Evolution and Human Behavior最新文献

Friendship is an important aspect of children's social lives. However, little is known about how it influences children's fairness behavior towards their peers. We tested (N = 183) pairs of children between 7 and 9 years of age from rural communities in India, Peru and Canada that are known to have divergent norms of fairness. Participants were paired with either a close friend or an acquaintance and could accept or reject different allocations of valuable resources. We experimentally compared children's responses to disadvantageous allocations (less for self) and advantageous allocations (more for self). Results showed that across the three societies children were more likely to reject disadvantageous allocations when they were paired with a friend relative to an acquaintance. When they stood to gain a relative advantage, children in Canada and to some extent Peru were more likely to reject advantageous allocations with friends, yet children in India rejected these offers rarely regardless of who they were paired with. These findings suggest that friendship may shape the expression of fairness concerns in young children, though its influence varies across societies.

Acquiring knowledge and understanding of social relationships is a key task for the human child. Methods for studying social networks are constrained by pragmatic issues of time and informant accuracy. Here we report results from a study focused specifically on how measures of cognitive ability – fluid intelligence and working memory – are associated with children's perceptions of their social networks assessed by a consecutive pile-sort method. This focused study is part of a long-term research project on family relationships and child health in a rural Caribbean community that provides comparative ethnographic depth. Our general objective here is to better understand relations between cognition and children's understanding of their social worlds. Our secondary objective is to assess an intersection of research methodologies from ethnography and cognitive psychology. Results demonstrated that measures of children's working memory capacity, but not of fluid reasoning ability, associated positively with accurate reconstruction of their social networks--a finding with potential implications for the co-evolution of human sociality and cognitive abilities.

Hadza food-sharing is extremely generous and often extends to individuals outside the household. Some anthropologists have proposed that individuals, especially men, share food beyond the household in order to signal foraging skill. While correlational data have been used to both evidence and critique this hypothesis, it has less often been experimentally tested. Here, we conducted an incentivised experiment to test whether Hadza adults are indeed willing to forgo caloric resources in order to signal their foraging skills. In this study, 196 Hadza adults were given the opportunity to participate in two games - an aim game and a search game - designed to advertise their skill as foragers. We varied the incentive structure of both games, adjusting i) whether there was a caloric cost (i.e., honey) to play, and ii) whether success in each game was rewarded with a prize (i.e., a colored bracelet), which functioned as a visible signal of skill. Although the aim game was universally popular when there was no cost to play, we found that individuals generally valued calories more than signaling opportunities and were unwilling to forgo caloric resources to continue participation in either game. In line with signaling theory, we did observe age and gender difference in willingness to wager calories for signaling opportunities. Men were more likely than women to forgo calories in order to participate. Younger people (<37), especially younger men, were also more likely to forgo calories to play than older people.

Climatic shocks and economic insecurity challenge the wellbeing of livestock managers, globally. Scholars argue that ranchers pursue different economic strategies (herd composition and uses) because of the effects of variation in wealth on risk preferences. However, intergenerational wealth transfers and experiences of loss could also explain these outcomes. There are no tests comparing which of these interpretations more closely align with decisions ranchers employ. Accordingly, we examine how ranchers from rural Baja California Sur, Mexico adjust herd compositions and uses across varying economic (i.e. land security) and environmental conditions (i.e. drought vs non-drought years). Our results indicate 1) both socio-economic condition and intergenerational transfers are associated with herd composition – people on secure land and whose parents ranched cattle have more cattle, 2) herd composition influences consumption patterns – people focusing on goat production eat a greater percentage of their livestock relative to those with cattle regardless of ecological condition, 3) socio-economic variation influences sales and maintenance under normal ecological conditions – people living on secure land place proportionally more livestock into sales, while the land insecure focus on maintenance, and 4) experience with drought-induced livestock losses, but not land security, explains variation in how people respond to an ecological shock – those experiencing larger losses place greater effort in keeping herds alive rather than sales, suggesting they become risk averse. Our results indicate that socio-economic variability influences risk preferences under benign ecological conditions; however, these preferences are flexible in the face of economic losses.

For many abilities, such as vision or language, our conscious experience is one of simplicity: We open our eyes and the world appears; we open our mouths and grammatical sentences tumble out. Yet these abilities rely on immensely complex, unconscious computations. Is this also true of abilities related to cooperation or competition, like deciding whether to share food or spread gossip? We tested whether decisions like these are guided by precise psychological variables, called welfare tradeoff ratios. Welfare tradeoff ratios summarize information about multiple sources of social value (such as whether a specific other person is kin or is generous with the self) along with information about the situation (such as what's at stake or who else is watching). We evaluated these hypothesized variables in four societies: among college students in the USA and Argentina and among two groups of Amazonian forager-horticulturalists, the Shuar of Ecuador and the Tsimane of Bolivia (ns = 167, 131, 73, 23). In all societies people made a series of hypothetical decisions where they had to weigh help or harm for themselves versus others. We found strong evidence that people trade off their welfare for others with consistency—a signature of decisions being guided by precise variables in the mind. We also found evidence in three of the societies that people discriminate among different categories of others in their welfare tradeoffs (e.g., friends versus acquaintances). Although most decisions about helping or harming feel simple and intuitive, they appear to be underwritten by precise computations.

Friendship is a recurring feature of human sociality. Extant evidence has highlighted several axes upon which the formation and maintenance of friendships rest, and has emphasised the importance of market-like mechanisms and preferential assortment in such dynamics. Such evidence has emerged from qualitative ethnographic descriptions, and observational or experimental case studies in relatively homogeneous samples from Western and industrialised settings. Here, we provide one of the first empirical evaluations of the structure of friendship networks in a rural subsistence setting. We collected individual-level friendship network data, and detailed economic and demographic information from individuals in four communities in rural Colombia (N = 470). We analyse these data using a combined social relations and stochastic block model. Our results highlight the importance of preferential assortment on the basis of several socio-demographic traits in all study communities. The extent to which friendship and social support networks overlap appears to vary considerably across communities, with greater overlap being observed in more impoverished areas. Similarly, the extent of wealth homophily was greater in more impoverished areas. Such findings suggest that variation in the axes upon which friendship rests may be affected by community-level variation in economic and demographic composition.

Cooperative resource sharing is widespread across cultures, and it was likely critical during much of human evolutionary history for pooling risk. Need-based sharing specifically pools risk by following two cooperative rules: help others when asked, and only request help when in need. In a two-part study, we first expanded an agent-based model of need-based sharing partnerships, adding two types of defection and varying partnership sizes. We show that refusing to help always has a long-term cost, which increases with larger partnerships. In contrast, “greedy” requests that are not based on survival risk carry little-to-no cost. We then conducted an experimental vignette study of osotua, a need-based sharing tradition, with Tanzanian Maasai pastoralists. We found that participants generally complied with osotua requests, but shared larger amounts for requests that were based on survival risk. We conclude by proposing an expanded framework for evolutionary models involving need and fitness interdependence, where the cost asymmetry among types of defection generally favors a decision heuristic where individuals prefer sharing with those in need, but err on the side of generosity when need is uncertain.

In small-scale societies hunting is a high-risk, high-reward activity which impacts status and reproductive success. The question of whether men hunt to provision families or as a costly signal of their phenotypic qualities has been hotly debated in the anthropological literature. To shed new light on this question, we explored audience assessments of a hunter's phenotypic quality and desirability as a function of the composition of prey acquired by the hunter. A combination of ranking and forced-choice tasks were administered to 52 informants (46% female, aged 15–76 years) from the Aché hunter-gatherer tribe of Paraguay between May and July of 2015. Ratings of a hunter's provisioning ability, strength, fighting ability, disease resistance, and desirability as a mate or ally were all positively associated with killing large and hard-to-kill prey, and negatively associated with killing hard-to-find prey. However, killing a single large animal resulted in a worse assessment of hunter phenotype and desirability than killing an equivalent biomass of small animals. These findings highlight the potential of small prey hunting as a mechanism for advertising both quality and consistent provisioning ability. Critically, no conflict was observed between the goal of advertising quality/desirability and the goal of effective provisioning, since hunters who acquired more meat, even if the source of the meat was small game, were generally perceived as having better phenotypes and as more desirable.

Theoretical models of gossip's role in the evolution of cooperation in ancestral human communities, and its role in within-group competition for resources, require gossip to cause changes in individuals' reputations, which then cause changes in the likelihood of their receiving benefits. However, there is scant experimental evidence from small-scale societies supporting such causal relationships. There is also little experimental evidence that, when making decisions about the transfer of resources, gossip receivers weigh gossip according to its relevance to the social context in which such transfers occur. Using an experimental vignette study design, in a sample from MTurk (N = 120) and another sample from a remote horticultural population, the Ngandu of the Central African Republic (CAR) (N = 160), we test whether positive and negative gossip increase and decrease the likelihood of transferring resources, respectively, mediated by their effects on reputation. We also test whether gossip that is relevant to the context of the resource transfer has a larger impact on reputation than other gossip. We found strong significant, context-relevant effects of gossip on participant willingness to transfer benefits, mediated by gossip's effects on reputation. Then, in an exploratory observational study of Aka hunter-gatherers of CAR using peer-reports (N = 40), we investigate whether providing benefits to the group (such as working hard, parenting or alloparenting, or sharing) and genetic relatedness to the group, were associated with reputations and receiving benefits. We found that, although having a good reputation was associated with receiving more benefits, there was a stark sex difference, with almost all women scoring higher than almost all men on a dimension involving better parenting, good reputations, and receipt of more benefits.