Traditionally, discussions of moral participation – and in particular moral agency – have focused on fully formed human actors. There has been some interest in the development of morality in humans, as well as interest in cultural differences when it comes to moral practices, commitments, and actions. However, until relatively recently, there has been little focus on the possibility that nonhuman animals have any role to play in morality, save being the objects of moral concern. Moreover, when nonhuman cases are (...) considered as evidence of moral agency or subjecthood, there has been an anthropocentric tendency to focus on those behaviors that inform our attributions of moral agency to humans. For example, some argue that the ability to evaluate the principles upon which a moral norm is grounded is required for full moral agency. Certainly, if a moral agent must understand what makes an action right or wrong, then most nonhuman animals would not qualify (and perhaps some humans too). However, if we are to understand the evolution of moral psychology and moral practice, we need to turn our attention to the foundations of full moral agency. We must first pay attention to the more broadly normative practices of other animals. Here, we begin that project by considering evidence that great apes and cetaceans participate in normative practices. (shrink)

Evolutionary debunking arguments, notably Sharon Street’s Darwinian Dilemma (2006), allege that moral realists need to explain the reliability of our moral judgments, given their evolutionary sources. David Copp (2008) and David Enoch (2010) take up the challenge. I argue on empirical grounds that realists have not met the challenge and moreover cannot do so. The outcome is that there are empirically-motivated reasons for thinking moral realists cannot explain moral reliability, given our current empirical understanding.

The evolution of altruism in human societies has been intensively investigated in social and natural sciences. A widely acknowledged recent idea is the “parochial altruism model,” which suggests that inter- group hostility and intragroup altruism can coevolve through lethal intergroup conflicts. The current article critically examines this idea by reviewing research relevant to intergroup conflicts in human evolutionary history from evolutionary biology, psychology, cultural anthropology, and archaeology. After a brief intro- duction, section 2 illustrates the mathematical model of parochial altruism (...) and some critiques of the model and its interpretation, primarily from an evolutionary biology point of view. Section 3 delves into the archaeological evidence of prehistoric intergroup conflicts in the Japanese archipelago’s Jomon and Yayoi periods, Europe’s Mesolithic period, and North America’s Pacific period as counter examples of the paro- chial altruism model. In section 4, the ethnographies of intergroup relationships and conflicts reveal that intergroup relationships in many ethnic groups are not as simple as the assumption in the mathematical model of parochial altruism. In section 5, we outline psychological research on intergroup conflicts which suggest that intergroup hostility and ingroup altruism are not necessarily correlated. In conclusion, we argue that the assumption and parameter settings of the parochial altruism model are inconsistent with empirical data. (shrink)

While some species have affiliative and even cooperative interactions between individuals of different social groups, humans are alone in having durable, positive-sum, interdependent relationships across unrelated social groups. Our capacity to have harmonious relationships that cross group boundaries is an important aspect of our species' success, allowing for the exchange of ideas, materials, and ultimately enabling cumulative cultural evolution. Knowledge about the conditions required for peaceful intergroup relationships is critical for understanding the success of our species and building a more (...) peaceful world. How do humans create harmonious relationships across group boundaries and when did this capacity emerge in the human lineage? Answering these questions involves considering the costs and benefits of intergroup cooperation and aggression, for oneself, one's group, and one's neighbor. Taking a game theoretical perspective provides new insights into the difficulties of removing the threat of war and reveals an ironic logic to peace – the factors that enable peace also facilitate the increased scale and destructiveness of conflict. In what follows, I explore the conditions required for peace, why they are so difficult to achieve, and when we expect peace to have emerged in the human lineage. I argue that intergroup cooperation was an important component of human relationships and a selective force in our species history beginning at least 300 thousand years. But the preconditions for peace only emerged in the past 100 thousand years and likely coexisted with intermittent intergroup violence which would have also been an important and selective force in our species' history. (shrink)

Violence and warfare in prehistory have been intensely discussed in various disciplines recently. Especially, some controversies are found on whether prehistoric hunter-gatherers had been already engaged in inter-group violence and warfare. Japanese archaeology has traditionally argued that warfare has begun in the Yayoi period with an introduction of full-fledged agriculture though people in the Jomon period, when subsistence system had been mainly hunting and gathering, had not been involved in inter-group violence and warfare. However, Lawrence Keeley, Samuel Bowles, Steven Pinker, (...) and others have recently focused on archaeological data of human skeletal remains, especially remains with some injures, arguing that prehistoric hunter-gatherers had already initiated inter-group violence and warfare. This paper aims to summarize and examine recent arguments that address the relationship between violence or warfare and human evolution based on archaeological data. In the first place, we summarize the claims of Keeley, Bowles, and Pinker that warfare had already been initiated and was commonplace among prehistoric hunter-gatherers, as well as the counter-argument of Ferguson that these original data are problematic. Secondly, we summarize the available data on human skeletal remains from the Mesolithic period in Europe (between 11,000 BC and 3,500 BC) and argue that these data support Ferguson’s argument. In particular, the available data suggests that even though inter-group violence and warfare was present during this period, it was very sporadic. Finally, we discuss the implications of this study and suggest future directions. (shrink)

Human societies are examined as distinct and coherent groups. This trait is most parsimoniously considered a deeply rooted part of our ancestry rather than a recent cultural invention. Our species is the only vertebrate with society memberships of significantly more than 200. We accomplish this by using society-specific labels to identify members, in what I call an anonymous society. I propose that the human brain has evolved to permit not only the close relationships described by the social brain hypothesis, but (...) also, at little mental cost, the anonymous societies within which such alliances are built. The human compulsion to discover or invent labels to “mark” group memberships may originally have been expressed in hominins as vocally learned greetings only slightly different in function from chimpanzee pant hoots (now known to be society-specific). The weight of evidence suggests that at some point, conceivably early in the hominin line, the distinct groups composed of several bands that were typical of our ancestors came to be distinguished by their members on the basis of multiple labels that were socially acquired in this way, the earliest of which would leave no trace in the archaeological record. Often overlooked as research subjects, these sizable fission-fusion communities, in recent egalitarian hunter-gatherers sometimes 2,000 strong, should consistently be accorded the status of societies, in the same sense that this word is used to describe tribes, chiefdoms, and other cultures arising later in our history. The capacity of hunter-gatherer societies to grow sufficiently populous that not all members necessarily recognize one another would make the transition to larger agricultural societies straightforward. Humans differ from chimpanzees in that societal labels are essential to the maintenance of societies and the processes giving birth to new ones. I propose that anonymous societies of all kinds can expand only so far as their labels can remain sufficiently stable. (shrink)

Based on evidence of selection against alpha-male behavior in the earliest Homo sapiens, I suggest that by 300,000 ya (years ago) language would have been sufficiently sophisticated to contribute to peacemaking between groups. Language also influenced the social landscape of peace and war, and groups' ability to form coalitions.

Göbekli Tepe is an archaeological site that has challenged much prior thought on human history with respect to our Neolithic revolution from animistic, egalitarian, hunter-gatherers to settled, socially stratified, and religious peoples. In the present paper we review the structures and possible purposes of Göbekli Tepe, summarize past considerations of the connection between psychological concepts and matters found thereat, and then introduce social identity theory as an apt theoretical perspective from which to best understand the peoples who constructed and utilized (...) the site. Throughout we show that social-cognitive processes and concepts have merit in interpreting the advent and utility of Göbekli Tepe, suggesting then a greater use for psychology within the framework of cognitive archaeology. (shrink)

This paper contributes to two debates: the debate about language evolution and the debate about the foundations of human collaboration. While both cooperation and language may give the impression of being adaptations that evolved for the “good of the group,” it is well established that the evolution of complex traits cannot be a direct result of group selection. In this paper I suggest how this tension can be solved: both language and cooperation evolved in a unique two-level evolutionary system which (...) was triggered by a well-documented geological event—the drying out of the climate—in East Africa, which subsequently reduced the intermating between groups and thus made it possible that the mechanism that produced differences between groups could be the target of natural selection on the group level. If a social form of selection produced differences in fitness between groups, the displacement process between groups would indirectly select those forms of social selection that produce groups that would displace all others. The main hypothesis presented in this paper is that, in this situation, a backchannel between the two levels of selection naturally evolves. A backchannel between the two levels would, for example, emerge when sexual selection was sensitive to the individual’s contribution to the group. Examples of systems utilizing a backchannel are nerve cells being better nourished when used more frequently, enabling them to be conducive to the survival of the whole organism, or a law firm in which all employees get paid to the extent that they contribute to the survival and success of the firm. In both cases, the selection on the higher level informs the selection on the lower level. The aim of the paper is to illuminate these rather opaque claims, to which the reader probably has many objections in this abridged form. (shrink)

There is a large body of research on conflict in nonhuman animal groups that measures the costs and benefits of intergroup conflict, and we suggest that much of this evidence is missing from De Dreu and Gross's interesting article. It is a shame this work has been missed, because it provides evidence for interesting ideas put forward in the article.

There has been a long-standing debate about the roles of San in the militaries of southern Africa and the prevalence of violence among the Ju/'hoansi and other San people. The evolutionary anthropology and social anthropological debates over the contexts in which violence and warfare occurs among hunters and gatherers are considered, as is the “tribal zone theory” of warfare between states and indigenous people. This paper assesses the issues that arise from these discussions, drawing on data from San in Angola, (...) Namibia, Botswana, and Zimbabwe. Utilizing cases of how San have been affected by military forces and wildlife conservation agencies in what became protected areas in southern Africa, this article shows that indigenous peoples have been treated differentially by state and nongovernmental organizations involved in anti-poaching, shoot-to-kill, and forced resettlement policies. Particular emphasis is placed on the!Xun and Khwe San of southern Angola and northern Namibia and the Tshwa San of western Zimbabwe and northern Botswana, who have been impacted by militarization and coercive conservation efforts since the late nineteenth century. Principal conclusions are that conservation and militarization efforts have led to a reduction in land and resources available to indigenous people, higher levels of poverty, increased socioeconomic stratification, and lower levels of physical well-being. San have responded to these trends by engaging in social activism, forming community-based institutions, and pursuing legal actions aimed at obtaining human rights and equitable treatment. (shrink)

Modern attitudes to meat in both men and women reflect a strong meat-masculinity association. Sex differences in the relationship between meat and masculinity have not been previously explored. In the current study we used two IATs (implicit association tasks), a visual search task, and a questionnaire to measure implicit and explicit attitudes toward meat in men and women. Men exhibited stronger implicit associations between meat and healthiness than did women, but both sexes associated meat more strongly with ‘healthy’ than ‘unhealthy’ (...) concepts. As ‘healthy’ was operationalized in the current study using terms such as “virile” and “powerful,” this suggests that a meat-strength/power association may mediate the meat-masculinity link readily observed across western cultures. The sex difference was not related to explicit attitudes to meat, nor was it attributable to a variety of other factors, such as a generally more positive disposition toward meat in men than women. Men also exhibited an attention bias toward meats, compared to non-meat foods, while females exhibited more caution when searching for non-meat foods, compared to meat. These biases were not related to implicit attitudes, but did tend to increase with increasing hunger levels. Potential ultimate explanations for these differences, including sex differences in bio-physiological needs and receptivity to social signals are discussed. (shrink)

Chimpanzees, bonobos, and human foragers share a fission-fusion social system and a mating system of joint male resource defense polygyny. Within-community skew in male strength varies among and within species. In this study, we extend a mathematical model of within-group male coalition formation among primates to derive the conditions for between-community conflicts in the form of raids. We show that the main factor affecting the presence of successful raiding is the likelihood of major discrepancies in party strength, which are set (...) by party size distributions (and thus community size) and the skew in strength. This study confirms the functional similarities between the raiding of chimpanzees and human foragers, and it supports the “imbalance of power” hypothesis for raiding. However, it also proposes two amendments to this model. First, the absence of raiding in bonobos may be attributable more to potential female involvement in defense against raids, which increases the size of defensive coalitions. Second, the model attributes some of the raiding in humans to major contrasts in instantaneous fighting ability created by surprise raids on unarmed victims; it also draws attention to the distinction between minor raids and major raids that involve multiple bands of the same community. (shrink)

The papers in this special issue of Human Nature collectively consider societal and individual responses to a wide variety of environmental and social risks. The first paper considers societal level effects of pathogen risk on collectivism and conformity, avoidance of outsiders, and in-group loyalty in a worldwide cross-cultural sample. The second deals with societal-level effects of resource unpredictability on the nature and conduct of warfare in eastern Africa. The third deals with effects of volcanic eruptions and earthquakes and mediating factors (...) on individual perceptions of risk in Mexico and Ecuador. The final paper deals with effects of various types of father absence on women’s reproductive life histories in Bangladesh. (shrink)

In the absence of explicit punitive sanctions, why do individuals voluntarily participate in intergroup warfare when doing so incurs a mortality risk? Here we consider the motivation of individuals for participating in warfare. We hypothesize that in addition to other considerations, individuals are incentivized by the possibility of rewards. We test a prediction of this “cultural rewards war-risk hypothesis” with ethnographic literature on warfare in small-scale societies. We find that a greater number of benefits from warfare is associated with a (...) higher rate of death from conflict. This provides preliminary support for the relationship between rewards and participation in warfare. (shrink)

The papers in this volume present varying approaches to human aggression, each from an evolutionary perspective. The evolutionary studies of aggression collected here all pursue aspects of patterns of response to environmental circumstances and consider explicitly how those circumstances shape the costs and benefits of behaving aggressively. All the authors understand various aspects of aggression as evolved adaptations but none believe that this implies we are doomed to continued violence, but rather that variation in aggression has evolutionary roots. These papers (...) reveal several similarities between human and nonhuman aggression, including our response to physical strength as an indicator of fighting ability, testosterone response to competition, a sensitivity to paternity, and baseline features of intergroup aggression in foragers and chimps. There is also one paper tackling the phylogeny of these traits. The many differences between human and nonhuman aggression are also pursued here. Topics here include the impact of modern weapons and extremes of wealth and power on both the costs and benefits of fighting, and the scale to which coercion can promote aggression that acts against a fighter’s own interests. Also the implications of large-scale human sociality are discussed. (shrink)

There is a well-entrenched schism on the frequency, intensity, and evolutionary significance of warfare among hunter-gatherers compared with large-scale societies. To simplify, Rousseauians argue that warfare among prehistoric and contemporary hunter-gatherers was nearly absent and, if present, was a late cultural invention. In contrast, so-called Hobbesians argue that violence was relatively common but variable among hunter-gatherers. To defend their views, Rousseauians resort to a variety of tactics to diminish the apparent frequency and intensity of hunter-gatherer warfare. These tactics include redefining (...) war, censoring ethnographic accounts of warfare in comparative analyses, misconstruing archaeological evidence, and claiming that outside contact inflates the intensity of warfare among hunter-gatherers. These tactics are subject to critical analysis and are mostly found to be wanting. Furthermore, Hobbesians with empirical data have already established that the frequency and intensity of hunter-gatherer warfare is greater compared with large-scale societies even though horticultural societies engage in warfare more intensively than hunter-gatherers. In the end I argue that although war is a primitive trait we may share with chimpanzees and/or our last common ancestor, the ability of hunter-gatherer bands to live peaceably with their neighbors, even though war may occur, is a derived trait that fundamentally distinguishes us socially and politically from chimpanzee societies. It is a point often lost in these debates. (shrink)

Previous research on warfare in a worldwide sample of societies by Ember and Ember (Journal of Conflict Resolution, 36, 242–262, 1992a) found a strong relationship between resource unpredictability (particularly food scarcity caused by natural disasters) in nonstate, nonpacified societies and overall warfare frequency. Focusing on eastern Africa, a region frequently plagued with subsistence uncertainty as well as violence, this paper explores the relationships between resource problems, including resource unpredictability, chronic scarcity, and warfare frequencies. It also examines whether resource scarcity predicts (...) more resource-taking in land, movable property, and people, as well as the commission of atrocities. Results support previous worldwide results regarding the relationship between resource unpredictability and warfare frequency. Results regarding resource-taking and atrocities are more nuanced and complex. In almost all findings, relationships are generally in opposite directions in nonstate and state societies. In post-hoc analyses, atrocities are significantly more likely to be committed in states than in nonstates. (shrink)

Reconstructing pathways to human peace can be hampered by superficial evaluations of similar processes in nonhuman species. A deeper understanding of bonobo social systems allows us to reevaluate the preconditions for peace to gain a greater insight on the evolutionary timescale of peace emergence.

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: (i) Altruism in this model is more likely to evolve (...) for defenses than for attacks. (ii) Risk aversion, deterrence and the interplay of migration levels and brutality can change evolutionary dynamics substantially. (iii) Unexpectedly, one occasional simulation outcome is a dynamically stable state of ‘tolerated intergroup theft’, raising the question if corresponding patterns also exist in real intergroup conflicts. Finally, possible implications for theories of the co-evolution of bellicosity and altruism in humans are discussed. (shrink)

As proposed for the emergence of modern languages, we argue that modern uses of languages (pragmatics) also evolved gradually in our species under the effects of human self‐domestication, with three key aspects involved in a complex feedback loop: (a) a reduction in reactive aggression, (b) the sophistication of language structure (with emerging grammars initially facilitating the transition from physical aggression to verbal aggression); and (c) the potentiation of pragmatic principles governing conversation, including, but not limited to, turn‐taking and inferential abilities. (...) Our core hypothesis is that the reduction in reactive aggression, one of the key factors in self‐domestication processes, enabled us to fully exploit our cognitive and interactional potential as applied to linguistic exchanges, and ultimately to evolve a specific form of communication governed by persuasive reciprocity—a trait of human conversation characterized by both competition and cooperation. In turn, both early crude forms of language, well suited for verbal aggression/insult, and later more sophisticated forms of language, well suited for persuasive reciprocity, significantly contributed to the resolution and reduction of (physical) aggression, thus having a return effect on the self‐domestication processes. Supporting evidence for our proposal, as well as grounds for further testing, comes mainly from the consideration of cognitive disorders, which typically simultaneously present abnormal features of self‐domestication (including aggressive behavior) and problems with pragmatics and social functioning. While various approaches to language evolution typically reduce it to a single factor, our approach considers language evolution as a multifactorial process, with each player acting upon the other, engaging in an intense mutually reinforcing feedback loop. Moreover, we see language evolution as a gradual process, continuous with the pre‐linguistic cognitive abilities, which were engaged in a positive feedback loop with linguistic innovations, and where gene‐culture co‐evolution and cultural niche construction were the main driving forces. (shrink)

De Dreu and Gross predict that attackers will have more difficulty winning conflicts than defenders. As their analysis is presumed to capture the dynamics of decentralized conflict, we consider how their framework compares with ethnographic evidence from small-scale societies, as well as chimpanzee patterns of intergroup conflict. In these contexts, attackers have significantly more success in conflict than predicted by De Dreu and Gross's model. We discuss the possible reasons for this disparity.

Thermoregulation is often thought to be a key motivating factor behind the origins of clothing. Less attention has been given, however, to the production and use of clothing across traditional societies in contexts outside of thermoregulatory needs. Here I investigate the use of disguises, modesty coverings, and body armor among the 10 hunter-gatherer societies in the Probability Sample Files (PSF) within the Human Relations Area Files (HRAF) World Cultures database, with a particular focus on disguise cases and how they compare (...) with strategies of deception across other taxa. The employment of disguises—defined as altering one’s appearance for purposes of deceiving conspecifics or other animals—is noted for eight of the 10 societies, with their use occurring in contexts of hunting, religious or cult practices, and war or interpersonal violence. Most hunter-gatherer disguises demonstrated clear similarities to cases of visual deception found in other species, with the majority of examples fitting categories of animal mimicry, masquerading as plants, disruptive coloration (camouflage), or background matching (camouflage), while disguises unique to humans involved the impersonation of culture-specific “spirit-beings.” Clothing for modesty purposes (nine societies) and body armor (six societies) are also noted. I propose that strategic initiatives by individuals or groups to disguise or conceal themselves represents one possible initial pathway to the cultural evolution of clothing. There are likely multiple potential (nonexclusive) social and functional pathways to the emergence of clothing outside of thermoregulatory needs. (shrink)

We propose that human self-domestication favored the emergence of a less aggressive phenotype in our species, more precisely phenotype prone to replace (reactive) physical aggression with verbal aggression. In turn, the (gradual) transition to verbal aggression and to more sophisticated forms of verbal behavior favored self-domestication, with the two processes engaged in a reinforcing feedback loop, considering that verbal behavior entails not only less violence and better survival, but also more opportunities to interact longer and socialize with more conspecifics, ultimately (...) enabling the emergence of more complex forms of language. Whereas in the case of self-domestication sexual selection has been proposed to work against physical aggression traits, in the case of verbal insult the selection has been proposed to work in favor of verbal aggression. The tension between these two seemingly opposing forces gets resolved/alleviated by a tendency to replace physical aggression with verbal aggression, and with verbal behavior more generally. This also helps solve the paradox of the Self Domestication Hypothesis regarding aggression, more precisely why aggression in humans has been reduced only when it comes to reactive aggression, but not when it comes to proactive aggression, the latter exhibiting an increase with the advent of modern language. We postulate that this feedback loop was particularly important during the time period before 50 kya, arguably between 200 kya and 50 kya, when humans were not fully modern, neither in terms of their skull/brain morphology and their behavior/culture, nor in terms of their self-domestication. The novelty of our approach lies (i) in giving an active role to early forms of language in interacting with self-domestication processes; (ii) in providing specific linguistic details and functions of this early stage of grammar (including insult and humor); (iii) in supplying neurobiological, ontogenetic, and clinical evidence of a link between (reactive) aggression and (reactive) verbal behavior; (iv) in identifying a variety of proxies of the earlier stages in evolution among cognitive disorders; and (v) in identifying specific points of contact and mutual reinforcement between these two processes (self-domestication and early language evolution), including reduction in physical aggression and stress/tension, as well as sexual selection. (shrink)

Historians play it safe. Complex issues are dissected while analytical distance keeps stakeholders at bay. But the relevance of historical research may be lost in caution and failure to engage with a wider audience. We can't afford that. We have too much to offer and too much at stake. We need to take the discussion of science and religion beyond our own professional circles. Peter Harrison's The Territories of Science and Religion gives us an opportunity to do so. We can (...) use his book to understand why people consistently get the relation wrong. However, we need to take the next step ourselves, involve historians in the common academic goal, across disciplines, to make sense of the world around us and make that combined knowledge truly useful. Evolution and natural history might help to that effect. (shrink)

Evolutionary models consider hunting and food sharing to be milestones that paved the way from primate to human societies. Because fossil evidence is scarce, hominoid primates serve as referential models to assess our common ancestors’ capacity in terms of communal use of resources, food sharing, and other forms of cooperation. Whereas chimpanzees form male-male bonds exhibiting resource-defense polygyny with intolerance and aggression toward nonresidents, bonobos form male-female and female-female bonds resulting in relaxed relations with neighboring groups. Here we report the (...) first known case of meat sharing between members of two bonobo communities, revealing a new dimension of social tolerance in this species. This observation testifies to the behavioral plasticity that exists in the two Pan species and contributes to scenarios concerning the traits of the last common ancestor of Pan and Homo. It also contributes to the discussion of physiological triggers of in-group/out-group behavior and allows reconsideration of the emergence of social norms in prehuman societies. (shrink)

De Dreu and Gross offer a compelling synthesis of a growing literature on the psychology of attack and defense. I argue that human raiding ecology suggests the need to endogenize attacker-defender move order as well as opportunities for tactical mismatch available to defenders. Perhaps most significantly, I draw attention to the surprising lacunae in sex differences across attack and defense.

Coalitions and alliances are core aspects of human behavior. All societies recognize alliances among communities, usually based in part on kinship and marriage. Aggression between groups is ubiquitous, often deadly, fueled by revenge, and can have devastating effects on general human welfare. Given its significance, it is surprising how little we know about the neurobiological and hormonal mechanisms that underpin human coalitionary behavior. Here we first briefly review a model of human coalitionary behavior based on a process of runaway social (...) selection. We then present several exploratory analyses of neuroendocrine responses to coalitionary social events in a rural Dominican community, with the objective of understanding differences between in-group and out-group competition in adult and adolescent males. Our analyses indicate: (1) adult and adolescent males do not elevate testosterone when they defeat their friends, but they do elevate testosterone when they defeat outsiders; (2) pre-competition testosterone and cortisol levels are negatively associated with strength of coalitionary ties; and (3) adult males usually elevate testosterone when interacting with adult women who are potential mates, but in a striking reversal, they have lower testosterone if the woman is a conjugal partner of a close friend. These naturalistic studies hint that reciprocity, dampening of aggression, and competition among friends and allies may be biologically embedded in unique ways among humans. (shrink)