Cooperation is widespread and arguably a pivotal evolutionary force in maintaining animal societies. Yet, proximately, what underlying motivators drive individuals to cooperate remains relatively unclear. Since \'free-riders\' can exploit the benefits by cheating, selecting the right partner is paramount. Such decision rules need not be based on complex calculations and can be driven by cognitively less-demanding mechanisms, like social relationships (e.g. kinship, non-kin friendships, dyadic tolerance), social status (e.g. dominance hierarchies) and personalities (social and non-social traits); however, holistic evidence related to those mechanisms is scarce. Using the classical \'loose-string paradigm\', we tested cooperative tendencies of a hierarchical primate, the long-tailed macaque (Macaca fascicularis). We studied three groups (n = 21) in their social settings, allowing partner choice. We supplemented cooperation with observational and experimental data on social relationships, dominance hierarchies and personality. Friendship and dissimilarities in non-social \'exploration\' and \'activity-sociability\' personality traits predicted the likelihood of cooperative dyad formation. Furthermore, the magnitude of cooperative success was positively associated with friendship, low rank-distance and dissimilarity in the activity-sociability trait. Kinship did not affect cooperation. While some findings align with prior studies, the evidence of (non-social) personality heterophily promoting cooperation may deepen our understanding of the proximate mechanisms and, broadly, the evolution of cooperation.

In human cooperation, people prefer to choose partners with high willingness and ability-while both are valued by partners, individuals often prioritize willingness. Two event-related potential (ERP) experiments were conducted to discern the neural processes underpinning this preference. In the first experiment, participants made a choice between two potential partners and received feedback on the selected partner\'s willingness to cooperate. This was followed by feedback on the partner\'s task performance (ability) or a gambling outcome. In contrast, the second experiment first provided feedback on ability, then presented feedback on willingness or a gambling outcome. This study revealed that a potential partner\'s willingness trait significantly influences individuals\' emotional evaluations and monetary allocations than the ability trait. Electrophysiological data indicated that low-willingness feedback elicited a diminished feedback-related negative (FRN) and an amplified P3 compared to high-willingness feedback. In contrast, no such difference was discernible between high- and low-ability feedback. Moreover, the P3 difference from high versus low willingness was considerably more pronounced than that from gambling outcomes, whereas the difference wave between high and low ability paralleled gambling outcomes. These findings bolster the novel finding that partner willingness may provide more substantial social rewards than ability. Furthermore, this study provides the first ERP evidence of willingness and ability trait perceptions in partner choice decisions.

Despite increasing diversity within many societies, ethnically endogamous unions remain common. In contexts where one ethnic minority has lived alongside the majority for centuries, understanding who partners with whom is central to understanding how ethnic boundaries are maintained or dissolved. This study examines the role of own and parental ethnolinguistic affiliation for the first partner choice in Finland. We provide a unique test of the relevance of ethnic endogamy across two generations, in a context where both groups are native, but one (Finnish speakers) overwhelmingly outnumbers the other (Swedish speakers). Using register data on the total population, we examine how a person\'s ethnolinguistic affiliation and background affect the choice of the first cohabiting partner in terms of the partner\'s ethnolinguistic affiliation and background. We apply discrete-time competing risk models for men and women born 1970-1983. Results indicate that Swedish-registered individuals with two Swedish-registered parents are, by far, the most likely to partner with another Swedish-registered person with endogamous background. Partnering with a Swedish-registered person with exogamous background is most likely among individuals who themselves come from mixed unions. Patterns are remarkably consistent across gender, and adjustments for education and residential area only marginally alter the results.

Many fungi that form ectomycorrhizas exhibit a degree of host specialisation, and individual trees are frequently colonised by communities of mycorrhizal fungi comprising species that fall on a gradient of specialisation along genetic, functional and taxonomic axes of variation. By contrast, arbuscular mycorrhizal fungi exhibit little specialisation. Here, we propose that host tree root morphology is a key factor that gives host plants fine-scale control over colonisation and therefore opportunities for driving specialisation and speciation of ectomycorrhizal fungi. A gradient in host specialisation is likely driven by four proximate mechanistic \'filters\' comprising partner availability, signalling recognition, competition for colonisation, and symbiotic function (trade, rewards and sanctions), and the spatially restricted colonisation seen in heterorhizic roots enables these mechanisms, especially symbiotic function, to be more effective in driving the evolution of specialisation. We encourage manipulation experiments that integrate molecular genetics and isotope tracers to test these mechanisms, alongside mathematical simulations of eco-evolutionary dynamics in mycorrhizal symbioses.

Prudently choosing who to interact with and who to avoid is an important ability to ensure that we benefit from a cooperative interaction. While the role of others\' preferences, attributes, and values in partner choice have been established (Rossetti, Hilbe & Hauser, 2022), much less is known about whether the manner in which a potential partner plans and implements a decision provides helpful cues for partner choice. We used a partner choice paradigm in which participants chose who to interact with in the Prisoners\' Dilemma. Before choosing a cooperation partner, participants were presented with information about the potential partners\' decision-related actions in another round of the Prisoners\' Dilemma. They received either information about the potential partners\' planning during decision making (i.e., decision-time; Experiment 1) or action execution during decision implementation (i.e., movement directness; Experiment 2). Across both games, participants preferred to interact with those who planned actions quickly or executed actions with direct and smooth movements, indicating that they were cooperating confidently and without deliberation. This demonstrates that action cues present in either the planning or implementation of economic decisions influence partner choice. We discuss implications of this finding for human decision-making and perception-action coupling in action understanding.

Why does human prosociality vary around the world? Evolutionary models and laboratory experiments suggest that possibilities for partner choice (i.e. the ability to leave unprofitable relationships and strike up new ones) should promote cooperation across human societies. Leveraging the Global Preferences Survey (n = 27,125; 27 countries) and the World Values Survey (n = 54,728; 32 countries), we test this theory by estimating the associations between relational mobility, a socioecological measure of partner choice, and a wide variety of prosocial attitudes and behaviours, including impersonal altruism, reciprocity, trust, collective action and moral judgements of antisocial behaviour. Contrary to our pre-registered predictions, we found little evidence that partner choice is related to prosociality across countries. After controlling for shared causes of relational mobility and prosociality - environmental harshness, subsistence style and geographic and linguistic proximity - we found that only altruism and trust in people from another religion are positively related to relational mobility. We did not find positive relationships between relational mobility and reciprocity, generalised trust, collective action or moral judgements. These findings challenge evolutionary theories of human cooperation which emphasise partner choice as a key explanatory mechanism, and highlight the need to generalise models and experiments to global samples.

Performing costly helpful behaviours can allow individuals to improve their reputation. Those who gain a good reputation are often preferred as interaction partners and are consequently better able to access support through cooperative relationships with others. However, investing in prosocial displays can sometimes yield social costs: excessively generous individuals risk losing their good reputation, and even being vilified, ostracised or antisocially punished. As a consequence, people frequently try to downplay their prosocial actions or hide them from others. In this review, we explore when and why investments in prosocial behaviour are likely to yield social costs. We propose two key features of interactions that make it more likely that generous individuals will incur social costs when: (a) observers infer that helpful behaviour is motivated by strategic or selfish motives; and (b) observers infer that helpful behaviour is detrimental to them. We describe how the cognition required to consider ulterior motives emerges over development and how these tendencies vary across cultures - and discuss how the potential for helpful actions to result in social costs might place boundaries on prosocial behaviour as well as limiting the contexts in which it might occur. We end by outlining the key avenues and priorities for future research.

Microfinance is an economic development tool that provides loans to low-income borrowers to stimulate economic growth and reduce financial hardship. Lenders typically require joint liability, where multiple borrowers share the responsibility of repaying a group loan. We propose that this lending practice creates a cooperation dilemma similar to that faced by humans and other organisms in nature across many domains. This could offer a real-world test case for evolutionary theories of cooperation from the biological sciences. In turn, such theories could provide new insights into loan repayment behaviour. We first hypothesise how group loan repayment efficacy should be affected by mechanisms of assortment from the evolutionary literature on cooperation, i.e. common ancestry (kin selection), prior interaction (reciprocity), partner choice, similarity of tags, social learning, and ecology and demography. We then assess selected hypotheses by reviewing 41 studies from 32 countries on micro-borrowers\' loan repayment, evaluating which characteristics of borrowers are associated with credit repayment behaviour. Surprisingly, we find that kinship is mostly negatively associated with repayment efficacy, but prior interaction and partner choice are both more positively associated. Our work highlights the scope of evolutionary theory to provide systematic insight into how humans respond to novel economic institutions and interventions.

Reputation is a powerful driver of human behavior. Reputation systems incentivize \'actors\' to take reputation-enhancing actions, and \'evaluators\' to reward actors with positive reputations by preferentially cooperating with them. This article proposes a reputation framework that centers the perspective of evaluators by suggesting that reputation systems can create two fundamentally different incentives for evaluators to reward positive reputations. Evaluators may be pulled towards \'good\' actors to benefit directly from their reciprocal cooperation, or pushed to cooperate with such actors by normative pressure. I discuss how psychology and behavior might diverge under pull versus push mechanisms, and use this framework to deepen our understanding of the empirical reputation literature and suggest ways that we may better leverage reputation for social good.

The body size of an animal plays a crucial role in determining its trophic level and position within the food web, as well as its interactions with other species. In the symbiosis between Termitomyces and fungus-growing termites, termites rely on nutrition of fungal nodules produced by Termitomyces. To understand whether the size of termites and fungal nodules are related to their partner specificity, we quantified the size of termite farmer caste, and the size and density of nodules in termite nests of four genera of fungus-growing termites, and identified their cultivated Termitomyces fungus species based on internal transcribed spacer regions and partial large subunit ribosomal RNA gene sequences. The results showed that the size and density of fungal nodules were different among Termitomyces clades and revealed a constant trade-off between size and density among clades. The nodule size of each clade has low variation and fits normal distribution, indicating that size is a stabilized trait. Moreover, we found larger termite genera cultivated Termitomyces with larger but less numerous nodules. Based on these results, we concluded that there is a size specificity between Termitomyces and fungus-growing termites, which may lead to diversification of Termitomyces as adaptations to different termite genera.