Altruistic cooperation during foraging by the Ache, and the evolved human predisposition to cooperate

Women's subsistence networks scaffold cultural transmission among BaYaka foragers in the Congo Basin

In hunter-gatherer societies, women's subsistence activities are crucial for food provisioning and children's social learning but are understudied relative to men's activities. To understand the structure of women's foraging networks, we present 230 days of focal-follow data in a BaYaka community. To analyze these data, we develop a stochastic blockmodel for repeat observations with uneven sampling. We find that women's subsistence networks are characterized by cooperation between kin, gender homophily, and mixed age-group composition. During early childhood, individuals preferentially coforage with adult kin, but those in middle childhood and adolescence are likely to coforage with nonkin peers, providing opportunities for horizontal learning. By quantifying the probability of coforaging ties across age classes and relatedness levels, our findings provide insights into the scope for social learning during women's subsistence activities in a real-world foraging population and provide ground-truth values for key parameters used in formal models of cumulative culture.

Tax thresholds yield multiple optimal cooperation levels in the spatial public goods game

Income redistribution, which involves transferring income from certain individuals to others, plays a crucial role in human societies. Previous research has indicated that tax-based redistribution can promote cooperation by enhancing incentives for cooperators. In such a tax system, all individuals, irrespective of their income levels, contribute to the tax system, and the tax revenue is subsequently redistributed to everyone. In this study, we relax this assumption by introducing a tax threshold, signifying that only individuals with incomes exceeding the threshold will be subject to taxation. In particular, we employ the spatial public goods game to investigate the influence of tax rates-the percentage of income allocated to tax-and tax thresholds, which determine the income level at which individuals become taxable, on the evolution of cooperation. Our extensive numerical simulations disclose that tax thresholds produce complex outcomes for the evolution of cooperation, depending on tax rates. Notably, at low tax rates (i.e., below 0.41), as the tax threshold increases, discontinuous phase transitions in cooperation performance suggest the presence of multiple intervals of effective tax thresholds that promote peak cooperation levels. Nevertheless, irrespective of the chosen tax rate, once the tax threshold surpasses a critical threshold, the redistribution mechanism fails, causing the collapse of cooperation. Evolutionary snapshots show that self-organized redistribution forms an intermediary layer on the peripheries of cooperative clusters, effectively shielding cooperators from potential defectors. Quantitative analyses shed light on how self-organized redistribution narrows the income gap between cooperators and defectors through precise identification of tax-exempt entities, thereby amplifying the cooperative advantage. Collectively, these findings enhance our comprehension of how income redistribution influences cooperation, highlighting the pivotal role of tax thresholds.

Inter-household transfers of material goods among Sama "sea nomads" of the Philippines: Reciprocity, helping, signaling, or something else?

The extent to which humans share with both kin and non-kin is a defining characteristic of our species. Evolutionary research suggests that pervasive reliance on inter-individual transfers of goods and services may have evolved to support a cooperative breeding adaptation in humans. However, while intensive food sharing between individuals and families has frequently been investigated in small-scale human societies, a comprehensive analysis of the daily transfers of all material goods has not been attempted. Likewise, while much previous research on cooperative transfers focused on terrestrial foraging populations, less attention is paid to other small-scale economic modalities traditionally inhabited by humans. Drawing on over three years' worth of interviews and observational data from a community of primarily ethnic Sama people residing along the coast of Southern Mindanao Island in the Philippines, this paper examines the overall transfer patterns of material goods in a marine foraging economy. A quantitative description of resource acquisition is followed by an in-depth exploration of the characteristics of individual households and household dyads who gave and/or received more during the study period. Results indicate that a household's age and income are consistently correlated with increased inflow and outflow of material goods. Results also suggest differential motivations underlie inter-household sharing of food, money, and other goods in the study community. Most importantly, we find that both daily and long-term reciprocity overwhelmingly drive sharing within household dyads in the study community, despite secondary effects of kinship, relative need, and relative household age between household dyads.

Female cooperative labour networks in hunter-gatherers and horticulturalists

Cooperation in food acquisition is a hallmark of the human species. Given that costs and benefits of cooperation vary among production regimes and work activities, the transition from hunting-and-gathering to agriculture is likely to have reshaped the structure of cooperative subsistence networks. Hunter-gatherers often forage in groups and are generally more interdependent and experience higher short-term food acquisition risk than horticulturalists, suggesting that cooperative labour should be more widespread and frequent for hunter-gatherers. Here we compare female cooperative labour networks of Batek hunter-gatherers of Peninsular Malaysia and Tsimane forager-horticulturalists of Bolivia. We find that Batek foraging results in high daily variation in labour partnerships, facilitating frequent cooperation in diffuse networks comprised of kin and non-kin. By contrast, Tsimane horticulture involves more restricted giving and receiving of labour, confined mostly to spouses and primary or distant kin. Tsimane women also interact with few individuals in the context of hunting/fishing activities and forage mainly with spouses and primary kin. These differences give rise to camp- or village-level networks that are more modular (have more substructure when partitioned) among Tsimane horticulturalists. Our findings suggest that subsistence activities shape the formation and extent of female social networks, particularly with respect to connections with other women and non-kin. We discuss the implications of restricted female labour networks in the context of gender relations, power dynamics and the adoption of farming in humans. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

Human cooperation in changing groups in a large-scale public goods game

How people cooperate to provide public goods is an important scientific question and relates to many societal problems. Previous research studied how people cooperate in stable groups in repeated or one-time-only encounters. However, most real-world public good problems occur in groups with a gradually changing composition due to old members leaving and new members arriving. How group changes are related to cooperation in public good provision is not well understood. To address this issue, we analyze a dataset from an online public goods game comprising approximately 1.5 million contribution decisions made by about 135 thousand players in about 11.3 thousand groups with about 234 thousand changes in group composition. We find that changes in group composition negatively relate to cooperation. Our results suggest that this is related to individuals contributing less in the role of newcomers than in the role of incumbents. During the process of moving from newcomer status to incumbent status, individuals cooperate more and more in line with incumbents.

Parochial cooperation in wild chimpanzees: a model to explain the evolution of parochial altruism

Parochial altruism, taking individual costs to benefit the in-group and harm the out-group, has been proposed as one of the mechanisms underlying the human ability of large-scale cooperation. How parochial altruism has evolved remains unclear. In this review paper, we formulate a parochial cooperation model in small-scale groups and examine the model in wild chimpanzees. As suggested for human parochial altruism, we review evidence that the oxytocinergic system and in-group cooperation and cohesion during out-group threat are integral parts of chimpanzee collective action during intergroup competition. We expand this model by suggesting that chimpanzee parochial cooperation is supported by the social structure of chimpanzee groups which enables repeated interaction history and established social ties between co-operators. We discuss in detail the role of the oxytocinergic system in supporting parochial cooperation, a pathway that appears integral already in chimpanzees. The reviewed evidence suggests that prerequisites of human parochial altruism were probably present in the last common ancestor between Pan and Homo. This article is part of the theme issue 'Intergroup conflict across taxa'.

Social foraging can benefit artisanal fishers who interact with wild dolphins

Abstract

Social foraging decisions depend on individual payoffs. However, it is unclear how individual variation in phenotypic and behavioural traits can influence these payoffs, thereby the decisions to forage socially or individually. Here, we studied how individual traits influence foraging tactics of net-casting fishers who interact with wild dolphins. While net-casting is primarily an individual activity, in the traditional fishery with dolphins, fishers can choose between fishing in cooperative groups or solitarily. Our semi-structured interviews with fishers show their social network is mapped onto these foraging tactics. By quantifying the fishers’ catch, we found that fishers in cooperative groups catch more fish per capita than solitary fishers. By quantifying foraging and social traits of fishers, we found that the choice between foraging tactics—and whom to cooperate with—relates to differences in peer reputation and to similarities in number of friends, propensity to fish with relatives, and frequency of interaction with dolphins. These findings indicate different payoffs between foraging tactics and that by choosing the cooperative partner fishers likely access other benefits such as social prestige and embeddedness. These findings reveal the importance of not only material but also non-material benefits of social foraging tactics, which can have implications for the dynamics of this rare fishery. Faced with the current fluctuation in fishing resource availability, the payoffs of both tactics may change, affecting the fishers’ social and foraging decisions, potentially threatening the persistence of this century-old fishery involving humans and wildlife.

Significance statement

Social foraging theory proposes that decisions to forage in groups are primarily driven by cost–benefit trade-offs that individuals experience, but it remains unclear whether, and how much, individual foragers’ characteristics influence these trade-offs and consequently the choice to forage in social groups. We study the artisanal net-casting fishers who choose between cooperating with each other or fishing alone when engaging in a rare interaction with wild dolphins. Our findings suggest that cooperative fishers capture more fish than solitary fishers, and that by choosing cooperative partners based on similarities and differences in key social (peer reputation, kinship, friendships) and foraging (fishing frequency) traits; these fishers also experience higher social prestige and more social embeddedness. These results suggest that material gains from foraging—but also non-material benefits accrued from socializing with like-minded individuals—can influence the dynamics of human social foraging.

Intergroup bias in punishing behaviors of adults with autism spectrum disorder

Groups are essential elements of society, and humans, by nature, commonly manifest intergroup bias (i.e., behave more positively toward an ingroup member than toward an outgroup member). Despite the growing evidence of various types of altered decision-making in individuals with autism spectrum disorder (ASD), their behavior under the situation involving group membership remains largely unexplored. By modifying a third-party punishment paradigm, we investigated intergroup bias in individuals with ASD and typical development (TD). In our experiment, participants who were considered as the third party observed a dictator game wherein proposers could decide how to distribute a provided amount of money while receivers could only accept unconditionally. Participants were confronted with two different group situations: the proposer was an ingroup member and the recipient was an outgroup member (IN/OUT condition) or the proposer was an outgroup member and the recipient was an ingroup member (OUT/IN condition). Participants with TD punished proposers more severely when violating social norms in the OUT/IN condition than in IN/OUT condition, indicating that their decisions were influenced by the intergroup context. This intergroup bias was attenuated in individuals with ASD. Our findings deepen the understanding of altered decision-making and socioeconomic behaviors in individuals with ASD.

Gains to cooperation drive the evolution of egalitarianism

What conditions favour egalitarianism, that is, muted hierarchies with relatively equal distributions of resources? Here, we combine the hawk-dove and prisoner's dilemma games to model the effects of economic defensibility, costs of competition and gains from cooperation on egalitarianism, operationalized as the absence of hawks. We show that a 'leveller' strategy, which punishes hawkishness in the hawk-dove game with defection in the prisoner's dilemma, can be evolutionarily stable provided that the gains from cooperation are high relative to the benefits of hawkishness. Under these conditions, rare mutant levellers select for hawks that acquiesce to punishment by playing dove. If these 'acquiescent hawks' become common, levellers outperform hawks and establish a new egalitarian equilibrium. An analysis of human foraging groups corroborates these results, as groups with a greater reliance on cooperation are more egalitarian. Cooperation fosters greater equality when individuals can withhold its benefits from would-be dominant individuals.

Adult Hippocampal Neurogenesis and Affective Disorders: New Neurons for Psychic Well-Being

A paradigm shift in neuroscience was the discovery that new neurons are constantly produced in the adult mammalian brain of several species, including Homo sapiens. These new-born cells are formed in some main neurogenic niches, including the subventricular zone (SVZ) at the margin of the lateral ventricle and subgranular zone (SGZ) in the hippocampal dentate gyrus (DG). In the DG, neuroblasts derive from SGZ progenitors and migrate to the hippocampal granular layer becoming adult granule cells, which are integrated into functional adult circuits. It has been confirmed that adult hippocampal neurogenesis (AHN) is a long-lasting phenomenon in the human brain. The functions of hippocampal new-born cells are not fully established. Experimental studies suggest that they have unique electrophysiological properties, including hyperexcitability, which enable them to regulate adult granule cells. Their specific function depends on the anatomical hippocampal location along the hippocampal dorsal-ventral axis. Dorsal hippocampus plays a more defined role on spatial learning and contextual information, while the ventral hippocampus is more related to emotional behavior, stress resilience and social interaction. Several reports suggest a role for AHN in pattern separation, cognitive flexibility, forgetting and reversal learning. It has been proposed that deficits in AHN might impair normal DG function, including pattern separation and cognitive flexibility, which could play a role on the etiology of affective disorders, such as depression, anxiety and post-traumatic stress disorder (PTSD). In this paper, we review recent scientific evidence suggesting that impairment of AHN may underlie the pathophysiology of affective disorders even in humans and that neurogenesis-inspired therapies may be a promising approach to reduce symptoms of affective disorders in humans.

Future-Orientation (as Trait and State) Promotes Reputation-Protective Choice in Moral Dilemmas

By neglecting the lifelong importance of having a good reputation, humans can profit in the short run from immoral behavior. Thus, reputation-protection is an intertemporal choice. In three preregistered studies ( N = 1,492 Americans), we tested the hypothesis that future-orientation would increase people’s willingness to protect their reputation from harm. In hypothetical scenarios, people had to choose whether they would pay an immediate cost to prevent a devastating rumor from spreading. Study 1 found a positive correlation between future-orientation and reputation-protection. Study 2 manipulated time perspective experimentally and found that future focus (vs. present focus) had a positive and causal effect on reputation-protection. Study 3 was a high-powered replication, showing that the effect of future focus on reputation-protective choice was robust and mediated by reputational concern. In line with common advice around the world of protecting one’s reputation from harm, one way people may achieve that goal is to think about the future before acting.

Voluntary restrictions on self-reliance increase cooperation and mitigate wealth inequality

Humans are considered a highly cooperative species. Through cooperation, we can tackle shared problems like climate change or pandemics and cater for shared needs like shelter, mobility, or healthcare. However, cooperation invites free-riding and can easily break down. Maybe because of this reason, societies also enable individuals to solve shared problems individually, like in the case of private healthcare plans or private retirement planning. Such "self-reliance" allows individuals to avoid problems related to public goods provision, like free-riding or underprovision, and decreases social interdependence. However, not everyone can equally afford to be self-reliant, and amid shared problems, self-reliance may lead to conflicts within groups on how to solve shared problems. In two preregistered studies, we investigate how the ability of self-reliance influences collective action and cooperation. We show that self-reliance crowds out cooperation and exacerbates inequality, especially when some heavily depend on collective action while others do not. However, we also show that groups are willing to curtail their ability of self-reliance. When given the opportunity, groups overwhelmingly vote in favor of abolishing individual solutions to shared problems, which, in turn, increases cooperation and decreases inequality, particularly between group members that differ in their ability to be self-reliant. The support for such endogenously imposed interdependence, however, reduces when individual solutions become more affordable, resonating with findings of increased individualism in wealthier societies and suggesting a link between wealth inequality and favoring individual independence and freedom over communalism and interdependence.

Timing Uncertainty in Collective Risk Dilemmas Encourages Group Reciprocation and Polarization

Social dilemmas are often shaped by actions involving uncertain returns only achievable in the future, such as climate action or voluntary vaccination. In this context, uncertainty may produce non-trivial effects. Here, we assess experimentally — through a collective risk dilemma — the effect of timing uncertainty, i.e. how uncertainty about when a target needs to be reached affects the participants' behaviors. We show that timing uncertainty prompts not only early generosity but also polarized outcomes, where participants' total contributions are distributed unevenly. Furthermore, analyzing participants' behavior under timing uncertainty reveals an increase in reciprocal strategies. A data-driven game-theoretical model captures the self-organizing dynamics underpinning these behavioral patterns. Timing uncertainty thus casts a shadow on the future that leads participants to respond early, whereas reciprocal strategies appear to be important for group success. Yet, the same uncertainty also leads to inequity and polarization, requiring the inclusion of new incentives handling these societal issues.

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Timing uncertainty influences experimental observations in the collective risk game

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It induces subjects to contribute earlier and in a polarized manner

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Successful players adopt reciprocal strategies, responding in kind to past actions

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Coordination gets more difficult under high timing uncertainty

Self-reliance crowds out group cooperation and increases wealth inequality

Humans establish public goods to provide for shared needs like safety or healthcare. Yet, public goods rely on cooperation which can break down because of free-riding incentives. Previous research extensively investigated how groups solve this free-rider problem but ignored another challenge to public goods provision. Namely, some individuals do not need public goods to solve the problems they share with others. We investigate how such self-reliance influences cooperation by confronting groups in a laboratory experiment with a safety problem that could be solved either cooperatively or individually. We show that self-reliance leads to a decline in cooperation. Moreover, asymmetries in self-reliance undermine social welfare and increase wealth inequality between group members. Less dependent group members often choose to solve the shared problem individually, while more dependent members frequently fail to solve the problem, leaving them increasingly poor. While self-reliance circumvents the free-rider problem, it complicates the governing of the commons.

Cooperation among humans is threatened by the free-rider problem. Here the authors identify another challenge to human cooperation: self-reliance, the ability to solve shared problems individually. The experiment reveals that self-reliance crowds out cooperation and increases wealth inequality.

A new paradigm for Prosocial Behavior and Reciprocity, assessed in WT and HET rats for the DAT gene

Most behavioral studies on animals focus on observation of individual subjects. Current paradigms of sociability set aside the social-operant dimension, i.e. acting in favor of another conspecific. We focused on prosocial behavior and reciprocity of male, adult Wild-Type (WT) and Heterozygous (HET) rats for the dopamine-transporter (DAT) gene. METHOD: The experiment consisted of 24 rats, of WT (n = 12) and HET (n = 12) genotypes. During training, rats were daily introduced, individually, into an apparatus hosting a suspended syringe, which they learnt to push in order to obtain food therein. Then, twice daily along several weeks, we introduced two rats separated by a grid in the same structure: by syringe-pushing, each subject had the opportunity to donate and receive donations of food. We tested pairs with similar versus different genotype. Eventually, we replaced food reward with polystyrene pieces, to understand if they pushed for actual reward or like a habit. RESULTS: In general, WT rats had better performance, regardless of reward type, than HET ones. When we crossed partner rats' genotype (WT-HET pairs), WT rats pushed at peak levels, regardless of food pellet received back (in fact, HET companions pushed less). Couples of WT rats achieved better results than HET ones even when polystyrene, instead of food, was used. Thus,WT rats seem to be a better model for altruistic behavior than HET ones. For this reason, HET rats could represent a model for studies on altered prosocial behavior, to understand the role of DAT gene for impaired social mechanisms.

How food brings us together: The ties between attachment and food behaviors

Interpersonal food behaviors, such as food sharing and food offering, are associated with closeness between people and thought to facilitate the formation and strengthening of social bonds. Despite the importance of food and food-related behavior in general and for social bonding specifically, there is relatively little research about the psychological underpinnings of food-related behavior within relationship science. To fill this gap, we conducted three studies focusing on food behaviors and attachment style. In Study 1, we found attachment avoidance to be negatively associated with engagement in food sharing behaviors, and with a lower likelihood of dating a potential partner due to the partner's food preferences. In Study 2, we found that enhancing attachment security increased the tendency to offer one's food to a fellow participant. In Study 3, we found that the tendency to accept food from a fellow participant was positively associated with attachment anxiety, but security priming did not affect this tendency. Implications for attachment, close relationships, and food sciences are discussed.

Can Development Programs Shape Cooperation? : Results from a Framed Field Experiment in Indonesia

Empirical studies among small-scale societies show that participation in national development programs impact traditional norms of community cooperation. We explore the extent to which varying levels of village and individual involvement in development policies relate to voluntary cooperation within community settings. We used a field experiment conducted in seven villages (208 participants) from an indigenous society in Indonesia known for their strong traditional cooperative norms, the Punan Tubu. We framed the experiment in terms of an ongoing government house-building program. The results indicate that there were synergistic and antagonistic interactions between existing cooperative norms and government development policies. Participants' cooperation in the experimental setting was low, probably because the Punan Tubu are used to cooperating and sharing both under demand and in a context in which uncooperative behavior is largely unpunished. Variation in experimental behavior was related to both village- and individual-level variables, with participants living in resettlement villages and participants living in a house constructed under the government program displaying more cooperative behavior. The cooperation evident in resettled villages may indicate that people in these villages are more comfortable interacting in anonymous settings and less committed to the demand-sharing norms still prevalent in the upstream villages. The more cooperative behavior among villagers who have previously received a house might indicate that they recognize that they are now better off than others and feel more obliged to cooperate. Policies aiming to capitalize on existing cooperative behavior to stimulate community collective action should consider the specific conditions under which cooperation occurs in real settings since traditional norms that regulate cooperative behavior might not translate well to cooperation in government-led programs.

Snapshots of human anatomy, locomotion, and behavior from Late Pleistocene footprints at Engare Sero, Tanzania

Fossil hominin footprints preserve data on a remarkably short time scale compared to most other fossil evidence, offering snapshots of organisms in their immediate ecological and behavioral contexts. Here, we report on our excavations and analyses of more than 400 Late Pleistocene human footprints from Engare Sero, Tanzania. The site represents the largest assemblage of footprints currently known from the human fossil record in Africa. Speed estimates show that the trackways reflect both walking and running behaviors. Estimates of group composition suggest that these footprints were made by a mixed-sex and mixed-age group, but one that consisted of mostly adult females. One group of similarly-oriented trackways was attributed to 14 adult females who walked together at the same pace, with only two adult males and one juvenile accompanying them. In the context of modern ethnographic data, we suggest that these trackways may capture a unique snapshot of cooperative and sexually divided foraging behavior in Late Pleistocene humans.

The many faces of obligation

My response to the commentaries focuses on four issues: (1) the diversity both within and between cultures of the many different faces of obligation; (2) the possible evolutionary roots of the sense of obligation, including possible sources that I did not consider; (3) the possible ontogenetic roots of the sense of obligation, including especially children's understanding of groups from a third-party perspective (rather than through participation, as in my account); and (4) the relation between philosophical accounts of normative phenomena in general - which are pitched as not totally empirical - and empirical accounts such as my own. I have tried to distinguish comments that argue for extensions of the theory from those that represent genuine disagreement.

Long-term evidence on cooperation and cultural differences in public goods dilemmas

It is hard to isolate human cooperation features outside the prevalent experimental laboratory context in a controlled setting. Since cooperation rates are very context-sensitive, a new setting-a public goods game in an online game-is analysed. This unique dataset addresses multiple critical issues: it is more realistic, individuals have an intrinsic motivation to play, there are no observer effects, and data are available for more than 18 000 individuals playing over 10 months. Participants come from 10 countries, which allows us to test for cultural influences on cooperation. We find small differences in cooperation rates between countries, ranging from 8.5% (Argentina) to 14.1% (Greece). Moreover, cooperation remains stable over long periods of time. Different cultural, economic and religious backgrounds do not seem to have a discernible influence on cooperation rates. Instead, individual differences seem to play a larger role. Finally, cooperation levels may be lower than suggested by experiments from the classical laboratory context.

Hunter-Gatherers and Human Evolution: New Light on Old Debates

One of the most persistent debates in anthropology and related disciplines has been over the relative weight of aggression and competition versus nonaggression and cooperation as drivers of human behavioral evolution. The literature on hunting and gathering societies—past and present—has played a prominent role in these debates. This review compares recent literature from both sides of the argument and evaluates how accurately various authors use or misuse the ethnographic and archaeological research on hunters and gatherers. Whereas some theories provide a very poor fit with the hunter-gatherer evidence, others build their arguments around a much fuller range of the available data. The latter make a convincing case for models of human evolution that place at their center cooperative breeding and child-rearing, as well as management of conflict, flexible land tenure, and balanced gender relations.

Threat and parochialism in intergroup relations: lab-in-the-field evidence from rural Georgia

Competition between groups is widely considered to foster cooperation within groups. Evidence from laboratory experiments hints at the existence of a proximate mechanism by which humans increase their level of cooperation with their ingroup when faced with an external threat. Further work suggests that ingroup cooperation should go along with aggressive behaviour towards the outgroup, although these theories are at odds with others that see high investments in outgroup relations as important means of stabilizing intergroup relations. Surprisingly, few of these arguments have been tested in the field, and existing studies are also limited by the lack of a direct measure of threat perception and aggressive behaviour. This study presents lab-in-the-field results from a rural context where exposure to an ethnic outgroup varies between villages. This context makes it possible to capture levels of threat perception, aggressive behaviour and cooperation without inducing intergroup competition artificially in the laboratory. All concepts are measured behaviourally. In- and outgroup cooperation was measured with a standard public goods game, and a novel experimental protocol was developed that measures perceived threat and aggressive behaviour: the threat game. The results show that levels of perceived threat, ingroup cooperation and aggressive behaviour are higher in regions more strongly exposed to ethnic outsiders. However, exposed regions also show high levels of outgroup cooperation and a concomitant lack of elevated ingroup bias. This pattern is explained by theorizing that communities show parochial altruism when faced with an ethnic outgroup, but balance aggressive behaviour with cooperative offers to diffuse tensions and to keep open channels of mutually beneficial exchange.

Hunter-gatherer diets and human behavioral evolution

Human behavior and physiology evolved under conditions vastly different from those which most humans inhabit today. This paper summarizes long-term dietary studies conducted on contemporary hunter-gatherer populations (sometimes referred to as foragers). Selected studies for the most part that use evolutionary theoretical perspectives and data collection methods derived from the academic field of human behavioral ecology, which derives relatively recently from the fields of evolutionary biology, ethology, population biology and ecological anthropology. I demonstrate how this body of research illuminates ancestral patterns of food production, consumption and sharing, infant feeding, and juvenile subsistence contributions in hunter-gatherer economies. Insights from hunter-gatherer studies are then briefly discussed within the context of better-studied human populations that are Westernized, Educated, Industrialized, Rich, and Democratic (WEIRD).

Sports and the human brain: an evolutionary perspective

An evolutionary perspective helps explain a conundrum faced by sports neurologists: why is the human brain dependent on physical activity to function optimally, yet simultaneously susceptible to harm from particular types of athletics? For millions of years, human bodies and brains co-evolved to meet the physical and cognitive demands of the uniquely human subsistence strategy of hunting and gathering. Natural selection favored bodies with adaptations for endurance-based physical activity patterns, whereas brains were selected to be big and powerful to navigate the complex cultural and ecologic landscapes of hunter-gatherers. Human brains require physical activity to function optimally because their physiology evolved among individuals who were rarely able to avoid regular physical activity. Moreover, because energy from food was limited, human brains, like most energetically costly physiologic systems, evolved to require stimuli from physical activity to adjust capacity to demand. Consequently, human brains are poorly adapted to excessive physical inactivity. In addition, while brain enlargement during human evolution was vital to successful hunting and gathering, it came at the cost of a decreased ability to withstand brain accelerations and decelerations, which commonly occur during contact/collision sports.

Death Before Dishonor

Predicated on the notion that people’s survival depends greatly on participation in cooperative society, and that reputation damage may preclude such participation, four studies with diverse methods tested the hypothesis that people would make substantial sacrifices to protect their reputations. A “big data” study found that maintaining a moral reputation is one of people’s most important values. In making hypothetical choices, high percentages of “normal” people reported preferring jail time, amputation of limbs, and death to various forms of reputation damage (i.e., becoming known as a criminal, Nazi, or child molester). Two lab studies found that 30% of people fully submerged their hands in a pile of disgusting live worms, and 63% endured physical pain to prevent dissemination of information suggesting that they were racist. We discuss the implications of reputation protection for theories about altruism and motivation.

Emergence and Evolution of Cooperation Under Resource Pressure

We study the influence that resource availability has on cooperation in the context of hunter-gatherer societies. This paper proposes a model based on archaeological and ethnographic research on resource stress episodes, which exposes three different cooperative regimes according to the relationship between resource availability in the environment and population size. The most interesting regime represents moderate survival stress in which individuals coordinate in an evolutionary way to increase the probabilities of survival and reduce the risk of failing to meet the minimum needs for survival. Populations self-organise in an indirect reciprocity system in which the norm that emerges is to share the part of the resource that is not strictly necessary for survival, thereby collectively lowering the chances of starving. Our findings shed further light on the emergence and evolution of cooperation in hunter-gatherer societies.

Competition for Cooperation: variability, benefits and heritability of relational wealth in hunter-gatherers

Many defining human characteristics including theory of mind, culture and language relate to our sociality, and facilitate the formation and maintenance of cooperative relationships. Therefore, deciphering the context in which our sociality evolved is invaluable in understanding what makes us unique as a species. Much work has emphasised group-level competition, such as warfare, in moulding human cooperation and sociality. However, competition and cooperation also occur within groups; and inter-individual differences in sociality have reported fitness implications in numerous non-human taxa. Here we investigate whether differential access to cooperation (relational wealth) is likely to lead to variation in fitness at the individual level among BaYaka hunter-gatherers. Using economic gift games we find that relational wealth: a) displays individual-level variation; b) provides advantages in buffering food risk, and is positively associated with body mass index (BMI) and female fertility; c) is partially heritable. These results highlight that individual-level processes may have been fundamental in the extension of human cooperation beyond small units of related individuals, and in shaping our sociality. Additionally, the findings offer insight in to trends related to human sociality found from research in other fields such as psychology and epidemiology.

Footprints reveal direct evidence of group behavior and locomotion in Homo erectus

Bipedalism is a defining feature of the human lineage. Despite evidence that walking on two feet dates back 6-7 Ma, reconstructing hominin gait evolution is complicated by a sparse fossil record and challenges in inferring biomechanical patterns from isolated and fragmentary bones. Similarly, patterns of social behavior that distinguish modern humans from other living primates likely played significant roles in our evolution, but it is exceedingly difficult to understand the social behaviors of fossil hominins directly from fossil data. Footprints preserve direct records of gait biomechanics and behavior but they have been rare in the early human fossil record. Here we present analyses of an unprecedented discovery of 1.5-million-year-old footprint assemblages, produced by 20+ Homo erectus individuals. These footprints provide the oldest direct evidence for modern human-like weight transfer and confirm the presence of an energy-saving longitudinally arched foot in H. erectus. Further, print size analyses suggest that these H. erectus individuals lived and moved in cooperative multi-male groups, offering direct evidence consistent with human-like social behaviors in H. erectus.

Human Altruism – Proximate Patterns and Evolutionary Origins

Are people selfish or altruistic? Throughout history this question has been answered on the basis of much introspection and little evidence. It has been at the heart of many controversial debates in politics, science, and philosophy. Some of the most fundamental questions concerning our evolutionary origins, our social relations, and the organization of society are centered around issues of altruism and selfishness. Experimental evidence indicates that human altruism is a powerful force and unique in the animal world. However, there is much individual heterogeneity and the interaction between altruists and selfish individuals is key for understanding the evolutionary dynamics as well as the proximate patterns of human cooperation. Depending on the environment, a minority of altruists can force a majority of selfish individuals to cooperate or, conversely, a few egoists can induce a large number of altruists to defect. Current gene-based evolutionary theories cannot explain important patterns of human altruism pointing towards the need for theories of cultural evolution and gene-culture coevolution.

Partner choice creates fairness in humans

Many studies demonstrate that partner choice has played an important role in the evolution of human cooperation, but little work has tested its impact on the evolution of human fairness. In experiments involving divisions of money, people become either over-generous or over-selfish when they are in competition to be chosen as cooperative partners. Hence, it is difficult to see how partner choice could result in the evolution of fair, equal divisions. Here, we show that this puzzle can be solved if we consider the outside options on which partner choice operates. We conduct a behavioural experiment, run agent-based simulations and analyse a game-theoretic model to understand how outside options affect partner choice and fairness. All support the conclusion that partner choice leads to fairness only when individuals have equal outside options. We discuss how this condition has been met in our evolutionary history, and the implications of these findings for our understanding of other aspects of fairness less specific than preferences for equal divisions of resources.

Cooperative hunting roles among taï chimpanzees

All known chimpanzee populations have been observed to hunt small mammals for meat. Detailed observations have shown, however, that hunting strategies differ considerably between populations, with some merely collecting prey that happens to pass by while others hunt in coordinated groups to chase fast-moving prey. Of all known populations, Taï chimpanzees exhibit the highest level of cooperation when hunting. Some of the group hunting roles require elaborate coordination with other hunters as well as precise anticipation of the movements of the prey. The meat-sharing rules observed in this community guarantee the largest share of the meat to hunters who perform the most important roles leading to a capture. The learning time of such hunting roles is sometimes especially long. Taï chimpanzee males begin hunting monkeys at about age 10. The hunters' progress in learning the more sophisticated hunting roles is clearly correlated with age; only after 20 years of practice are they able to perform them reliably. This lengthy learning period has also been shown in some hunter-gatherer societies and confirms the special challenge that hunting represents.

Kinship, lineage, and an evolutionary perspective on cooperative hunting groups in Indonesia

Work was conducted among traditional, subsistence whale hunters in Lamalera, Indonesia, in order to test if strict biological kinship or lineage membership is more important for explaining the organization of cooperative hunting parties ranging in size from 8 to 14 men. Crew identifications were collected for all 853 hunts that occurred between May 3 and August 5, 1999. Lineage identity and genetic relatedness were determined for a sample of 189 hunters. Results of matrix regression show that genetic kinship explains little of the hunters' affiliations independent of lineage identity. Crew members are much more closely related to each other than expected by chance, but this is due to the correlation between lineage membership and genetic kinship. Lineage members are much more likely to affiliate in crews, but kin with r<0.5 are just as likely not to affiliate. The results are discussed vis-à-vis the evolution of cooperation and group identity.

Morality: An Evolutionary Account

Refinements in Darwin's theory of the origin of a moral sense create a framework equipped to organize and integrate contemporary theory and research on morality. Morality originated in deferential, cooperative, and altruistic "social instincts," or decision-making strategies, that enabled early humans to maximize their gains from social living and resolve their conflicts of interest in adaptive ways. Moral judgments, moral norms, and conscience originated from strategic interactions among members of groups who experienced confluences and conflicts of interest. Moral argumentation buttressed by moral reasoning is equipped to generate universal and impartial moral standards. Moral beliefs and standards are products of automatic and controlled information-processing and decision-making mechanisms. To understand how people make moral decisions, we must understand how early evolved mechanisms in the old brain and recently evolved mechanisms in the new brain are activated and how they interact. Understanding what a sense of morality is for helps us understand what it is.

Using multilevel models to estimate variation in foraging returns. Effects of failure rate, harvest size, age, and individual heterogeneity

Distributions of human foraging success across age have implications for many aspects of human evolution. Estimating the distribution of foraging returns is complicated by (1) the zero-inflated nature of hunting returns, as many if not most trips fail, and (2) the substantial variation among hunters, independent of age. We develop a multilevel mixture analysis of human foraging data to address these difficulties. Using a previously published 20-year record of hunts by 147 individual Aché hunters in eastern Paraguay, we estimate returns-by-age functions for both hunting failures and the size of harvests, while also estimating the heterogeneity among hunters. Consistent with previous analyses, we find that most hunters peak around 40 years of age. We can also show, however, that much more of the variation among Aché hunters arises from heterogeneity in failure rates (zero returns), not harvest sizes. We also introduce a new R package, glmer2stan, to assist in defining and fitting similar multilevel mixture models.

Hunter-gatherer inter-band interaction rates: implications for cumulative culture

Our species exhibits spectacular success due to cumulative culture. While cognitive evolution of social learning mechanisms may be partially responsible for adaptive human culture, features of early human social structure may also play a role by increasing the number potential models from which to learn innovations. We present interview data on interactions between same-sex adult dyads of Ache and Hadza hunter-gatherers living in multiple distinct residential bands (20 Ache bands; 42 Hadza bands; 1201 dyads) throughout a tribal home range. Results show high probabilities (5%–29% per year) of cultural and cooperative interactions between randomly chosen adults. Multiple regression suggests that ritual relationships increase interaction rates more than kinship, and that affinal kin interact more often than dyads with no relationship. These may be important features of human sociality. Finally, yearly interaction rates along with survival data allow us to estimate expected lifetime partners for a variety of social activities, and compare those to chimpanzees. Hadza and Ache men are estimated to observe over 300 men making tools in a lifetime, whereas male chimpanzees interact with only about 20 other males in a lifetime. High intergroup interaction rates in ancestral humans may have promoted the evolution of cumulative culture.

The development of human female competition: allies and adversaries

Throughout their lives, women provide for their own and their children's and grandchildren's needs and thus must minimize their risk of incurring physical harm. Alliances with individuals who will assist them in attaining these goals increase their probability of survival and reproductive success. High status in the community enhances access to physical resources and valuable allies. Kin, a mate, and affines share a mother's genetic interests, whereas unrelated women constitute primary competitors. From early childhood onwards, girls compete using strategies that minimize the risk of retaliation and reduce the strength of other girls. Girls' competitive strategies include avoiding direct interference with another girl's goals, disguising competition, competing overtly only from a position of high status in the community, enforcing equality within the female community and socially excluding other girls.