Costly punishment prevails in intergroup conflict

Understanding how societies resolve conflicts between individual and common interests remains one of the most fundamental issues across disciplines. The observation that humans readily incur costs to sanction uncooperative individuals without tangible individual benefits has attracted considerable attention as a proximate cause as to why cooperative behaviours might evolve. However, the proliferation of individually costly punishment has been difficult to explain. Several studies over the last decade employing experimental designs with isolated groups have found clear evidence that the costs of punishment often nullify the benefits of increased cooperation, rendering the strong human tendency to punish a thorny evolutionary puzzle. Here, we show that group competition enhances the effectiveness of punishment so that when groups are in direct competition, individuals belonging to a group with punishment opportunity prevail over individuals in a group without this opportunity. In addition to competitive superiority in between-group competition, punishment reduces within-group variation in success, creating circumstances that are highly favourable for the evolution of accompanying group-functional behaviours. We find that the individual willingness to engage in costly punishment increases with tightening competitive pressure between groups. Our results suggest the importance of intergroup conflict behind the emergence of costly punishment and human cooperation.     

Oxytocin modulates selection of allies in intergroup conflict

In intergroup competition and conflict, humans benefit from coalitions with strong partners who help them to protect their in-group and prevail over competing out-groups. Here, we link oxytocin, a neuropeptide produced in the hypothalamus, to ally selection in intergroup competition. In a double-blind placebo-controlled experiment, males self-administered oxytocin or placebo, and made selection decisions about six high-threat and six low-threat targets as potential allies in intergroup competition. Males given oxytocin rather than placebo viewed high-threat targets as more useful allies and more frequently selected them into their team than low-threat targets.     

Evolution and the psychology of intergroup conflict: the male warrior hypothesis

The social science literature contains numerous examples of human tribalism and parochialism-the tendency to categorize individuals on the basis of their group membership, and treat ingroup members benevolently and outgroup members malevolently. We hypothesize that this tribal inclination is an adaptive response to the threat of coalitional aggression and intergroup conflict perpetrated by 'warrior males' in both ancestral and modern human environments. Here, we describe how male coalitional aggression could have affected the social psychologies of men and women differently and present preliminary evidence from experimental social psychological studies testing various predictions from the 'male warrior' hypothesis. Finally, we discuss the theoretical implications of our research for studying intergroup relations both in humans and non-humans and discuss some practical implications.     

Duration and outcome of intergroup conflict influences intragroup affiliative behaviour

Theoreticians have long suggested that the amount of intergroup conflict in which a group is involved could influence the level of cooperation or affiliation displayed by its members. Despite the prevalence of intergroup conflicts in many social animal species, however, few empirical studies have investigated this potential link. Here, I show that intragroup allopreening rates are highest in green woodhoopoe (Phoeniculus purpureus) groups that have the greatest involvement in intergroup conflict. One reason for this relationship is a post-conflict increase in allopreening, and I demonstrate for the first time that both conflict duration and outcome influence subsequent allopreening rates: group members allopreened more following long conflicts and those they lost compared with short conflicts and those they won, perhaps because the former are more stressful. The increase in affiliative behaviour was the result of more allopreening of subordinate helpers by the dominant breeding pair, which may be because the breeders are trying to encourage helpers to participate in future conflicts; relative group size influences conflict outcome and helpers contribute more to conflicts than do the breeding pair. These results emphasize that our understanding of cooperation and group dynamics can be enhanced by investigations of how intergroup interactions affect intragroup processes.     

Indirect reciprocity in asymmetric interactions: when apparent altruism facilitates profitable exploitation

Intraspecific cooperation and interspecific mutualism often feature an asymmetry in the scope for exploitation. We investigate the evolution of indirect reciprocity in an asymmetric game, loosely modelled on interactions between cleaner fishes and clients, in which 'actors' can choose to help or to exploit a 'recipient' that approaches them, while recipients can only choose whether or not to approach an actor (based on the observation of its behaviour towards others). We show that when actors vary in state over time, in a manner that influences the potential gains from exploitation, an equilibrium is possible at which recipients avoid actors whom they have observed exploiting others in the past, and actors help when the potential gains from exploitation are low but choose to exploit when the potential gains are high. In this context, helping is favoured not because it elicits reciprocal altruism ('help so that you may be helped'), but because it facilitates profitable exploitation ('help so that you may gain the opportunity to harm'). The cost of helping one recipient is thereby recouped through exploitation of another. Indirect reciprocity is thus possible even in asymmetric interactions in which one party cannot directly 'punish' exploitation or 'reward' helping by the other.     

The evolutionary interplay of intergroup conflict and altruism in humans: a review of parochial altruism theory and prospects for its extension

Drawing on an idea proposed by Darwin, it has recently been hypothesized that violent intergroup conflict might have played a substantial role in the evolution of human cooperativeness and altruism. The central notion of this argument, dubbed ‘parochial altruism’, is that the two genetic or cultural traits, aggressiveness against the out-groups and cooperativeness towards the in-group, including self-sacrificial altruistic behaviour, might have coevolved in humans. This review assesses the explanatory power of current theories of ‘parochial altruism’. After a brief synopsis of the existing literature, two pitfalls in the interpretation of the most widely used models are discussed: potential direct benefits and high relatedness between group members implicitly induced by assumptions about conflict structure and frequency. Then, a number of simplifying assumptions made in the construction of these models are pointed out which currently limit their explanatory power. Next, relevant empirical evidence from several disciplines which could guide future theoretical extensions is reviewed. Finally, selected alternative accounts of evolutionary links between intergroup conflict and intragroup cooperation are briefly discussed which could be integrated with parochial altruism in the future.     

Oscillations in the evolution of reciprocity

A game-theoretical analysis of the Iterated Prisoner's Dilemma shows that the evolution of ensembles of stochastic strategies displays a dynamics of high complexity and unpredictability.     

Evolution of indirect reciprocity in groups of various sizes and comparison with direct reciprocity

Recently many studies have investigated the evolution of indirect reciprocity through which cooperative action is returned by a third individual, e.g. individual A helped B and then receives help from C. Most studies on indirect reciprocity have presumed that only two individuals take part in a single interaction (group), e.g. A helps B and C helps A. In this paper, we investigate the evolution of indirect reciprocity when more than two individuals take part in a single group, and compare the result with direct reciprocity through which cooperative action is directly returned by the recipient. Our analyses show the following. In the population with discriminating cooperators and unconditional defectors, whether implementation error is included or not, (i) both strategies are evolutionarily stable and the evolution of indirect reciprocity becomes more difficult as group size increases, and (ii) the condition for the evolution of indirect reciprocity under standing reputation criterion where the third individuals distinguish between justified and unjustified defections is more relaxed than that under image scoring reputation criterion in which the third individuals do not distinguish with. Furthermore, in the population that also includes unconditional cooperators, (iii) in the presence of errors in implementation, the discriminating strategy is evolutionarily stable not only under standing but also under image scoring if group size is larger than two. Finally, (iv) in the absence of errors in implementation, the condition for the evolution of direct reciprocity is equivalent to that for the evolution of indirect reciprocity under standing, and, in the presence of errors, the condition for the evolution of direct reciprocity is very close to that for the evolution of indirect reciprocity under image scoring.     

Upstream reciprocity and the evolution of gratitude

If someone is nice to you, you feel good and may be inclined to be nice to somebody else. This every day experience is borne out by experimental games: the recipients of an act of kindness are more likely to help in turn, even if the person who benefits from their generosity is somebody else. This behaviour, which has been called ‘upstream reciprocity’, appears to be a misdirected act of gratitude: you help somebody because somebody else has helped you. Does this make any sense from an evolutionary or a game theoretic perspective? In this paper, we show that upstream reciprocity alone does not lead to the evolution of cooperation, but it can evolve and increase the level of cooperation if it is linked to either direct or spatial reciprocity. We calculate the random walks of altruistic acts that are induced by upstream reciprocity. Our analysis shows that gratitude and other positive emotions, which increase the willingness to help others, can evolve in the competitive world of natural selection.     

Intragroup cohesion and intergroup hostility: the relation between grooming distributions and intergroup competition among female primates

In some primate species, females interact affinitively withmany related and unrelated females, whereas in other speciesfemales interact with only a small subset of available partners.One explanation for high rates of affinitive interactions amongfemale members of the same group is that they function to maintainthe group's cohesion in competition for resources against othergroups. Here, I attempt to determine if grooming is more "egalitarian"or diverse in groups that compete aggressively with their neighborsthan in groups in which females rarely take an active role inbetween-group competition. Three types of data are considered.The first concerns grooming and intergroup encounters in onepopulation of free-ranging vervet monkeys. The second concernsgrooming interactions in a captive population of vervets beforeand after females in adjacent cages began to respond aggressivelyto one another. The third involves a literature survey of avariety of species. When only female-bonded species are considered,there is no relation between the diversity of grooming withingroups and female participationin intergroup encounters. Therealso appears to be no clear relation between the strength ofthe female dominance hierarchy and the diversity of groomingamong females. Female-bonded groups are apparently composedof subgroups allied in a loose confederation against other groups.Female members of the same group may compete against other groupsas a cohesive unit, but their grooming relationships are oftensharply differentiated.     

Network homophily and the evolution of the pay-it-forward reciprocity

The pay-it-forward reciprocity is a type of cooperative behavior that people who have benefited from others return favors to third parties other than the benefactors, thus pushing forward a cascade of kindness. The phenomenon of the pay-it-forward reciprocity is ubiquitous, yet how it evolves to be part of human sociality has not been fully understood. We develop an evolutionary dynamics model to investigate how network homophily influences the evolution of the pay-it-forward reciprocity. Manipulating the extent to which actors carrying the same behavioral trait are linked in networks, the computer simulation model shows that strong network homophily helps consolidate the adaptive advantage of cooperation, yet introducing some heterophily to the formation of network helps advance cooperation's scale further. Our model enriches the literature of inclusive fitness theory by demonstrating the conditions under which cooperation or reciprocity can be selected for in evolution when social interaction is not confined exclusively to relatives.     

Evolution of direct and indirect reciprocity

Indirect reciprocity (IR) occurs when individuals help those who help others. It is important as a potential explanation for why people might develop cooperative reputations. However, previous models of IR are based on the assumption that individuals never meet again. Yet humans and other animals often interact repeatedly within groups, thereby violating the fundamental basis of these models. Whenever re-meeting can occur, discriminating reciprocators can decide whether to help those who helped others (IR) or those who helped them (direct reciprocity, DR). Here I used simulation models to investigate the conditions in which we can expect the different forms of reciprocity to predominate. I show that IR through image scoring becomes unstable with respect to DR by experience scoring as the probability of re-meeting increases. However, using the standing strategy, which takes into account the context of observed defections, IR can be stable with respect to DR even when individuals interact with few partners many times. The findings are important in showing that IR cannot explain a concern for reputation in typical societies unless reputations provide as reliable a guide to cooperative behaviour as does experience.     

Combining genetic variation and phenotypic plasticity in tradeoff modelling

Tradeoffs lead to antagonistic relationships between phenotypic traits and are thought to be determined both genetically and environmentally. We present here an allocation model that distinguishes between the genetic and environmental components of variation in resource allocation. In this model we introduced plasticity of resource allocation which was considered to be an adaptive response to environmental variations. The results show that resource allocation plasticity is a key parameter for the existence of environmental (i.e. inter environments and intra genotype) correlations and is therefore necessary to detect environment-induced tradeoffs. We also investigated the impact of the resource allocation plasticity and other factors on genetic (i.e. inter genotypes) correlations. Our results show that resource allocation plasticity induces a masking effect of tradeoffs when studying genetic correlations and increases the masking effect of resource variation by making apparent correlations positive when negative correlations are expected. In addition, by simulating different sources of resource acquisition variation, we demonstrated that resource variation might have different effects on correlations according to the experimental design and the studied biological material. Further development of this model may be used to investigate the theoretical implications of tradeoffs in evolutionary biology and to improve design and interpretation of experimental studies.     

Evolutionary tradeoff and equilibrium in an aquatic predator-prey system

Due to the conventional distinction between ecological (rapid) and evolutionary (slow) timescales, ecological and population models have typically ignored the effects of evolution. Yet the potential for rapid evolutionary change has been recently established and may be critical to understanding how populations persist in changing environments. In this paper we examine the relationship between ecological and evolutionary dynamics, focusing on a well-studied experimental aquatic predator-prey system (Fussmann et al., 2000, Science, 290, 1358–1360; Shertzer et al., 2002, J. Anim. Ecol., 71, 802–815; Yoshida et al., 2003, Nature, 424, 303–306). Major properties of predator-prey cycles in this system are determined by ongoing evolutionary dynamics in the prey population. Under some conditions, however, the populations tend to apparently stable steady-state densities. These are the subject of the present paper. We examine a previously developed model for the system, to determine how evolution shapes properties of the equilibria, in particular the number and identity of coexisting prey genotypes. We then apply these results to explore how evolutionary dynamics can shape the responses of the system to ‘management’: externally imposed alterations in conditions. Specifically, we compare the behavior of the system including evolutionary dynamics, with predictions that would be made if the potential for rapid evolutionary change is neglected. Finally, we posit some simple experiments to verify our prediction that evolution can have significant qualitative effects on observed population-level responses to changing conditions.     

Cancer invasion and the microenvironment: plasticity and reciprocity

Cancer invasion is a cell- and tissue-driven process for which the physical, cellular, and molecular determinants adapt and react throughout the progression of the disease. Cancer invasion is initiated and maintained by signaling pathways that control cytoskeletal dynamics in tumor cells and the turnover of cell-matrix and cell-cell junctions, followed by cell migration into the adjacent tissue. Here, we describe the cell-matrix and cell-cell adhesion, protease, and cytokine systems that underlie tissue invasion by cancer cells. We explain how the reciprocal reprogramming of both the tumor cells and the surrounding tissue structures not only guides invasion, but also generates diverse modes of dissemination. The resulting "plasticity" contributes to the generation of diverse cancer invasion routes and programs, enhanced tumor heterogeneity, and ultimately sustained metastatic dissemination.