Shane J. Macfarlan

Sex Differences in Cooperation: A Meta-Analytic Review of Social Dilemmas

Although it is commonly believed that women are kinder and more cooperative than men, there is conflicting evidence for this assertion. Current theories of sex differences in social behavior suggest that it may be useful to examine in what situations men and women are likely to differ in cooperation. Here, we derive predictions from both sociocultural and evolutionary perspectives on context-specific sex differences in cooperation, and we conduct a unique meta-analytic study of 272 effect sizes-sampled across 50 years of research-on social dilemmas to examine several potential moderators. The overall average effect size is not statistically different from zero (d = -0.05), suggesting that men and women do not differ in their overall amounts of cooperation. However, the association between sex and cooperation is moderated by several key features of the social context: Male-male interactions are more cooperative than female-female interactions (d = 0.16), yet women cooperate more than men in mixed-sex interactions (d = -0.22). In repeated interactions, men are more cooperative than women. Women were more cooperative than men in larger groups and in more recent studies, but these differences disappeared after statistically controlling for several study characteristics. We discuss these results in the context of both sociocultural and evolutionary theories of sex differences, stress the need for an integrated biosocial approach, and outline directions for future research.

Lethal coalitionary aggression and long-term alliance formation among Yanomamö men

Significance Humans, like chimpanzees, engage in coalitionary violence: Members of both species coordinate lethal activity against conspecifics. The origin and adaptive functions of this behavior are poorly understood, and data from tribal populations are rare. We examine the composition of lethal coalitions from the Yanomamö, a tribal society in Amazonia. In contrast to chimpanzees, Yanomamö coalitions are composed of individuals from different lineages and natal communities. Many coalition partners are ideal marriage exchange partners. Men who kill together more often are more likely to live together in the same village later in life and to engage in marriage exchange. Our results highlight connections between coalitionary aggression and alliance formation and illuminate differences in social structure distinguishing humans from other primates. Some cross-cultural evidence suggests lethal coalitionary aggression in humans is the product of residence and descent rules that promote fraternal interest groups, i.e., power groups of coresident males bonded by kinship. As such, human lethal coalitions are hypothesized to be homologous to chimpanzee (Pan troglodytes) border patrols. However, humans demonstrate a unique metagroup social structure in which strategic alliances allow individuals to form coalitions transcending local community boundaries. We test predictions derived from the fraternal interest group and strategic alliance models using lethal coalition data from a lowland South American population, the Yanomamö. Yanomamö men who kill an enemy acquire a special status, termed unokai. We examine the social characteristics of co-unokais or men who jointly kill others. Analyses indicate co-unokais generally are (i) from the same population but from different villages and patrilines, (ii) close age mates, and (iii) maternal half-first cousins. Furthermore, the incident rate for co-unokai killings increases if men are similar in age, from the same population, and from different natal communities. Co-unokais who have killed more times in the past and who are more genetically related to each other have a higher probability of coresidence in adulthood. Last, a relationship exists between lethal coalition formation and marriage exchange. In this population, internal warfare unites multiple communities, and co-unokais strategically form new residential groups and marriage alliances. These results support the strategic alliance model of coalitionary aggression, demonstrate the complexities of human alliance formation, and illuminate key differences in social structure distinguishing humans from other primates.

Kinship, family, and gender effects in the ultimatum game

Kinship and reciprocity are two main predictors of altruism. The ultimatum game has been used to study altruism in many small-scale societies. We used the ultimatum game to examine effects of individuals’ family and kin relations on altruistic behavior in a kin-based horticultural community in rural Dominica. Results show sex-specific effects of kin on ultimatum game play. Average coefficient of relatedness to the village was negatively associated with women’s ultimatum game proposals and had little effect on men’s proposals. Number of brothers in the village was positively associated with men’s ultimatum game proposals and negatively associated with women’s proposals. Similarly, presence of father in the village was associated with higher proposals by men and lower proposals by women. We interpret the effect of brothers on men’s proposals as a consequence of local competition among brothers. We speculate that daughter-biased parental care in this community creates a sense of entitlement among women with brothers, which may explain the inverse relation between number of brothers and women’s ultimatum game proposals. The pattern of results may be consistent with how matrifocality affects cultural models of fairness differently along gender and family lines.

Competitive Altruism Explains Labor Exchange Variation in a Dominican Community

Smallholder farmers rely on labor exchange to generate agricultural work when cash is rare and credit unavailable. Reciprocal altruism, biased by genetic kinship, has been implicated as the mechanism responsible for labor exchange; however, few empirical tests confirm this proposition. Competitive altruism could be operating if people differ in ability and use this information as a criterion for partnership selection. Labor exchange data are presented from a Dominican smallholder village over a 10-month period within the village’s primary cash economic opportunity, bay oil production. Results indicate that competitive altruism better explains variation in labor exchange relationships and group size than reciprocal altruism and kinship, suggesting the presence of a biologic market for male exchange relationships. Bay oil laborers vary in altruistic behaviors, causing reputations for altruism to emerge. Men with reputations as high-quality altruists generate larger labor groups in bay oil production than do poor-quality ones. Larger groups induce bargaining wars, causing men to compete through altruistic acts, which allows high-quality individuals to discriminate potential partners for labor exchange relationships. Men with better reputations achieve more same-sex reciprocal partnerships but not a greater incidence of conjugal partnership, suggesting that male altruism is intra- but not intersexually selected.

Anointing variation across wild capuchin populations: a review of material preferences, bout frequency and anointing sociality in Cebus and Sapajus

The frequency of anointing bouts and the materials used for self‐ and social anointing vary across capuchin species in captivity, but there is little published data on capuchin anointing in the wild. Here we present previously unpublished data on anointing behaviors from capuchin monkey populations at ten different field sites and incorporate these data into a review of the anointing literature for captive and wild capuchins. Using a comparative phylogenetic framework, we test four hypotheses derived primarily from captive literature for variation in anointing between wild untufted capuchins (Cebus) and tufted capuchins (Sapajus), including that (1) the frequency of anointing is higher in Cebus, (2) Cebus uses a higher proportion of plant species to insect species for anointing compared with Sapajus, (3) anointing material diversity is higher in Cebus, and (4) social indices of anointing are higher in Cebus. We found that wild Cebus anoints more with plant parts, including fruits, whereas wild Sapajus anoints more with ants and other arthropods. Cebus capucinus in particular uses more plant species per site for anointing compared with other capuchins and may specialize in anointing as an activity independent from foraging, whereas most other capuchin species tend to eat the substances they use for anointing. In agreement with captive studies, we found evidence that wild Cebus anoints at a significantly higher frequency than Sapajus. However, contrary to the captive literature, we found no difference in the range of sociality for anointing between Cebus and Sapajus in the wild. We review anointing in the context of other Neotropical primate rubbing behaviors and consider the evidence for anointing as self‐medication; as a mechanism for enhanced sociality; and as a behavioral response to chemical stimuli. Am. J. Primatol. 74:299–314, 2012.

Cooperative behaviour and prosocial reputation dynamics in a Dominican village

Prosocial reputations play an important role, from the evolution of language to Internet transactions; however, questions remain about their behavioural correlates and dynamics. Formal models assume prosocial reputations correlate with the number of cooperative acts one performs; however, if reputations flow through information networks, then the number of individuals one assists may be a better proxy. Formal models demonstrate indirect experience must track behaviour with the same fidelity as direct experience for reputations to become viable; however, research on corporate reputations suggests performance change does not always affect reputation change. Debate exists over the cognitive mechanisms employed for assessing reputation dynamics. Image scoring suggests reputations fluctuate relative to the number of times one fails to assist others in need, while standing strategy claims reputations fluctuate relative to the number of times one fails to assist others in good standing. This study examines the behavioural correlates of prosocial reputations and their dynamics over a 20-month period in an Afro-Caribbean village. Analyses suggest prosocial reputations: (i) are correlated with the number of individuals one assists in economic production, not the number of cooperative acts; (ii) track cooperative behaviour, but are anchored across time; and (iii) are captured neither by image scoring nor standing strategy-type mechanisms.

Multiple reputation domains and cooperative behaviour in two Latin American communities

Reputations are a ubiquitous feature of human social life, and a large literature has been dedicated to explaining the relationship between prosocial reputations and cooperation in social dilemmas. However, humans form reputations in domains other than prosociality, such as economic competency that could affect cooperation. To date, no research has evaluated the relative effects of multiple reputation domains on cooperation. To bridge this gap, we analyse how prosocial and competency reputations affect cooperation in two Latin American communities (Bwa Mawego, Dominica, and Pucucanchita, Peru) across a number of social contexts (Dominica: labour contracting, labour exchange and conjugal partnership formation; Peru: agricultural and health advice network size). First, we examine the behavioural correlates of prosocial and competency reputations. Following, we analyse whether prosocial, competency, or both reputation domains explain the flow of cooperative benefits within the two communities. Our analyses suggest that (i) although some behaviours affect both reputation domains simultaneously, each reputation domain has a unique behavioural signature; and (ii) competency reputations affect cooperation across a greater number of social contexts compared to prosocial reputations. Results are contextualized with reference to the social markets in which behaviour is embedded and a call for greater theory development is stressed.

Why Do Humans Help Their Friends? Proximal and Ultimate Hypotheses from Evolutionary Theory

Humans frequently cultivate friendships, or enduring and selective social bonds in which partners help each other in times of need, often at great cost to themselves. In recent years, researchers have proposed several evolutionary explanations for the increased helping and sharing among close friends. In this chapter, we review how these evolutionary explanations answer two questions: (1) what proximate psychological mechanisms or cognitive algorithms do people use in choosing to bestow exceptional aid to friends? (2) What evolutionary challenges are likely selected for this tendency to help friends? We then assess how these hypotheses fare in the light of existing evidence from psychology, anthropology, and biology on how close friends help each other and how people cultivate friendships.

The Dual-Role Method and Ultimatum Game Performance

The dual-role method (DRM) extracts subject preferences as two players in experimental economic games. Previous research indicates mixed effects of DRM on game performance. A two (role)-by-two (role order) design for the ultimatum game (UG) was implemented in a naturalistic setting across 2 years. Subjects played according to the role they were assigned (proposer or responder). Immediately after, they played as the other role. Meta-analyses show no mean effect of role order on performance across studies or between laboratory and naturalistic settings. Power analyses indicate sample sizes needed to find an effect using the DRM is larger than many small-scale populations.

Social Evolution: The Force of the Market

Biological market forces shape patterns of cooperation typical of small-scale human societies that are organized by division of labor based on age and gender. Labor specialization promotes trade, while supply and demand affect the amount individuals exchange for commodities.