Successful Neighbors Research Articles

Imitating the Most Successful Neighbor in Social Networks

We study a model of observational learning in a set of agents who are connected through a social network. The agents face identical decision problems under uncertainty, where they are not aware of the relative profitability of their alternative choices. They choose repeatedly from a common set of actions with uncertain payoffs and observe the actions chosen by their neighbors, as well as the payoffs that they received. In each period, they update their choice myopically, imitating the choice of their most successful neighbor in the preceding period. We show that in finite networks, regardless of the network structure, the population converges to a monomorphic steady state, i.e. one at which every agent chooses the same action, and it can never be predicted which this state is going to be. Moreover, in arbitrarily large networks with bounded neighborhoods, an action is diffused to the whole population if it is the only one chosen initially by a non-negligible share of the population. If there exists more than one such action, we provide an additional sufficient condition in the payoff structure, which ensures convergence to a monomorphic steady state for any network. Furthermore, we show that without the assumption of bounded neighborhoods, (i) an action can survive even if only a single agent chooses it initially, and (ii) a network can be in steady state without this being monomorphic.

Coevolutionary Dynamics of Cultural Markers, Parochial Cooperation, and Networks

The emergence and institutionalization of cooperation in sizable groups without reciprocity receives considerable attention in game-theoretical modeling. Agents in our study play the Prisoner’s Dilemma game cooperating with tolerably similar neighbors. They may imitate cultural markers (tags) and tolerance from more successful neighbors. Alternatively, they break ties to out-group neighbors. New partners are selected either from neighbors’ neighbors (clustering) or randomly. Variations in network plasticity (the likelihood of changing partners rather than being influenced) and clustering are explored. With high plasticity and high clustering, networks tend to fragment. With low plasticity and low clustering, networks tend toward global cooperation, but with severe losses of cultural diversity and tolerance. Cooperation in such regimes also proves to be vulnerable to defection. Between, there is a space displaying relatively stable and widespread cooperation with diversity and tolerance. We note some important structural characteristics of the networks evolving in this space.

Altruism and Local Interaction

This paper studies altruistic behavior in a model of local interaction. Individuals live on a circle and choose whether to altruistically provide a local public good. Choices are based on imitation of successful neighbors and experimentation. The public good is assumed to be less local than imitation. In the absence of experimentation altruism can prevail and coexist with egoism as long as the public good is non-global. With experimentation altruism persists in the long run only if the population is large relative to the reach of the public good.

Prospectors combine social and environmental information to improve habitat selection and breeding success in the subsequent year

1. Because habitats have profound effects on individual fitness, there is strong selection for improving the choice of breeding habitat. One possible mechanism is for individuals to use public information when prospecting future breeding sites; however, to our knowledge, no study has shown prospecting behaviour to be directly linked to subsequent choice of breeding site and future reproductive success. 2. We collected long-term data on territory-specific prospecting behaviour and subsequent breeding in the short-lived northern wheatear (Oenanthe oenanthe). Non-breeders established prospecting territories (<2 ha) that overlapped the breeding territories of conspecifics. We tested whether: (i) prospectors used social and environmental cues that predicted territory-specific breeding success in the following year, and (ii) the prospecting territory was tightly linked to the subsequent breeding territory of the prospector, and whether this link would be weakened by intraspecific competition with original territory owners if they also survived. 3. As expected, prospectors were attracted to a combination of site-specific cues that predicted future breeding success, i.e. short ground vegetation, a successfully breeding focal pair and successful close neighbours. 4. Prospecting behaviour was directly linked to the choice of the following year's breeding territory: 79% of surviving prospectors established a breeding territory at their prospecting site in the following year, with their breeding success being higher than other individuals of the same age. As predicted, fidelity to the prospected site was strongly dependent on whether the original territory owner of the same sex had died or moved. 5. Our findings suggest that the use of multiple cues reduces the negative impact of stochasticity on the reliability of social cues at small spatial scales (e.g. territories) and hence increases the probability of breeding success in the next year. Also, the use of conspecific attraction (i.e. the preference for breeding aggregations) is selectively advantageous because individuals are more likely to find a vacancy in an aggregation as compared to a solitary site. By extension, we hypothesize that species life-history traits may influence the spatial scale of prospecting behaviour and habitat selection strategies.

Effects of asynchronism on evolutionary games

We analyze the influence of the update dynamics on symmetric 2-player evolutionary games, which are among the most used tools to study the emergence of cooperation in populations of interacting agents. A synchronous dynamics means that, at each time step, all the agents of the population update their strategies simultaneously. An extreme case of asynchronism is sequential dynamics, in which only one agent is updated each time. We first show that these two opposite update dynamics can lead to very different outcomes and that sequential dynamics is detrimental to the emergence of cooperation only when the probability of imitating the most successful neighbors is high. In this sense, we can say that, when the update dynamics has some influence, in general asynchronism is beneficial to the emergence of cooperation. We then explore the consequences of using intermediate levels of asynchronism, where only a fraction of the agents update their behavior each time. In general, the level of cooperation changes smoothly and monotonically as we gradually go from synchronous to sequential dynamics. However, there are some exceptions that should be taken into account. In addition, the results show that the possibility of agents taking irrational decisions has a key role in the sensitivity of these models to changes in the update dynamics. Explanations for the observed behaviors are advanced.

Memory-based prisoner’s dilemma on square lattices

We investigate the evolutionary prisoner’s dilemma with memory-based agents on a square lattice. By introducing memory effects into this game, we assume that individuals’ performance is evaluated in terms of the accumulative payoffs in their memories. It is shown that if individuals behave as their successful neighbors, then cooperation can be significantly promoted. The mechanism responsible for the promotion of cooperation is discussed in detail. We confirm that the promotion of cooperation induced by memory effects remains effective when a preferential selection rule or an asynchronous updating rule is employed. Our work may shed some new light on the study of evolutionary games in real-world situations where the effects of individuals’ memories play a key role in the evolution of cooperation.

A Tag-Based Evolutionary Prisoner's Dilemma Game on Networks with Different Topologies

Researchers from many disciplines have been interested in the maintenance of cooperation in animal and human societies using the Prisoner's Dilemma game. Recent studies highlight the roles of cognitively simple agents in the evolution of cooperation who read tags to interact either discriminately or selectively with tolerably similar partners. In our study on a one-shot Prisoner's Dilemma game, artificial agents with tags and tolerance perceive dissimilarities to local neighbors to cooperate with in-group and otherwise defect. They imitate tags and learn tolerance from more successful neighbors. In terms of efficiency, society-wide cooperation can evolve even when the benefits of cooperation are relatively low. Meanwhile, tolerance however decreases as agents become homogenized. In terms of stability, parochial cooperators are gullible to the deviants - defectors displaying tolerably similar tags. We find that as the benefits of cooperation increase and the dimensions of tag space become larger, emergent societies can be more tolerant towards heterogeneous others. We also identify the effects of clustering and small-world-ness on the dynamics of tag-based parochial cooperation in spite of its fundamental vulnerability to those deviants regardless of network topology. We discuss the issue of tag mutability in search for alternative societies in which tag-based parochial cooperation is not only efficient but also robust.

Gradual learning and the evolution of cooperation in the spatial Continuous Prisoner’s Dilemma

The usual mechanism for modeling learning in spatially structured evolutionary games has to date been imitation of some successful neighbor. However, it seems natural that individuals hesitate to imitate their neighbor’s acts, specially if they can imply high costs. Here we study the effect of incorporating resistance to imitation on these models. Our framework is the spatial Continuous Prisoner’s Dilemma. For this evolutionary game, it has been reported that occasional errors in the imitation process can explain the emergence of cooperation from a non-cooperative initial state. In this work, we show that this only occurs for particular regimes of low costs of cooperation. Furthermore, we display how resistance gets greater the range of scenarios where cooperative individuals can invade selfish populations. In this context, where resistance to imitation can be interpreted as a general rule of gradual learning, our results show that the less that is learnt in a single step from a successful neighbors, the larger the degree of global cooperation finally attained. In general, the effect of step-by-step learning can be more efficient for the evolution of cooperation than a full blast one.

Scale‐sensitive landscape complementation determines habitat suitability for a territorial generalist

Evaluating habitat suitability is often complex, as habitat effects may be scale‐dependent, critical resources may be spatially segregated, and resource availability may also depend on intra‐ and inter‐specific interactions. Using analyses that spanned multiple years and spatial scales, we investigated habitat requirements of a territorial generalist, the common raven Corvus corax, in a relatively pristine woodland, Białowieża Forest (E Poland). We compiled data from multiple raven surveys conducted between 1985 and 2001. Ravens were regularly distributed over the entire study area but declined in density over 50% within the 16 yr interval. In the same period game and forest management significantly reduced ungulate densities and likely diminished the habitat quality with regard to food supply, especially carrion. To better understand habitat requirements of ravens we studied breeding performance in relation to three different habitat types across multiple scales: open areas, coniferous‐dominated forest, and deciduous‐dominated forest. We found a prominent dissimilarity between raven nesting and foraging habitats highlighting the importance of resource complementation for ravens. On a fine scale, large old pines were exclusively selected as nesting trees and nesting areas were generally coniferous‐dominated. However, at increasingly broader scales, coniferous habitats were negatively associated with raven reproductive success as those habitats likely provide a lower food supply. Only where the coniferous nesting areas at smaller scales were complemented with high percentages of deciduous forests and open areas at broader scales did the breeding performance increase. In addition to habitat composition, intra‐specific interactions were important determinants of reproductive performance and very successful neighbors decreased reproductive performance of a focal pair. Most of previous studies have investigated resource complementation in terms of habitat edges or proximity of complementing resource patches. Our study demonstrates that the concept of landscape complementation also applies to gradients in landscape composition and emphasizes the importance of scales and intraspecific interactions in habitat analyses.

Evolutionary prisoner's dilemma games with one-dimensional local interaction and imitation

In this paper we explore the impact of imitation rules on players' long-run behaviors in evolutionary prisoner's dilemma games. All players sit sequentially and equally spaced around a circle. Players are assumed to interact only with their neighbors, and to imitate either their successful neighbors and/or themselves or the successful actions taken by their neighbors and/or themselves. In the imitating-successful-player dynamics, full defection is the unique long-run equilibrium as the probability of players' experimentations (or mutations) tend to 0. By contrast, full cooperation could emerge in the long run under the imitating-successful-action dynamics. Moreover, it is discovered that the convergence rate to equilibrium under local interaction could be slower than that under global interaction.

Evolutionary prisoner's dilemma games with one-dimensional local interaction and imitation

In this paper we explore the impact of imitation rules on players' long-run behaviors in evolutionary prisoner's dilemma games. All players sit sequentially and equally spaced around a circle. Players are assumed to interact only with their neighbors, and to imitate either their successful neighbors and/or themselves or the successful actions taken by their neighbors and/or themselves. In the imitating-successful-player dynamics, full defection is the unique long-run equilibrium as the probability of players' experimentations (or mutations) tend to 0. By contrast, full cooperation could emerge in the long run under the imitating-successful-action dynamics. Moreover, it is discovered that the convergence rate to equilibrium under local interaction could be slower than that under global interaction.

The evolution of cooperation with memory, learning and dynamic preferential selection in spatial prisoner's dilemma game

The prisoner's dilemma is widely used to research the evolution of cooperation. Here we suppose players use non-discriminative strategies within their neighborhoods and they have different memories to remember the previous interaction histories. In each round, a player copies his most successful neighbor's strategy or most successful strategy in his neighborhood within memory history as his strategy in the coming round, and the player uses preferential selection rule to select a neighbor to learn from. By the use of MC simulations, research results have been given as following: the preferential selection rule greatly promotes the cooperation level in heterogeneity networks while it inhibits the emergence of cooperation in homogeneous regular network; and different learning rules and memory sizes strongly affect the evolution of cooperation especially in homogeneous case.

Evolution of imitation networks in Minority Game model

The Minority Game is adapted to study the “imitation dilemma”, i.e. the tradeoff between local benefit and global harm coming from imitation. The agents are placed on a substrate network and are allowed to imitate more successful neighbours. Imitation domains, which are oriented trees, are formed. We investigate size distribution of the domains and in-degree distribution within the trees. We use four types of substrate: one-dimensional chain; Erdos-Renyi graph; Barabasi-Albert scale-free graph; Barabasi-Albert 'model A' graph. The behaviour of some features of the imitation network strongly depend on the information cost epsilon, which is the percentage of gain the imitators must pay to the imitated. Generally, the system tends to form a few domains of equal size. However, positive epsilon makes the system stay in a long-lasting metastable state with complex structure. The in-degree distribution is found to follow a power law in two cases of those studied: for Erdos-Renyi substrate for any epsilon and for Barabasi-Albert scale-free substrate for large enough epsilon. A brief comparison with empirical data is provided.

Making meaning happen

What is it for a sound or gesture to have a meaning, and how does it come to have one? In this paper, a range of simulations are used to extend the tradition of theories of meaning as use. The authors work throughout with large spatialized arrays of sessile individuals in an environment of wandering food sources and predators. Individuals gain points by feeding and lose points when they are hit by a predator and are not hiding. They can also make sounds heard by immediate neighbours in the array, and can respond to sounds from immediate neighbours. No inherent meaning for these sounds is built into the simulation; under what circumstances they are sent, if any, and what the response to them is, if any, vary initially with the strategies randomized across the array. These sounds do take on a specific function for communities of individuals, however, with any of three forms of strategy change: direct imitation of strategies of successful neighbours, a localized genetic algorithm in which strategies are ‘crossed’ with those of successful neighbours, and neural net training on the behaviour of successful neighbours. Starting from an array randomized across a large number of strategies, and using any of these modes of strategy change, communities of ‘communicators’ emerge. Within these evolving communities the sounds heard from immediate neighbours, initially arbitrary across the array, come to be used for very specific communicative functions. ‘Communicators’ make a particular sound on feeding and respond to that same sound from neighbours by opening their mouths; they make a different sound when hit with a predator and respond to that sound by hiding. Robustly and persistently, even in simple computer models of communities of self-interested agents, something suggestively like signalling emerges and spreads.

Uncertainty in territory quality affects the benefits of usurpation in a Mediterranean wrasse

Individuals should defend sites when the expected benefits of the territory exceed the cost of defense. However, if territory qulaity is unpredictable or difficult to assess, the expected pattern of territorial behavior is less clear. In a Mediterranean wrasse, Symphodus ocellatus, mating success is skewed with 2% of nesting males getting more than 20% of the spawning success. Yet, variation in mating success is not explained by any known physical characteristic of males or their territories. Instead, females prefer nests with a recent history of mating success because males are less likely to desert the offspring she leaves behind. Thus, territory quality is transient and determined by interactions between the sexes. I measured the frequency of territorial takeovers and the uncertainty in mating success among days at a nest. Observations indicated that S. ocellatus males usurped their neighbor’s successful nests when males were unsuccessful and larger than their successful neighbor. Sites that achieved mating success had a significantly higher probability (0.84) of remaining sucessful between consecutive days than unsuccessful territories had of becoming successful (0.30). Unsuccessful males obtained higher and more certain fitness returns if they usurped a successful neighbor’s territory. Interactions within and between the sexes drive uncertainty in success, which influences territorial behavior in this species. Key words: territoriality, stochasticity, labridae, sexual conflict, assessment. [Behav Ecol 15:278–285 (2004)]

Reflections on AIDS, 1981–2031

Reflections on AIDS, 1981–2031

Learning to Communicate: The Emergence of Signaling in Spatialized Arrays of Neural Nets

We work with a large spatialized array of individuals in an environment of drifting food sources and predators. The behavior of each individual is generated by its simple neural net; individuals are capable of making one of two sounds and are capable of responding to sounds from their immediate neighbors by opening their mouths or hiding. An individual whose mouth is open in the presence of food is fed and gains points; an individual who fails to hide when a predator is present is hurt by losing points. Opening mouths, hiding, and making sounds each exact an energy cost. There is no direct evolutionary gain for acts of cooperation or communication per se.In such an environment we start with a spatialized array of neural nets with randomized weights. Using standard learning algorithms, our individuals train up on the behavior of successful neighbors at regular intervals. Given that simple setup, will a community of neural nets evolve a simple language for signaling the presence of food and predators? With important qualifications, the answer is yes. In a simple spatial environment, pursuing individualistic gains and using partial training on successful neighbors, randomized neural nets can learn to communicate.

Learning to Communicate: The Emergence of Signaling in Spatialized Arrays of Neural Nets

We work with a large spatialized array of individuals in an environment of drifting food sources and predators. The behavior of each individual is generated by its simple neural net; individuals arecapable of making one of two sounds and are capable of responding to sounds from their immediate neighbors by opening their mouths or hiding. An individual whose mouth is open in the presence of food is 'fed' and gains points; an individual who fails to hide when a predator is present is 'hurt' by losing points. Opening mouths, hiding, and making sounds each exact an energy cost. There is no direct evolutionary gain for acts of cooperation or 'successful communication' per se. In such an environment we start with a spatialized array of neural nets with randomized weights. Using standard learning algorithms, our individuals 'train up' on the behavior of successful neighbors at regular intervals. Given that simple setup, will a community of neural nets evolve a simple language for signaling the presence of food and predators? With important qualifications, the answer is yes.'In a simple spatial environment, pursuing individualistic gains and using partial training on successful neighbors, randomized neural nets can learn to communicate.

Social games in a social network.

We study an evolutionary version of the Prisoner's Dilemma game, played by agents placed in a small-world network. Agents are able to change their strategy, imitating that of the most successful neighbor. We observe that different topologies, ranging from regular lattices to random graphs, produce a variety of emergent behaviors. This is a contribution towards the study of social phenomena and transitions governed by the topology of the community.

Cooperation, Mimesis, and Local Interaction

In a population with a local interaction structure, where individuals interact with their neighbors and learning is by way of imitating a successful neighbor, cooperation is shown to be a stable strategy that cannot easily be eliminated from the population.