Spread of two linked social norms on complex interaction networks

In this paper, we study the spread of social norms, such as rules and customs that are components of human cultures. We consider the spread of two social norms, which are linked through individual behaviors. Spreading social norms depend not only on the social network structure, but also on the learning system. We consider four social network structures: (1) complete mixing, in which each individual interacts with the others at random, (2) lattice, in which each individual interacts with its neighbors with some probability and with the others at random, (3) power-law network, in which a few influential people have more social contacts than the others, and (4) random graph network, in which the number of contacts follows a Poisson distribution. Using the lattice model, we also investigate the effect of the small-world phenomenon on the dynamics of social norms. In our models, each individual learns a social norm by trial and error (individual learning) and also imitates the other's social norm (social learning). We investigate how social network structure and learning systems affect the spread of two linked social norms. Our main results are: (1) Social learning does not lead to coexistence of social norms. Individual learning produces coexistence, and the dynamics of coexistence depend on which social norms are learned individually. (2) Social norms spread fastest in the power-law network model, followed by the random graph model, the complete mixing model, the two-dimensional lattice model and the one-dimensional lattice. (3) We see a "small world effect" in the one-dimensional model, but not in two dimensions.     

Runaway selection for cooperation and strict-and-severe punishment

Punishing defectors is an important means of stabilizing cooperation. When levels of cooperation and punishment are continuous, individuals must employ suitable social standards for defining defectors and for determining punishment levels. Here we investigate the evolution of a social reaction norm, or psychological response function, for determining the punishment level meted out by individuals in dependence on the cooperation level exhibited by their neighbors in a lattice-structured population. We find that (1) cooperation and punishment can undergo runaway selection, with evolution towards enhanced cooperation and an ever more demanding punishment reaction norm mutually reinforcing each other; (2) this mechanism works best when punishment is strict, so that ambiguities in defining defectors are small; (3) when the strictness of punishment can adapt jointly with the threshold and severity of punishment, evolution favors the strict-and-severe punishment of individuals who offer slightly less than average cooperation levels; (4) strict-and-severe punishment naturally evolves and leads to much enhanced cooperation when cooperation without punishment would be weak and neither cooperation nor punishment are too costly; and (5) such evolutionary dynamics enable the bootstrapping of cooperation and punishment, through which defectors who never punish gradually and steadily evolve into cooperators who punish those they define as defectors.     

Subtle social cues, explicit incentives and cooperation in social dilemmas

Subtle contextual social cues and the explicit incentive structure of social dilemmas are two important, but fundamentally different classes of determinants of cooperative behavior. The former provides subjective social information regarding the likelihood of attaining mutual cooperation by shaping one's expectations of the cooperative behavior of the interacting party. The latter provides objective, ecological information about the strategic opportunities offered by the situation. In prior research, both classes have generally been studied in isolation, hampering insight into how social judgments and ecological opportunities interact in shaping cooperation. To fill in this gap we set up a repeated mixed-motive game in which we manipulate subtle social cues and the incentive structure of the game simultaneously. We develop the hypothesis that social information is less important in shaping mutual cooperation of two team members when the incentive structure is such that it contains natural synergies. In contrast, when the incentive structure offers no synergy, “rational contracting” is hampered and social cues are essential for mutual cooperation to develop. The evidence we present in this paper is consistent with this hypothesis.     

Networks in ecology

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Recently, ecology has shown a strong interest in network theory. The question, as with any other emerging field, is to what extent we are making real progress in understanding ecological and evolutionary processes or just telling the same stories with fancy new words. I first present a biased overview of the development of network theory, focusing on its search for common patterns across seemingly different systems. I then proceed by discussing some applications of network theory in ecology, namely, species interactions, spatial ecology, epidemiology, and evolution in social groups . Finally, I suggest important contributions of the network approach such as identifying the consequences of heterogeneity for population and community dynamics, potential pitfalls, and future directions.     

The leading eight: social norms that can maintain cooperation by indirect reciprocity

The theory of indirect reciprocation explains the evolution of cooperation among unrelated individuals, engaging in one-shot interaction. Using reputation, a player acquires information on who are worth cooperating and who are not. In a previous paper, we formalized the reputation dynamics, a rule to assign a binary reputation (good or bad) to each player when his action, his current reputation, and the opponent's reputation are given. We then examined all the possible reputation dynamics, and found that there exist only eight reputation dynamics named "leading eight" that can maintain the ESS with a high level of cooperation, even if errors are included in executing intended cooperation and in reporting the observation to the public. In this paper, we study the nature of these successful social norms. First, we characterize the role of each pivot of the reputation dynamics common to all of the leading eight. We conclude that keys to the success in indirect reciprocity are to be nice (maintenance of cooperation among themselves), retaliatory (detection of defectors, punishment, and justification of punishment), apologetic, and forgiving. Second, we prove the two basic properties of the leading eight, which give a quantitative evaluation of the ESS condition and the level of cooperation maintained at the ESS.     

Synergy and discounting of cooperation in social dilemmas

The emergence and maintenance of cooperation by natural selection is an enduring conundrum in evolutionary biology, which has been studied using a variety of game theoretical models inspired by different biological situations. The most widely studied games are the Prisoner's Dilemma, the Snowdrift game and by-product mutualism for pairwise interactions, as well as Public Goods games in larger groups of interacting individuals. Here, we present a general framework for cooperation in social dilemmas in which all the traditional scenarios can be recovered as special cases. In social dilemmas, cooperators provide a benefit to the group at some cost, while defectors exploit the group by reaping the benefits without bearing the costs of cooperation. Using the concepts of discounting and synergy for describing how benefits accumulate when more than one cooperator is present in a group of interacting individuals, we recover the four basic scenarios of evolutionary dynamics given by (i) dominating defection, (ii) coexistence of defectors and cooperators, (iii) dominating cooperation and (iv) bi-stability, in which cooperators and defectors cannot invade each other. Generically, for groups of three or more interacting individuals further, more complex, dynamics can occur. Our framework provides the first unifying approach to model cooperation in different kinds of social dilemmas.     

Population structure,spite, and the iterated Prisoner's Dilemma

Evolutionary stability (sensu Maynard Smith: Evolution and the Theory of Games, Cambridge: Cambridge University Press, 1982) of TIT FOR TAT (TFT) under the social ecology of the iterated Prisoner's Dilemma is a function of the number of pure TFT groups (dyads) in the population, relative to the social position of a focal invading defector. Defecting against TFT always raises the defector's relative intragroup fitness; when Axelrod's (Am. Polit. Sci. Rev. 75:306–318, 1981; The Evolution of Cooperation. New York: Basic Books, 1984) Evolutionary stable strategy (ESS) conditions are met, defection also lowers the absolute fitness of the defector. Here the retaliatory (punishing) character of TFT converts defection into spite, permitting pure TFT groups to sufficiently outproduce the defector for the latter's evolutionary suppression. Increasing the relative impact of spiteful defection on a population lowers the range of evolutionary stability for TFT. When individuals participate in multiple dyads, those participating in the greatest number of dyads are most likely to provide a vehicle for the successful invasion of defection. Within social networks, ESS conditions for TFT are thus individual specific. This logic is generalized to the context of an interated n-person Prisoner's Dilemma, providing a cooperative solution conceptually identical with TFT in the two-person game.     

A mechanism of dynamical interactions for two-person social dilemmas

We propose a new mechanism of interactions between game-theoretical agents in which the weights of the connections between interacting individuals are dynamical, payoff-dependent variables. Their evolution depends on the difference between the payoff of the agents from a given type of encounter and their average payoff. The mechanism is studied in the frame of two models: agents distributed on a random graph, and a mean field model. Symmetric and asymmetric connections between the agents are introduced. Long time behavior of both systems is discussed for the Prisoner's Dilemma and the Snow Drift games.     

Oxytocin and cooperation under conditions of uncertainty: The modulating role of incentives and social information

The neuropeptide Oxytocin (OT) has been implicated in many aspects of mammalian social behavior. This study investigates how OT interacts with two well-studied determinants of cooperative behavior: incentives and social information. Participants received OT or a placebo and played two economic games: a Coordination Game (with strong incentives to cooperate) and a Prisoner's Dilemma (with weak cooperative incentives). OT enhanced cooperation only when social information was present, and this effect was significantly more pronounced in the Coordination Game. When social information was lacking, OT surprisingly decreased cooperation. Consistent with the well-established role of OT in trust-building and in social cognition, social information appears to be crucial for OT to boost cooperative expectations in an interdependent social interaction that provides incentives to cooperate. When these cues are absent, OT appears to instead elicit a risk-averse strategy.     

Feeding ecology of a large social antelope in the rainforest

We studied the feeding ecology of the bongo (Tragelaphus eurycerus) in the Dzanga National Park in the rainforest of the Central African Republic to understand better the trade-off between food selectivity, ranging behaviour and social organization of a large, forest-dwelling, social antelope. Food plants and vegetation types were registered along a 311-km route travelled by bongos. Food availability was determined by identifying and counting the plants in 19 randomly chosen forest plots. Bongos showed pronounced selectivity for 26 out of 100 woody forest species. They predominantly consumed younger leaves, which suggests that high protein and low fibre content influence plant choice. In addition to leaves, bongos also ate fruits of two and flowers of one species. Furthermore, the diet was supplemented by grasses and herbs consumed on large natural licks. Such licks were regularly visited by the bongos. According to Jarman's ecological classification of antelopes, selective browsers are relatively small and live alone or in pairs to avoid competition over food. The bongo's large size and gregariousness should not allow it to survive in the rainforest as a pure selective browser. Our data suggest that the bongo relies on the opportunity to graze in bulk which it finds on the natural licks. We hypothesize that such licks either limit the distribution of bongos in other rainforest areas or allow larger group sizes than in areas without licks. Received: 8 June 1998 / Accepted: 30 November 1998     

根瘤菌生态学研究中两个数学模型的探讨

以自然重壤土和无菌砂为培养基质,栽种大豆,采用MPN法测数,证明了大豆根瘤菌数量的增长符合逻辑斯谛模型并编写了相应的BASIC程序。通过6组试验数据的计算,发现根瘤菌接种量和其占瘤率之间存在一定的数学关系,并编写了相应的BASIC程序。     

Chimpanzees and children avoid mutual defection in a social dilemma

Cooperation often comes with the temptation to defect and benefit at the cost of others. This tension between cooperation and defection is best captured in social dilemmas like the Prisoner's Dilemma. Adult humans have specific strategies to maintain cooperation during Prisoner's Dilemma interactions. Yet, little is known about the ontogenetic and phylogenetic origins of human decision-making strategies in conflict scenarios. To shed light on this question, we compared the strategies used by chimpanzees and 5-year old children to overcome a social dilemma. In our task, waiting for the partner to act first produced the best results for the subject. Alternatively, they could mutually cooperate and divide the rewards. Our findings indicate that the two species differed substantially in their strategies to solve the task. Chimpanzees became more strategic across the study period by waiting longer to act in the social dilemma. Children developed a more efficient strategy of taking turns to reciprocate their rewards. Moreover, children used specific types of communication to coordinate with their partners. These results suggest that while both species behaved strategically to overcome a conflict situation, only children engaged in active cooperation to solve a social dilemma.     

Generalized additive models in plant ecology

Abstract. Generalized additive models (GAMs) are a non-parametric extension of generalized linear models (GLMs). They are introduced here as an exploratory tool in the analysis of species distributions with respect to climate. An important result is that the long-debated question of whether a response curve, in one dimension, is actually symmetric and bell-shaped or not, can be tested using GAMs. GAMs and GLMs are discussed and are illustrated by three examples using binary data. A grey-scale plot of one of the fits is constructed to indicate which areas on a map seem climatically suitable for that species. This is useful for species introductions. Further applications are mentioned.     

The volunteer's dilemma and the optimal size of a social group

If one or few individuals are enough to perform an action that produces a collective good and if this action has a cost, living in group can be beneficial because the cost can be shared with other individuals. Without coordination, however, the production of a collective good by the contribution of one or few individuals is inefficient and can be modelled as a volunteer's dilemma. In the volunteer's dilemma the individuals that pay the cost for the production of the collective good benefit from their action if nobody else volunteers, but the cost is wasted if too many individuals volunteer. Increasing group size reduces the need of volunteering for each member of the group; the overall benefit for the group, however, decreases too because the larger the group is, the less likely it is that the collective good is produced. This problem persists even with a high degree of relatedness between group members; an optimal, intermediate group size exists that maximizes the probability to produce the collective good.     

Animal models of social anxiety disorder and their validity criteria

Anxiety disorders pose one of the largest threats to global mental health, and they predominantly emerge early in life. Social anxiety disorder, also known as social phobia, is the most common of all anxiety disorders. Moreover, it has severe consequences and is a disabling disorder that can cause an individual to be unable to perform the tasks of daily life. Social anxiety disorder is associated with the subsequent development of major depression and other mental diseases, as well as increased substance abuse. Although some neurobiological alterations have been found to be associated with social anxiety disorder, little is known about this disorder. Animal models are useful tools for the investigation of this disorder, as well as for finding new pharmacological targets for treatment. Thus, this review will highlight the main animal models of anxiety associated with social phobia.     

Spatial effects in social dilemmas

Social dilemmas and the evolutionary conundrum of cooperation are traditionally studied through various kinds of game theoretical models such as the prisoner's dilemma, public goods games, snowdrift games or by-product mutualism. All of them exemplify situations which are characterized by different degrees of conflicting interests between the individuals and the community. In groups of interacting individuals, cooperators produce a common good benefitting the entire group at some cost to themselves, whereas defectors attempt to exploit the resource by avoiding the costly contributions. Based on synergistic or discounted accumulation of cooperative benefits a unifying theoretical framework was recently introduced that encompasses all games that have traditionally been studied separately (Hauert, Michor, Nowak, Doebeli, 2005. Synergy and discounting of cooperation in social dilemmas. J. Theor. Biol., in press.). Within this framework we investigate the effects of spatial structure with limited local interactions on the evolutionary fate of cooperators and defectors. The quantitative effects of space turn out to be quite sensitive to the underlying microscopic update mechanisms but, more general, we demonstrate that in prisoner's dilemma type interactions spatial structure benefits cooperation-although the parameter range is quite limited-whereas in snowdrift type interactions spatial structure may be beneficial too, but often turns out to be detrimental to cooperation.     

Institutions,information exchange,and migrant social networks in Rome

While a considerable body of work examines immigrant networks, inadequate attention has been devoted to understanding how networks regulate the relationship between immigrants and host institutions. A rich immigrant process may reinforce current power structures by providing a convenient buffer between the elite and challengers. Conversely, immigration may challenge the status quo. I employ social network analysis to examine three understudied immigrant groups in Rome, Bangladeshis, Filipinos, and Peruvians. I find that they have developed systems of problem-solving and sense-making that often interrupt their interactions with host institutions. The state tolerates this because it situates immigrants’ concerns outside its sphere of responsibility.     

Lattice deformation and thermal stability of crystals in spider silk

The X-ray diffraction of dragline silks, produced by Nephila and Cyrtophora spiders, were measured by synchrotron radiation in their original states or in situ during stretching and heating. Nephila pilipes spiders construct a two-dimensional orb web that must be rebuilt in one or 2 days, but Cyrtophora spiders form a three-dimensional tent web that can exist for several weeks in a tropical forest. Diffraction patterns of N. pilipes and Cyrtophora draglines resemble each other. Crystals of two kinds are identified in these draglines; one is aligned parallel to the silk direction and another is less oriented. The less oriented crystal in Cyrtophora dragline is aligned better than that in N. pilipes dragline, which generates about three times stronger diffract intensity. Crystals in N. pilipes and C. moluccensis dragline silks have remarkable thermal stability. Equatorial reflections remain undiminished until 350 and 450 °C for N. pilipes and C. moluccensis, respectively. In contrast, the meridional reflections S and (0 0 2), which are parallel to the silk thread, disappear at a temperature less than 100 °C for C. moluccensis but remain for Nephila up to 100 °C. Meridional reflections S and (0 0 2) shift to a smaller angle during stretching, whereas equatorial reflections remain constant in a range 1.0–1.3 times the original length. The position of the S reflection shifts rapidly in the first 10% of elongation from the original length but remains constant during subsequent stretching, whereas the (0 0 2) reflection shifts rapidly during the first 5% elongation from the original length and continues to shift subsequently. In contrast, the features of N. pilipes dragline alter insignificantly during stretching. Examination of the composition of amino acids of the draglines of N. pilipes and C. moluccensis indicates that a dragline of N. pilipes contains more glycine, but much less alanine, than that of C. moluccensis.     

Comparison between perfect information and passive–adaptive social learning models of forest harvesting

This paper compares perfect information and passive–adaptive social learning models of forest harvesting using a simple Markov chain model for land-use dynamics. A perfect information model assumes that landowners know true utility values of forest conservation and harvesting. In contrast, a passive–adaptive social learning model assumes that landowners do not know true utility values and they learn these values by their past experiences and by exchanging information with others in a society. We determine conditions under which the same consequences expected from perfect information and passive–adaptive social learning models. We found that the outcome from a perfect information model resembles that from passive–adaptive social learning model only when the perfect information model incorporates little discounting for future values. The stability analysis of landscape dynamics predicts a cyclic overexploitation of forest resources in a passive–adaptive social learning model with short-term memory, while instability of landscapes is never expected in a perfect information model. We discuss the role of discounting the future and discounting the past in the context of forest management.