods, then one aims to reduce it
by finding a moment closure relation expressing "higher-order moments" in terms
of "lower-order moments". In this brief review, we focus on highlighting how
moment closure methods occur in different contexts. We also conjecture via a
geometric explanation why it has been difficult to rigorously justify many
moment closure approximations although they work very well in practice.Comment: short survey paper (max 20 pages) for a broad audience in
mathematics, physics, chemistry and quantitative biolog
A synergy of costly punishment and commitment in cooperation dilemmas
To ensure cooperation in the Prisoner’s Dilemma, individuals may require prior commitments from others, subject to compensations when agreements to cooperate are violated. Alternatively, individuals may prefer to behave reactively, without arranging prior commitments, by simply punishing those who misbehave. These two mechanisms have been shown to promote the emergence of cooperation, yet are complementary in the way they aim to promote cooperation. Although both mechanisms have their specific limitations, either one of them can overcome the problems of the other. On one hand, costly punishment requires an excessive effect-to-cost ratio to be successful, and this ratio can be significantly reduced by arranging a prior commitment with a more limited compensation. On the other hand, commitment-proposing strategies can be suppressed by free-riding strategies that commit only when someone else is paying the cost to arrange the deal, whom in turn can be dealt with more effectively by reactive punishers. Using methods from Evolutionary Game Theory, we present here an analytical model showing that there is a wide range of settings for which the combined strategy outperforms either strategy by itself, leading to significantly higher levels of cooperation. Interestingly, the improvement is most significant when the cost of arranging commitments is sufficiently high and the penalty reaches a certain threshold, thereby overcoming the weaknesses of both mechanisms.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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