Author's response: The challenge of peace.

The 30 commentators are largely sympathetic to the account I develop for the origins of peace in humans, though many suggest that peace has deeper roots and that humans share characteristics of peace with other species. Multiple commentators propose how to extend my framework or focus on the cognitive and psychological prerequisites for peace. In my reply, I discuss these considerations and further my account of why I think peace as defined here was unlikely prior to behavioral modernity which emerged approximately 100,000 years ago. In general, there seems to be a consensus that moving the debate beyond "war versus peace" in human evolution and instead focusing on the conditions that enable war or peace is a fruitful direction for the field to take.

Universal interpretations of vocal music.

Despite the variability of music across cultures, some types of human songs share acoustic characteristics. For example, dance songs tend to be loud and rhythmic, and lullabies tend to be quiet and melodious. Human perceptual sensitivity to the behavioral contexts of songs, based on these musical features, suggests that basic properties of music are mutually intelligible, independent of linguistic or cultural content. Whether these effects reflect universal interpretations of vocal music, however, is unclear because prior studies focus almost exclusively on English-speaking participants, a group that is not representative of humans. Here, we report shared intuitions concerning the behavioral contexts of unfamiliar songs produced in unfamiliar languages, in participants living in Internet-connected industrialized societies (n = 5,516 native speakers of 28 languages) or smaller-scale societies with limited access to global media (n = 116 native speakers of three non-English languages). Participants listened to songs randomly selected from a representative sample of human vocal music, originally used in four behavioral contexts, and rated the degree to which they believed the song was used for each context. Listeners in both industrialized and smaller-scale societies inferred the contexts of dance songs, lullabies, and healing songs, but not love songs. Within and across cohorts, inferences were mutually consistent. Further, increased linguistic or geographical proximity between listeners and singers only minimally increased the accuracy of the inferences. These results demonstrate that the behavioral contexts of three common forms of music are mutually intelligible cross-culturally and imply that musical diversity, shaped by cultural evolution, is nonetheless grounded in some universal perceptual phenomena.

Norm violations and punishments across human societies.

Punishments for norm violations are hypothesised to be a crucial component of the maintenance of cooperation in humans but are rarely studied from a comparative perspective. We investigated the degree to which punishment systems were correlated with socioecology and cultural history. We took data from the Standard Cross-Cultural Sample database and coded ethnographic documents from a sample of 131 largely non-industrial societies. We recorded whether punishment for norm violations concerned adultery, religion, food, rape or war cowardice and whether sanctions were reputational, physical, material or execution. We used Bayesian phylogenetic regression modelling to test for culture-level covariation. We found little evidence of phylogenetic signals in evidence for punishment types, suggesting that punishment systems change relatively quickly over cultural evolutionary history. We found evidence that reputational punishment was associated with egalitarianism and the absence of food storage; material punishment was associated with the presence of food storage; physical punishment was moderately associated with greater dependence on hunting; and execution punishment was moderately associated with social stratification. Taken together, our results suggest that the role and kind of punishment vary both by the severity of the norm violation, but also by the specific socio-economic system of the society.

Why did foraging, horticulture and pastoralism persist after the Neolithic transition? The oasis theory of agricultural intensification.

Despite the global spread of intensive agriculture, many populations retained foraging or mixed subsistence strategies until well into the twentieth century. Understanding why has been a longstanding puzzle. One explanation, called the marginal habitat hypothesis, is that foraging persisted because foragers tended to live in marginal habitats generally not suited to agriculture. However, recent empirical studies have not supported this view. The alternative but untested oasis hypothesis of agricultural intensification claims that intensive agriculture developed in areas with low biodiversity and a reliable water source not reliant on local rainfall. We test both the marginal habitat and oasis hypotheses using a cross-cultural sample drawn from the 'Ethnographic atlas' (Murdock 1967 Ethnology 6, 109-236). Our analyses provide support for both hypotheses. We found that intensive agriculture was unlikely in areas with high rainfall. Further, high biodiversity, including pathogens associated with high rainfall, appears to have limited the development of intensive agriculture. Our analyses of African societies show that tsetse flies, elephants and malaria are negatively associated with intensive agriculture, but only the effect of tsetse flies reached significance. Our results suggest that in certain ecologies intensive agriculture may be difficult or impossible to develop but that generally lower rainfall and biodiversity is favourable for its emergence. This article is part of the theme issue 'Evolutionary ecology of inequality'.

The Evolution of Peace.

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 in the past 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.

Acoustic regularities in infant-directed speech and song across cultures.

When interacting with infants, humans often alter their speech and song in ways thought to support communication. Theories of human child-rearing, informed by data on vocal signalling across species, predict that such alterations should appear globally. Here, we show acoustic differences between infant-directed and adult-directed vocalizations across cultures. We collected 1,615 recordings of infant- and adult-directed speech and song produced by 410 people in 21 urban, rural and small-scale societies. Infant-directedness was reliably classified from acoustic features only, with acoustic profiles of infant-directedness differing across language and music but in consistent fashions. We then studied listener sensitivity to these acoustic features. We played the recordings to 51,065 people from 187 countries, recruited via an English-language website, who guessed whether each vocalization was infant-directed. Their intuitions were more accurate than chance, predictable in part by common sets of acoustic features and robust to the effects of linguistic relatedness between vocalizer and listener. These findings inform hypotheses of the psychological functions and evolution of human communication.

Key individuals catalyse intergroup violence.

Intergroup violence is challenging to understand: why do individuals cooperate to harm members of other groups when they themselves may be killed or injured? Despite progress in understanding the evolutionary and proximate mechanisms that underlie violence, we still have little insight into the processes that lead to the emergence of coalitionary aggression. We argue that an overlooked component is the presence of individuals who have a crucial role in initiating violence. In instigating intergroup violence, these key individuals may expect to face lower costs, receive greater benefits, or garner benefits that have a greater value to them than others. Alternatively, key individuals may be motivated by individual traits such as increased boldness, propensity for aggression or exploratory behaviour. Key individuals catalyse the emergence of coalitionary violence through one of several processes including altering the costs and benefits that accrue to others, paying a greater share of the startup costs, signalling privileged knowledge, or providing coordination, among other factors. Here we integrate diverse lines of empirical research from humans and non-human animals demonstrating that inter-individual variation is an important factor in the emergence of intergroup violence. Focusing on the role of key individuals provides new insights into how and why violence emerges. This article is part of the theme issue 'Intergroup conflict across taxa'.

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'.

How small-scale societies achieve large-scale cooperation.

For most of our species' history, humans have lived in relatively small subsistence communities, often called small-scale societies. While these groups lack centralized institutions, they can and often do maintain large-scale cooperation. Here, we explore several mechanisms promoting cooperation in small-scale societies, including (a) the development of social norms that encourage prosocial behavior, (b) reciprocal exchange relationships, (c) reputation that facilitates high-cost cooperation, (d) relational wealth, and (e) risk buffering institutions. We illustrate these with ethnographic and psychological evidence from contemporary small-scale societies. We argue that these mechanisms for cooperation helped past and present small-scale communities adapt to diverse ecological and social niches.

Toward inclusive theories of the evolution of musicality.

We compare and contrast the 60 commentaries by 109 authors on the pair of target articles by Mehr et al. and ourselves. The commentators largely reject Mehr et al.'s fundamental definition of music and their attempts to refute (1) our social bonding hypothesis, (2) byproduct hypotheses, and (3) sexual selection hypotheses for the evolution of musicality. Instead, the commentators generally support our more inclusive proposal that social bonding and credible signaling mechanisms complement one another in explaining cooperation within and competition between groups in a coevolutionary framework (albeit with some confusion regarding terminologies such as "byproduct" and "exaptation"). We discuss the proposed criticisms and extensions, with a focus on moving beyond adaptation/byproduct dichotomies and toward testing of cross-species, cross-cultural, and other empirical predictions.

The emergence of locally adaptive institutions: Insights from traditional social structures of East African pastoralists.

Humans inhabit the widest range of ecological and social niches of any mammal. Yet each ecological and social environment presents a set of challenges that we must solve in order to successfully inhabit it. We are able to do so by building institutions that can flexibly respond to changing circumstances. Institutions that solve adaptive challenges necessary for human sociality, such as how to resolve conflicts, find mates, and extract and distribute resources, are termed locally adaptive institutions. The design of locally adaptive institutions promotes coordination and cooperation among unrelated individuals, reflecting the constraints of the particular ecological and social challenges to which they are responsive. Institutions generally are enabled by a suite of social and psychological mechanisms, including norm compliance, self-interested design, selective imitation, and cultural group selection among others. The development of locally adaptive institutions are likely to be especially shaped by self-interested design in which agents are sensitive to the payoffs from various norms and choose to enforce and follow those which they anticipate to be most beneficial to themselves. Exogenous shocks, including the advent of material and cultural technologies, population pressures, or even group conflict can contribute to the modification of existing social institutions and the development of new social structures. Using several case examples from traditional east African pastoralist societies, I illustrate how ecological and social pressures shape the development of social norms that underlie locally adaptive social institutions and facilitate continued cooperation in the face of change at scales ranging from local to global.

Music as a coevolved system for social bonding.

Why do humans make music? Theories of the evolution of musicality have focused mainly on the value of music for specific adaptive contexts such as mate selection, parental care, coalition signaling, and group cohesion. Synthesizing and extending previous proposals, we argue that social bonding is an overarching function that unifies all of these theories, and that musicality enabled social bonding at larger scales than grooming and other bonding mechanisms available in ancestral primate societies. We combine cross-disciplinary evidence from archeology, anthropology, biology, musicology, psychology, and neuroscience into a unified framework that accounts for the biological and cultural evolution of music. We argue that the evolution of musicality involves gene-culture coevolution, through which proto-musical behaviors that initially arose and spread as cultural inventions had feedback effects on biological evolution because of their impact on social bonding. We emphasize the deep links between production, perception, prediction, and social reward arising from repetition, synchronization, and harmonization of rhythms and pitches, and summarize empirical evidence for these links at the levels of brain networks, physiological mechanisms, and behaviors across cultures and across species. Finally, we address potential criticisms and make testable predictions for future research, including neurobiological bases of musicality and relationships between human music, language, animal song, and other domains. The music and social bonding hypothesis provides the most comprehensive theory to date of the biological and cultural evolution of music.

Universality and diversity in human song.

What is universal about music, and what varies? We built a corpus of ethnographic text on musical behavior from a representative sample of the world's societies, as well as a discography of audio recordings. The ethnographic corpus reveals that music (including songs with words) appears in every society observed; that music varies along three dimensions (formality, arousal, religiosity), more within societies than across them; and that music is associated with certain behavioral contexts such as infant care, healing, dance, and love. The discography-analyzed through machine summaries, amateur and expert listener ratings, and manual transcriptions-reveals that acoustic features of songs predict their primary behavioral context; that tonality is widespread, perhaps universal; that music varies in rhythmic and melodic complexity; and that elements of melodies and rhythms found worldwide follow power laws.

Making Pastoralists Count: Geospatial Methods for the Health Surveillance of Nomadic Populations.

Nomadic pastoralists are among the world's hardest-to-reach and least served populations. Pastoralist communities are difficult to capture in household surveys because of factors including their high degree of mobility over remote terrain, fluid domestic arrangements, and cultural barriers. Most surveys use census-based sampling frames which do not accurately capture the demographic and health parameters of nomadic populations. As a result, pastoralists are "invisible" in population data such as the Demographic and Health Surveys (DHS). By combining remote sensing and geospatial analysis, we developed a sampling strategy designed to capture the current distribution of nomadic populations. We then implemented this sampling frame to survey a population of mobile pastoralists in southwest Ethiopia, focusing on maternal and child health (MCH) indicators. Using standardized instruments from DHS questionnaires, we draw comparisons with regional and national data finding disparities with DHS data in core MCH indicators, including vaccination coverage, skilled birth attendance, and nutritional status. Our field validation demonstrates that this method is a logistically feasible alternative to conventional sampling frames and may be used at the population level. Geospatial sampling methods provide cost-affordable and logistically feasible strategies for sampling mobile populations, a crucial first step toward reaching these groups with health services.

Reasons to strike first.

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.

Form and Function in Human Song.

Humans use music for a variety of social functions: we sing to accompany dance, to soothe babies, to heal illness, to communicate love, and so on. Across animal taxa, vocalization forms are shaped by their functions, including in humans. Here, we show that vocal music exhibits recurrent, distinct, and cross-culturally robust form-function relations that are detectable by listeners across the globe. In Experiment 1, internet users (n = 750) in 60 countries listened to brief excerpts of songs, rating each song's function on six dimensions (e.g., "used to soothe a baby"). Excerpts were drawn from a geographically stratified pseudorandom sample of dance songs, lullabies, healing songs, and love songs recorded in 86 mostly small-scale societies, including hunter-gatherers, pastoralists, and subsistence farmers. Experiment 1 and its analysis plan were pre-registered. Despite participants' unfamiliarity with the societies represented, the random sampling of each excerpt, their very short duration (14 s), and the enormous diversity of this music, the ratings demonstrated accurate and cross-culturally reliable inferences about song functions on the basis of song forms alone. In Experiment 2, internet users (n = 1,000) in the United States and India rated three contextual features (e.g., gender of singer) and seven musical features (e.g., melodic complexity) of each excerpt. The songs' contextual features were predictive of Experiment 1 function ratings, but musical features and the songs' actual functions explained unique variance in function ratings. These findings are consistent with the existence of universal links between form and function in vocal music.

The cultural evolution of war rituals.

The cultural evolutionary processes outlined by Singh illuminate why ritualized behaviors aimed at controlling unseen forces and overcoming fear are common in warfare among many small-scale societies. They also suggest an explanation for the development of ritual specialists for war who are distinct from war leaders.

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.

Self-Interest and the Design of Rules.

Rules regulating social behavior raise challenging questions about cultural evolution in part because they frequently confer group-level benefits. Current multilevel selection theories contend that between-group processes interact with within-group processes to produce norms and institutions, but within-group processes have remained underspecified, leading to a recent emphasis on cultural group selection as the primary driver of cultural design. Here we present the self-interested enforcement (SIE) hypothesis, which proposes that the design of rules importantly reflects the relative enforcement capacities of competing parties. We show that, in addition to explaining patterns in cultural change and stability, SIE can account for the emergence of much group-functional culture. We outline how this process can stifle or accelerate cultural group selection, depending on various social conditions. Self-interested enforcement has important bearings on the emergence, stability, and change of rules.

Subsistence styles shape human social learning strategies.

Social learning is a fundamental element of human cognition. Learning from others facilitates the transmission of information that helps individuals and groups rapidly adjust to new environments and underlies adaptive cultural evolution1-6. While basic human propensities for social learning are traditionally assumed to be species-universal1,7, recent empirical studies show that they vary between individuals and populations8-13. Yet the causes of this variation remain poorly understood9. Here we show that interdependence in everyday social and economic activities can strongly amplify social learning. With an experimental decision-making task we examine individual versus social learning in three recently diverged populations of a single-ethnic group, whose subsistence styles require varying degrees of interdependence. Interdependent pastoralists and urban dwellers have markedly higher propensities for social learning than independent horticulturalists, who predominantly rely on individual payoff information. These results indicate that everyday social and economic practices can mould human social learning strategies and they highlight the flexibility of human cognition to change with local ecology. Our study further suggests that shifts in subsistence styles - which can occur when humans inhabit new habitats or cultural niches2 - can alter reliance on social learning and may therefore impact the ability of human societies to adapt to novel circumstances.