Sarah Mathew

Punishment sustains large-scale cooperation in prestate warfare

Understanding cooperation and punishment in small-scale societies is crucial for explaining the origins of human cooperation. We studied warfare among the Turkana, a politically uncentralized, egalitarian, nomadic pastoral society in East Africa. Based on a representative sample of 88 recent raids, we show that the Turkana sustain costly cooperation in combat at a remarkably large scale, at least in part, through punishment of free-riders. Raiding parties comprised several hundred warriors and participants are not kin or day-to-day interactants. Warriors incur substantial risk of death and produce collective benefits. Cowardice and desertions occur, and are punished by community-imposed sanctions, including collective corporal punishment and fines. Furthermore, Turkana norms governing warfare benefit the ethnolinguistic group, a population of a half-million people, at the expense of smaller social groupings. These results challenge current views that punishment is unimportant in small-scale societies and that human cooperation evolved in small groups of kin and familiar individuals. Instead, these results suggest that cooperation at the larger scale of ethnolinguistic units enforced by third-party sanctions could have a deep evolutionary history in the human species.

When does optional participation allow the evolution of cooperation

Altruistic punishment has been shown to invade when rare if individuals are allowed to opt out of cooperative ventures. Individuals that opt out do not contribute to the common enterprise or derive benefits from it. This result is potentially significant because it offers an explanation for the origin of large-scale cooperation in one-shot interactions among unrelated individuals. Here, we show that this result is not a general consequence of optional participation in cooperative activities, but depends on special assumptions about cooperative pay-offs. We extend the pay-off structure of optional participation models to consider the effects of economies and diseconomies of scale in public-goods production, rival and non-rival consumption of goods, and different orderings of the pay-offs of freeriding and opting out. This more general model highlights the kinds of pay-offs for which optional participation favours cooperation, and those in which it does not.

Dating the origin of language using phonemic diversity

Language is a key adaptation of our species, yet we do not know when it evolved. Here, we use data on language phonemic diversity to estimate a minimum date for the origin of language. We take advantage of the fact that phonemic diversity evolves slowly and use it as a clock to calculate how long the oldest African languages would have to have been around in order to accumulate the number of phonemes they possess today. We use a natural experiment, the colonization of Southeast Asia and Andaman Islands, to estimate the rate at which phonemic diversity increases through time. Using this rate, we estimate that present-day languages date back to the Middle Stone Age in Africa. Our analysis is consistent with the archaeological evidence suggesting that complex human behavior evolved during the Middle Stone Age in Africa, and does not support the view that language is a recent adaptation that has sparked the dispersal of humans out of Africa. While some of our assumptions require testing and our results rely at present on a single case-study, our analysis constitutes the first estimate of when language evolved that is directly based on linguistic data.

An evolutionary theory of large-scale human warfare: Group-structured cultural selection

When humans wage war, it is not unusual for battlefields to be strewn with dead warriors. These warriors typically were men in their reproductive prime who, had they not died in battle, might have gone on to father more children. Typically, they are also genetically unrelated to one another. We know of no other animal species in which reproductively capable, genetically unrelated individuals risk their lives in this manner. Because the immense private costs borne by individual warriors create benefits that are shared widely by others in their group, warfare is a stark evolutionary puzzle that is difficult to explain. Although several scholars have posited models of the evolution of human warfare, these models do not adequately explain how humans solve the problem of collective action in warfare at the evolutionarily novel scale of hundreds of genetically unrelated individuals. We propose that group‐structured cultural selection explains this phenomenon.

Behavioural variation in 172 small-scale societies indicates that social learning is the main mode of human adaptation.

The behavioural variation among human societies is vast and unmatched in the animal world. It is unclear whether this variation is due to variation in the ecological environment or to differences in cultural traditions. Underlying this debate is a more fundamental question: is the richness of humans’ behavioural repertoire due to non-cultural mechanisms, such as causal reasoning, inventiveness, reaction norms, trial-and-error learning and evoked culture, or is it due to the population-level dynamics of cultural transmission? Here, we measure the relative contribution of environment and cultural history in explaining the behavioural variation of 172 Native American tribes at the time of European contact. We find that the effect of cultural history is typically larger than that of environment. Behaviours also persist over millennia within cultural lineages. This indicates that human behaviour is not predominantly determined by single-generation adaptive responses, contra theories that emphasize non-cultural mechanisms as determinants of human behaviour. Rather, the main mode of human adaptation is social learning mechanisms that operate over multiple generations.

Cultural group selection follows Darwin's classic syllogism for the operation of selection

The main objective of our target article was to sketch the empirical case for the importance of selection at the level of groups on cultural variation. Such variation is massive in humans, but modest or absent in other species. Group selection processes acting on this variation is a framework for developing explanations of the unusual level of cooperation between non-relatives found in our species. Our case for cultural group selection (CGS) followed Darwins classic syllogism regarding natural selection: If variation exists at the level of groups, if this variation is heritable, and if it plays a role in the success or failure of competing groups, then selection will operate at the level of groups. We outlined the relevant domains where such evidence can be sought and characterized the main conclusions of work in those domains. Most commentators agree that CGS plays some role in human evolution, although some were considerably more skeptical. Some contributed additional empirical cases. Some raised issues of the scope of CGS explanations versus competing ones.

Solving the puzzle of human warfare requires an explanation of battle raids and cultural institutions.

Glowacki and Wrangham provide a valuable analysis of reproductive success and raiding among the Nyangatom pastoralists in Ethiopia (1). The authors divided raiding into low-risk stealth raids and high-risk battle raids and found that elder males reputed to participate in stealth raids as youths had more wives and offspring than other elder males. Glowacki and Wrangham (1) claim this result supports the “general proposition that warriors participating in small-scale warfare tend to receive fitness-enhancing benefits.” However, Glowacki and Wrangham’s analysis does not warrant such a general claim.

Cultural history, not ecological environment, is the main determinant of human behaviour

Towner et al. [[1][1]] question the methods and the theoretical framework of our study of behavioural variation among Native American tribes of Western North America [[2][2]]. Here we show that their concerns are unfounded and that our results are robust. We also clarify the theoretical issues that

Human Cooperation, Evolution of

Human cooperation is a novel evolutionary puzzle because we cooperate with genetically unrelated individuals in groups that comprise millions of people. Direct reciprocity, especially when considering errors in behavior, has shed light on pairwise cooperation among well-known but genetically unrelated people. Three mechanisms have been identified to explain large-scale cooperation in humans. Cooperation can evolve by indirect reciprocity in groups as long as people can observe each others behavior, or can garner honest information via gossip on who cooperates and defects in their interactions. Humans also have a disposition to cooperate and to punish those who do not, even at a cost to oneself, and such costly punishment can sustain cooperation in even larger groups of people who do not know much about each other. Cultural group selection explains the scale of human cooperation, why it is variable, and why norms enforced by sanctions are group-beneficial. Support for these theories has come from laboratory experiments using a variety of behavioral economic games, and from field studies in small-scale societies. Key open questions include understanding what characterizes goodness in indirect reciprocity, why gossip is sufficiently accurate, and why people are motivated to engage in costly punishment.

Are Cultural and Evolutionary Views of Human Warfare Converging? A Review of War, Peace and Human Nature: The Convergence of Evolutionary and Cultural Views, edited by Douglas P. Fry (Oxford University Press, 2015)

War, Peace and Human Nature: The Convergence of Evolutionary and Cultural Views, edited by Douglas P. Fry, provides a wealth of information on various topics related to human conflict. Copyright Information: Copyright 2015 by the article author(s). All rights reserved. Cliodynamics: The Journal of Quantitative History and Cultural Evolution Corresponding author’s e-mail: sarah.mathew@asu.edu Citation: Mathew, Sarah. 2015. Are cultural and evolutionary views of human warfare converging? 6: 108–110. Are Cultural and Evolutionary Views of Human Warfare Converging? A Review of War, Peace and Human Nature: The Convergence of Evolutionary and Cultural Views, edited by Douglas P. Fry (Oxford University Press, 2015) Sarah Mathew Arizona State University War, Peace and Human Nature: The Convergence of Evolutionary and Cultural Views, edited by Douglas P. Fry, provides a wealth of information on various topics related to human conflict. The book begins with chapters from an ecological and evolutionary perspective that build a foundation for thinking about human conflict based on the selective forces that shape conflict and restraint in other animals. It then moves to a discussion of prehistoric conflict, which includes detailed summaries of archaeological data that provide a window into the state of violence before complex, centralized societies predominated the world. Next is a valuable segment on the nature of conflict in contemporary, egalitarian hunter-gatherers, including the Hadza of Tanzania, the Batek of Malaysia, the Moriori of New Zealand, and aboriginal societies of Australia. A section on war and peace in other primates follows, covering chimpanzee warfare, conflict and peace-making among baboons, and a comparison of conflict resolution mechanisms in children and non-human primates. The book concludes with a section discussing the psychological basis of killing in combat—including an intriguing discussion on the aversion to killing that combatants in modern military have had to confront and overcome. For those interested in a review of conflict and mechanisms of conflict resolution in humans, this book consolidates a wide variety of pertinent information. Note that the book is not just about warfare, but about conflict in general, which can include a variety of inter-personal conflict, including physical aggression between same-sex individuals, domestic violence, conflict within social relationships, verbal aggression, and alcohol-induced fights. Some readers may find this problematic. The evolutionary forces that shape warfare differ from the evolutionary forces that shape inter-personal violence because warfare can occur only if the problem of collective action is solved. Thus, the evolution of warfare is tightly linked to the mechanisms underpinning the evolution of cooperation. This fact alone accounts for the rarity of warfare in most of the animal kingdom despite the prevalence of myriad other forms of conflict. So, for readers interested in the Mathew: Cultural and Evolutionary Views of Warfare. Cliodynamics 6:1 (2015) 109 evolution of warfare, the book may seem like a grab bag of too many unrelated phenomena. A weakness of the book is that it does not live up to its title, which promises a convergence between cultural and evolutionary views. The book does not present a mechanistic framework by which evolutionary and cultural viewpoints can be combined. This is disappointing, especially because over the last two decades, theoretical frameworks such as cultural group selection have been developed that explain how warfare (Turchin 2006; Turchin et al. 2013; Turchin 2010; Zefferman and Mathew 2015) as well as other social behaviors in humans (Boyd and Richerson 2010; Richerson et al. 2014; Henrich 2004) are shaped by selection operating on societies with different cultural norms. Cultural group selection provides an explicit mechanism by which the cultural norms and institutions of some societies evolve to motivate citizens to engage in warfare despite the tremendous personal risks to doing so. For instance, the Turkana, an egalitarian pastoral society in Kenya, launch highrisk cattle-raids in which they engage in a firefight to obtain livestock, take revenge, and get access to grazing and watering sites. Despite the risk (50% of adult male mortality in certain populations is due to warfare), Turkana men participate in combat because the cultural norms of their society promote doing so (Mathew and Boyd 2011; Mathew and Boyd 2014). Cowards and deserters on raids are sanctioned by their community whereas brave warriors are praised and rewarded. There is some evidence that these norms arose by cultural group selection: Turkana norms governing warfare prohibit raiding other Turkana, and only promote raiding other cultural groups. Thus, the norms benefit the cultural group, not smaller or larger social units. Cultural group selection can also explain why there is considerable variability in the prevalence, scale, and mode of warfare (Zefferman and Mathew 2015). The question of how prevalent warfare is across societies has long been a point of disagreement between cultural approaches and the traditional evolutionary approaches that have not taken cultural factors seriously. Some of the chapters in this volume continue to echo this divide while doing little to resolve it. Instead, the convergence in this book stems from including the distinct perspectives from both cultural and evolutionary literatures. This is at best a weak form of convergence—only a step beyond the scholarly context of the past, in which the two sides operated in isolation or open hostility of each other. Unfortunately, the reader is still left with the job of synthesizing these perspectives into a coherent framework. It is unclear what aspects of the cultural and evolutionary viewpoints are at odds: Are the perspectives different merely because they examine the phenomenon at different levels? Or are there contradictions between these perspectives that need to be resolved? Lastly, the book does not Mathew: Cultural and Evolutionary Views of Warfare. Cliodynamics 6:1 (2015) 110 sufficiently flesh out why convergence is necessary to better explain human warfare. Convergence is, in fact, imperative: it is increasingly evident that human warfare could not have evolved without culture. In no other species do large numbers of reproductively able, genetically unrelated individuals bear immense personal costs to fight together against opponents. So far, there is no way to explain this feature of human warfare without explicitly incorporating cultural factors into evolutionary models. Although this book provides an excellent starting point to discover the various perspectives arising from cultural and evolutionary viewpoints, the next step of fusing these together to produce the much-needed convergence still remains to be done.