Police for peace.

Glowacki's detailed account of small-scale societies' endogenously emerging tendencies to oscillate between phases of peace and war highlights a need for understanding better the incentives governing "internal" policing for "external" peacekeeping. Here, I sketch some of these incentives and point out a resulting dilemma which Glowacki's account leaves unresolved for the time being.

Costs, needs, and integration efforts shape helping behavior toward refugees.

Recent political instabilities and conflicts around the world have drastically increased the number of people seeking refuge. The challenges associated with the large number of arriving refugees have revealed a deep divide among the citizens of host countries: one group welcomes refugees, whereas another rejects them. Our research aim is to identify factors that help us understand host citizens' (un)willingness to help refugees. We devise an economic game that captures the basic structural properties of the refugee situation. We use it to investigate both economic and psychological determinants of citizens' prosocial behavior toward refugees. In three controlled laboratory studies, we find that helping refugees becomes less likely when it is individually costly to the citizens. At the same time, helping becomes more likely with the refugees' neediness: helping increases when it prevents a loss rather than generates a gain for the refugees. Moreover, particularly citizens with higher degrees of prosocial orientation are willing to provide help at a personal cost. When refugees have to exert a minimum level of effort to be eligible for support by the citizens, these mandatory "integration efforts" further increase prosocial citizens' willingness to help. Our results underscore that economic factors play a key role in shaping individual refugee helping behavior but also show that psychological factors modulate how individuals respond to them. Moreover, our economic game is a useful complement to correlational survey measures and can be used for pretesting policy measures aimed at promoting prosocial behavior toward refugees.

A note on the endogeneity of attacker and defender roles in asymmetric conflicts.

We argue that the roles of attacker and defender in asymmetric intergroup conflict are structurally ambiguous and their perception is likely to be subjectively biased. Although this allows for endogenous selection into each role, we argue that claiming the role of the defender likely is more advantageous for conflict participants.

Human agricultural economy is, and likely always was, largely based on kinship - Why?

We question the sequence of evolutionary transitions leading to ultrasociality in humans proposed by Gowdy & Krall. Evidence indicates that families are, and likely always have been, the primary productive units in human agricultural economies, suggesting that genetic relatedness is key to understanding when the suppression of individual autonomy to the benefit of subsistence groups, that is, extended families, evolved.

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.

Modelling behaviour in intergroup conflicts: a review of microeconomic approaches.

Microeconomic modelling offers a powerful formal toolbox for analysing the complexities of real-world intergroup relations and conflicts. One important class of models scrutinizes individuals' valuations of different group memberships, attitudes towards members of different groups and preferences for resource distribution in group contexts. A second broad class uses game theoretical methods to study strategic interactions within and between groups of individuals in contest and in conflict. After a concise discussion of some essential peculiarities of microeconomic modelling, this review provides an overview of the pertinent literatures in economics, highlights instructive examples of central model types and points out several ways forward. This article is part of the theme issue 'Intergroup conflict across taxa'.

Heroic behavior: A review of the literature on high-stakes altruism in the wild.

This review compiles and contextualizes the available empirical literature on natural occurrences of high-stakes altruism among nonrelatives, i.e., behaviors often called 'heroic'. Four domains are covered: exceptional bravery and self-sacrifice in war, heroism in civilian life, undirected organ donations, and rescues of persecuted persons during mass violence and genocides. The diversity of strategies used to identify instances of heroic behavior and the multiplicity of data collection methods employed in this relatively sparse literature currently preclude a systematic comparison of results. Therefore, the aim of this overview is to provide an exhaustive point of entry to a fascinating field of research with much untapped potential for testing theories about prosocial behavior.

Are strangers just enemies you have not yet met? Group homogeneity, not intergroup relations, shapes ingroup bias in three natural groups.

Humans often favour ingroup members over others, a bias that drives discrimination and intergroup conflicts. Hostile relations between groups and homogeneity within groups may affect such ingroup bias. In an experiment with members of three natural groups in Ethiopia, we vary intergroup relations (neutral versus enmity) and exploit the natural variation in the homogeneity of groups (homogeneous versus heterogeneous) to identify their effect on in- and outgroup concerns. We find that ingroup bias largely manifests as positive concern for ingroup members combined with no concern for outgroup members. Enmity has no effect on ingroup bias, whereas ingroup concern is amplified in homogeneous groups. Group homogeneity, thus, is the primary driver of concerns for others in our study's context. Our results are relevant to understanding the consequences of exclusionary group identities. This article is part of the theme issue 'Intergroup conflict across taxa'.

Spoils division rules shape aggression between natural groups.

Violent intergroup conflicts cause widespread harm; yet, throughout human history, destructive hostilities occur time and time again1,2. Benefits that are obtainable by victorious parties include territorial expansion, deterrence and ascendency in between-group resource competition3-6. Many of these are non-excludable goods that are available to all group members, whereas participation entails substantial individual risks and costs. Thus, a collective action problem emerges, raising the question why individuals participate in such campaigns at all7-9. Distinguishing offensive and defensive intergroup aggression provides a partial answer: defensive aggression is adaptive under many circumstances10-14. However, participation in offensive aggression, such as raids or wars of conquest, still requires an explanation. Here, we focus on one condition that is hypothesized to facilitate the emergence of offensive intergroup aggression: asymmetric division of a conflict's spoils may motivate those profiting from such inequality to initiate between-group aggression, even if doing so jeopardizes their group's welfare15-17. We test this hypothesis by manipulating how benefits among victors are shared in a contest experiment among three Ethiopian societies whose relations are either peaceful or violent. Under equal sharing, between-group hostility increased contest contributions. By contrast, unequal sharing prompted offensive contribution strategies in privileged participants, whereas disadvantaged participants resorted to defensive strategies, both irrespective of group relations.

The two sides of warfare: an extended model of altruistic behavior in ancestral human intergroup conflict.

Building on and partially refining previous theoretical work, this paper presents an extended simulation model of ancestral warfare. This model (1) disentangles attack and defense, (2) tries to differentiate more strictly between selfish and altruistic efforts during war, (3) incorporates risk aversion and deterrence, and (4) pays special attention to the role of brutality. Modeling refinements and simulation results yield a differentiated picture of possible evolutionary dynamics. The main observations are: (a) Altruism in this model is more likely to evolve for defenses than for attacks. (b) Risk aversion, deterrence, and the interplay of migration levels and brutality can change evolutionary dynamics substantially. (c) Unexpectedly, one occasional simulation outcome is a dynamically stable state of "tolerated intergroup theft," raising the question as to whether corresponding patterns also exist in real intergroup conflicts. Finally, possible implications for theories of the coevolution of bellicosity and altruism in humans are discussed.

Comparing forests across climates and biomes: qualitative assessments, reference forests and regional intercomparisons.

Communities, policy actors and conservationists benefit from understanding what institutions and land management regimes promote ecosystem services like carbon sequestration and biodiversity conservation. However, the definition of success depends on local conditions. Forests' potential carbon stock, biodiversity and rate of recovery following disturbance are known to vary with a broad suite of factors including temperature, precipitation, seasonality, species' traits and land use history. Methods like tracking over-time changes within forests, or comparison with "pristine" reference forests have been proposed as means to compare the structure and biodiversity of forests in the face of underlying differences. However, data from previous visits or reference forests may be unavailable or costly to obtain. Here, we introduce a new metric of locally weighted forest intercomparison to mitigate the above shortcomings. This method is applied to an international database of nearly 300 community forests and compared with previously published techniques. It is particularly suited to large databases where forests may be compared among one another. Further, it avoids problematic comparisons with old-growth forests which may not resemble the goal of forest management. In most cases, the different methods produce broadly congruent results, suggesting that researchers have the flexibility to compare forest conditions using whatever type of data is available. Forest structure and biodiversity are shown to be independently measurable axes of forest condition, although users' and foresters' estimations of seemingly unrelated attributes are highly correlated, perhaps reflecting an underlying sentiment about forest condition. These findings contribute new tools for large-scale analysis of ecosystem condition and natural resource policy assessment. Although applied here to forestry, these techniques have broader applications to classification and evaluation problems using crowdsourced or repurposed data for which baselines or external validations are not available.

Asymmetries in altruistic behavior during violent intergroup conflict.

Recent theoretical and experimental investigations of altruistic behavior in intergroup conflict in humans frequently make use of the assumption that warfare can be modeled as a symmetrical n-person prisoner's dilemma, abstracting away the strategic differences between attack and defense. In contrast, some empirical studies on intergroup conflict in hunter-gatherer societies and chimpanzees indicate that fitness relevant risks and potential benefits of attacks and defenses might have differed substantially under ancestral conditions. Drawing on these studies, it is hypothesized that the success of defenses was much more important for individual and kin survival and that a disposition to act altruistically during intergroup conflict is thus more likely to evolve for the strategic situation of defense. It is then investigated empirically if such asymmetries in the occurrence of altruistic behavior during intergroup conflict can be found. Analyzing detailed historical case data from 20th century wars, this study finds that altruistic behavior towards members of the in-group indeed seems to occur more frequently when soldiers are defending themselves and their comrades against enemy attacks. It is proposed that this asymmetry reflects adaptive behavioral responses to the materially different strategic character of attacks and defenses under ancestral conditions. If true, this would call for a refinement of theories of the evolutionary interaction of intergroup conflict and altruism.