Biological markets and the effects of partner choice on cooperation and friendship

Feelings of obligation are valuations of signaling-mediated social payoffs

We extend Tomasello's framework by addressing the functional challenge of obligation. If the long-run social consequences of a decision are sufficiently costly, obligation motivates the actor to forgo potential immediate benefits in favor of long-term social interests. Thus, obligation psychology balances the downstream socially-mediated payoffs from a decision. This perspective can predict when and why obligation will be experienced.

Children are sensitive to reputation when giving to both ingroup and outgroup members

Previous studies establish that reputation concerns play an important role in outgroup giving. However, it is unclear whether the same is true for ingroup giving, which by some accounts tends to be motivated by empathic concerns. To explore this question, we tested the extent to which 5 to 9-year-old children (Study 1: N = 164) and adults (Study 2: N = 80) shared resources with ingroup and outgroup members, either when being watched by an observer (where we expected reputation concerns to be salient) or in private (where we expected no effect of reputation concerns). We also assessed whether children and adults differ in their beliefs about which form of sharing (ingroup or outgroup giving) is nicer. Although we found that both children and adults exhibited an ingroup bias when sharing, there was no evidence in either group that reputation concerns were greater for outgroup members than for ingroup members. We also found that, in contrast to adults, children shared more resources when observed than in private. Additionally, children evaluated ingroup giving as nicer across different sharing scenarios, whereas adults identified outgroup giving as nicer when the two forms of giving were contrasted. These results are the first to suggest that reputational concerns influence children's sharing both with ingroup and outgroup members, and that children differ from adults in their reasoning about which form of group sharing is nicer.

Being the chosen one: social inclusion modulates decisions in the ultimatum game. An ERP study

In the present study, participants played a modified ultimatum game simulating a situation of inclusion/exclusion, in which either the participant or a rival could be selected to play as the responder. This selection was made either randomly by a computer (i.e. random pairing mode) or by the proposer (i.e. choice mode), based on physical appearance. Being chosen by the proposer triggered positive reciprocal behavior in participants, who accepted unfair offers more frequently than when they had been selected by the computer. Independently of selection mode, greater P200 amplitudes were found when participants received fair offers than when they received unfair offers and when unfair shares were offered to their rivals rather than to them, suggesting that receiving fair offers or observing a rival’s misfortune was rewarding for participants. While participants generally showed more interest in the offers they themselves received (i.e. greater P300 responses to these offers), observing their rivals receive fair shares after the latter had been chosen by the proposer triggered an increase in P300 amplitude likely to reflect a feeling of envy. This study provides new insights into both the cognitive and affective processes underpinning economic decision making in a context of social inclusion/exclusion.

Elevation, an emotion for prosocial contagion, is experienced more strongly by those with greater expectations of the cooperativeness of others

A unique emotion, elevation, is thought to underlie prosocial contagion, a process whereby witnessing a prosocial act leads to acting prosocially. Individuals differ in their propensity to experience elevation, and thus their proneness to prosocial contagion, but little is known about the causes of such variation. We introduce an adaptationist model wherein elevation marks immediate circumstances in which generalized prosociality is advantageous, with this evaluation of circumstances hinging in part on prior expectations of others' prosociality. In 15 studies, we add to evidence that elevation can reliably be elicited and mediates prosocial contagion. Importantly, we confirm a novel prediction-generated by our adaptationist account-that an idealistic attitude, which indexes others' expected degree of prosociality, moderates the relationship between exposure to prosocial cues and experiencing elevation. We discuss how our findings inform both basic theorizing in the affective sciences and translational efforts to engineer a more harmonious world, and we offer future research directions to further test and extend our model.

Large-scale cooperation driven by reputation, not fear of divine punishment

Reputational considerations favour cooperation and thus we expect less cooperation in larger communities where people are less well known to each other. Some argue that institutions are, therefore, necessary to coordinate large-scale cooperation, including moralizing religions that promote cooperation through the fear of divine punishment. Here, we use community size as a proxy for reputational concerns, and test whether people in small, stable communities are more cooperative than people in large, less stable communities in both religious and non-religious contexts. We conducted a donation game on a large naturalistic sample of 501 people in 17 communities, with varying religions or none, ranging from small villages to large cities in northwestern China. We found that more money was donated by those in small, stable communities, where reputation should be more salient. Religious practice was also associated with higher donations, but fear of divine punishment was not. In a second game on the same sample, decisions were private, giving donors the opportunity to cheat. We found that donors to religious institutions were not less likely to cheat, and community size was not important in this game. Results from the donation game suggest donations to both religious and non-religious institutions are being motivated by reputational considerations, and results from both games suggest fear of divine punishment is not important. This chimes with other studies suggesting social benefits rather than fear of punishment may be the more salient motive for cooperative behaviour in real-world settings.

Power in economic games

Economic games offer an analytic tool to examine strategic decision-making in social interactions. Here we identify four sources of power that can be captured and studied with economic games - asymmetric dependence, the possibility to reduce dependence, the ability to punish and reward, and the use of knowledge and information. We review recent studies examining these distinct forms of power, highlight that the use of economic games can benefit our understanding of the behavioral and neurobiological underpinnings of power, and illustrate how power differences within and between groups impact cooperation, exploitation, and conflict.

Why cooperation is not running away

A growing number of experimental and theoretical studies show the importance of partner choice as a mechanism to promote the evolution of cooperation, especially in humans. In this paper, we focus on the question of the precise quantitative level of cooperation that should evolve under this mechanism. When individuals compete to be chosen by others, their level of investment in cooperation evolves towards higher values, a process called competitive altruism, or runaway cooperation. Using a classic adaptive dynamics model, we first show that when the cost of changing partner is low, this runaway process can lead to a profitless escalation of cooperation. In the extreme, when partner choice is entirely frictionless, cooperation even increases up to a level where its cost entirely cancels out its benefit. That is, at evolutionary equilibrium, individuals gain the same payoff than if they had not cooperated at all. Second, importing models from matching theory in economics we, however, show that when individuals can plastically modulate their choosiness in function of their own cooperation level, partner choice stops being a runaway competition to outbid others and becomes a competition to form the most optimal partnerships. In this case, when the cost of changing partner tends towards zero, partner choice leads to the evolution of the socially optimum level of cooperation. This last result could explain the observation that human cooperation seems to be often constrained by considerations of social efficiency.

Biological market effects predict cleaner fish strategic sophistication

Market-like situations emerge in nature when trading partners exchange goods and services. However, how partner choice option contributes to the expression of social strategic sophistication (i.e., the ability to adjust behavior flexibly given the specifics of a situation) is still poorly understood. A suitable study system to explore this question is the “cleaner” fish Labroides dimidiatus. Cleaners trade parasite removal in exchange for food with a variety of “client” species. Previous research documented strong interindividual variation in two features of their strategic sophistication, namely, the ability to adjust service quality to the presence of an audience and to give priority to clients with access to alternative cleaners (“visitor clients”) over clients lacking such choice options (“resident clients”). Here, we sampled various demes (i.e., group of individuals) of the same population of cleaner fish in order to investigate the extent to which factors describing fish densities and cleaning interaction patterns predict the strategic sophistication in two laboratory experiments. These experiments tested whether cleaners could increase their food intake through reputation management and/or learning to provide service priority to a visitor-like ephemeral food plate. We found that high “outbidding competition,” characterized by high densities of cleaners and visitor clients, along with visitor’s behavior promoting such competition, consistently predicted high strategic sophistication in cleaners. A better understanding of the role of learning versus potential genetic factors, interacting with local market conditions to affect strategic sophistication, is needed to clarify how natural selection has promoted the evolution and maintenance of cooperation in this cleaning mutualism.

Common knowledge, coordination, and strategic mentalizing in human social life

Humans are an unusually cooperative species, and our cooperation is of 2 kinds: altruistic, when actors benefit others at a cost to themselves, and mutualistic, when actors benefit themselves and others simultaneously. One major form of mutualism is coordination, in which actors align their choices for mutual benefit. Formal examples include meetings, division of labor, and legal and technological standards; informal examples include friendships, authority hierarchies, alliances, and exchange partnerships. Successful coordination is enabled by common knowledge: knowledge of others’ knowledge, knowledge of their knowledge of one’s knowledge, ad infinitum. Uncovering how people acquire and represent the common knowledge needed for coordination is thus essential to understanding human sociality, from large-scale institutions to everyday experiences of civility, hypocrisy, outrage, and taboo.

People often coordinate for mutual gain, such as keeping to opposite sides of a stairway, dubbing an object or place with a name, or assembling en masse to protest a regime. Because successful coordination requires complementary choices, these opportunities raise the puzzle of how people attain the common knowledge that facilitates coordination, in which a person knows X, knows that the other knows X, knows that the other knows that he knows, ad infinitum. We show that people are highly sensitive to the distinction between common knowledge and mere private or shared knowledge, and that they deploy this distinction strategically in diverse social situations that have the structure of coordination games, including market cooperation, innuendo, bystander intervention, attributions of charitability, self-conscious emotions, and moral condemnation.

Incidental ostracism emerges from simple learning mechanisms

Ostracism, or social exclusion, is widespread and associated with a range of detrimental psychological and social outcomes. Ostracism is typically explained as instrumental punishment of free-riders or deviants. However, this instrumental account fails to explain many of the features of real-world ostracism, including its prevalence. Here we hypothesized that ostracism can emerge incidentally (non-instrumentally) when people choose partners in social interactions, and that this process is driven by simple learning mechanisms. We tested this hypothesis in four experiments (n = 456) with economic games in dynamic social networks. Contrary to the instrumental account of ostracism, we find that the targets of ostracism are not primarily free-riders. Instead, incidental initial variability in choosing partners for social interactions predicts later ostracism better than the instrumental account. Using computational modelling, we show that simple reinforcement learning mechanisms explain the incidental emergence of ostracism, and that they do so better than a formalization of the instrumental account. Finally, we leveraged these reinforcement learning mechanisms to experimentally reduce incidental ostracism. Our results demonstrate that ostracism is more incidental than previously assumed and can arise from basic forms of learning. They also show that the same mechanisms that result in incidental ostracism can help to reduce its emergence.

It is not all about mating: Attractiveness predicts partner value across multiple relationship domains

An account of the "beauty premium" based only on mating motivations overlooks adaptationist models of social valuation that have broader explanatory power. We suggest a broader approach based on evolved preferences for attractive partners in multiple cooperative domains (not just mating), which accounts for many observations of attractiveness-based preferential treatment more comfortably than does the target article's mating-specific account.

Hunter-Gatherers Maintain Assortativity in Cooperation despite High Levels of Residential Change and Mixing

Widespread cooperation is a defining feature of human societies from hunter-gatherer bands to nation states [1, 2], but explaining its evolution remains a challenge. Although positive assortment of cooperators is recognized as a basic requirement for the evolution of cooperation, the mechanisms governing assortment are debated. Moreover, the social structure of modern hunter-gatherers, characterized by high mobility, residential mixing, and low genetic relatedness [3], undermines assortment and adds to the puzzle of how cooperation evolved. Here, we analyze four years of data (2010, 2013, 2014, 2016) tracking residence and levels of cooperation elicited from a public goods game in Hadza hunter-gatherers of Tanzania. Data were collected from 56 camps, comprising 383 unique individuals, 137 of whom we have data for two or more years. Despite significant residential mixing, we observe a robust pattern of assortment that is necessary for cooperation to evolve; in every year, Hadza camps exhibit high between-camp and low within-camp variation in cooperation. We find little evidence that cooperative behavior within individuals is stable over time or that similarity in cooperation between dyads predicts their future cohabitation. Both sets of findings are inconsistent with models that assume stable cooperative and selfish types, including partner choice models. Consistent with social norms, culture, and reciprocity theories, the strongest predictor of an individual's level of cooperation is the mean cooperation of their current campmates. These findings underscore the adaptive nature of human cooperation-particularly its responsiveness to social contexts-as a feature that is important in generating the assortment necessary for cooperation to evolve.

Bottlenose dolphins can understand their partner's role in a cooperative task

In recent decades, a number of studies have examined whether various non-human animals understand their partner's role in cooperative situations. Yet the relatively tolerant timing requirements of these tasks make it theoretically possible for animals to succeed by using simple behavioural strategies rather than by jointly intended coordination. Here we investigated whether bottlenose dolphins could understand a cooperative partner's role by testing whether they could learn a button-pressing task requiring precise behavioural synchronization. Specifically, members of cooperative dyads were required to swim across a lagoon and each press their own underwater button simultaneously (within a 1 s time window), whether sent together or with a delay between partners of 1–20 s. We found that dolphins were able to work together with extreme precision even when they had to wait for their partner, and that their coordination improved over the course of the study, with the time between button presses in the latter trials averaging 370 ms. These findings show that bottlenose dolphins can learn to understand their partner's role in a cooperative situation, and suggest that the behavioural synchronization evident in wild dolphins' synchronous movement and coordinated alliance displays may be a generalized cognitive ability that can also be used to solve novel cooperative tasks.

Relational mobility predicts social behaviors in 39 countries and is tied to historical farming and threat

Biologists and social scientists have long tried to understand why some societies have more fluid and open interpersonal relationships and how those differences influence culture. This study measures relational mobility, a socioecological variable quantifying voluntary (high relational mobility) vs. fixed (low relational mobility) interpersonal relationships. We measure relational mobility in 39 societies and test whether it predicts social behavior. People in societies with higher relational mobility report more proactive interpersonal behaviors (e.g., self-disclosure and social support) and psychological tendencies that help them build and retain relationships (e.g., general trust, intimacy, self-esteem). Finally, we explore ecological factors that could explain relational mobility differences across societies. Relational mobility was lower in societies that practiced settled, interdependent subsistence styles, such as rice farming, and in societies that had stronger ecological and historical threats.

Bottlenose Dolphins Retain Individual Vocal Labels in Multi-level Alliances

Cooperation between allied individuals and groups is ubiquitous in human societies, and vocal communication is known to play a key role in facilitating such complex human behaviors [1, 2]. In fact, complex communication may be a feature of the kind of social cognition required for the formation of social alliances, facilitating both partner choice and the execution of coordinated behaviors [3]. As such, a compelling avenue for investigation is what role flexible communication systems play in the formation and maintenance of cooperative partnerships in other alliance-forming animals. Male bottlenose dolphins in some populations form complex multi-level alliances, where individuals cooperate in the pursuit and defense of an important resource: access to females [4]. These strong relationships can last for decades and are critical to each male's reproductive success [4]. Convergent vocal accommodation is used to signal social proximity to a partner or social group in many taxa [5, 6], and it has long been thought that allied male dolphins also converge onto a shared signal to broadcast alliance identity [5-8]. Here, we combine a decade of data on social interactions with dyadic relatedness estimates to show that male dolphins that form multi-level alliances in an open social network retain individual vocal labels that are distinct from those of their allies. Our results differ from earlier reports of signature whistle convergence among males that form stable alliance pairs. Instead, they suggest that individual vocal labels play a central role in the maintenance of differentiated relationships within complex nested alliances.

High voice pitch mitigates the aversiveness of antisocial cues in men's speech

Speech contains both explicit social information in semantic content and implicit cues to social behaviour and mate quality in voice pitch. Voice pitch has been demonstrated to have pervasive effects on social perceptions, but few studies have examined these perceptions in the context of meaningful speech. Here, we examined whether male voice pitch interacted with socially relevant cues in speech to influence listeners' perceptions of trustworthiness and attractiveness. We artificially manipulated men's voices to be higher and lower in pitch when speaking words that were either prosocial or antisocial in nature. In Study 1, we found that listeners perceived lower-pitched voices as more trustworthy and attractive in the context of prosocial words than in the context of antisocial words. In Study 2, we found evidence that suggests this effect was driven by stronger preferences for higher-pitched voices in the context of antisocial cues, as voice pitch preferences were not significantly different in the context of prosocial cues. These findings suggest that higher male voice pitch may ameliorate the negative effects of antisocial speech content and that listeners may be particularly avoidant of those who express multiple cues to antisociality across modalities.

Why Hate the Good Guy? Antisocial Punishment of High Cooperators Is Greater When People Compete To Be Chosen

When choosing social partners, people prefer good cooperators (all else being equal). Given this preference, people wishing to be chosen can either increase their own cooperation to become more desirable or suppress others’ cooperation to make them less desirable. Previous research shows that very cooperative people sometimes get punished (“antisocial punishment”) or criticized (“do-gooder derogation”) in many cultures. Here, we used a public-goods game with punishment to test whether antisocial punishment is used as a means of competing to be chosen by suppressing others’ cooperation. As predicted, there was more antisocial punishment when participants were competing to be chosen for a subsequent cooperative task (a trust game) than without a subsequent task. This difference in antisocial punishment cannot be explained by differences in contributions, moralistic punishment, or confusion. This suggests that antisocial punishment is a social strategy that low cooperators use to avoid looking bad when high cooperators escalate cooperation.

Broadening the role of "self-interest" in folk-economic beliefs

We extend Boyer & Petersen's (B&P's) model of folk-economic beliefs (FEBs) by suggesting FEBs serve self-interest (broadly defined), which includes indirect benefits such as creating alliances, advancing self-beneficial ideologies, and signaling one's traits. By expanding the definition of self-interest, the model can predict who will hold what FEBs, which FEBs will propagate, when they will change, why, and in which direction.

Folk-economic beliefs: An evolutionary cognitive model

The domain of "folk-economics" consists in explicit beliefs about the economy held by laypeople, untrained in economics, about such topics as, for example, the causes of the wealth of nations, the benefits or drawbacks of markets and international trade, the effects of regulation, the origins of inequality, the connection between work and wages, the economic consequences of immigration, or the possible causes of unemployment. These beliefs are crucial in forming people's political beliefs and in shaping their reception of different policies. Yet, they often conflict with elementary principles of economic theory and are often described as the consequences of ignorance, irrationality, or specific biases. As we will argue, these past perspectives fail to predict the particular contents of popular folk-economic beliefs and, as a result, there is no systematic study of the cognitive factors involved in their emergence and cultural success. Here we propose that the cultural success of particular beliefs about the economy is predictable if we consider the influence of specialized, largely automatic inference systems that evolved as adaptations to ancestral human small-scale sociality. These systems, for which there is independent evidence, include free-rider detection, fairness-based partner choice, ownership intuitions, coalitional psychology, and more. Information about modern mass-market conditions activates these specific inference systems, resulting in particular intuitions, for example, that impersonal transactions are dangerous or that international trade is a zero-sum game. These intuitions in turn make specific policy proposals more likely than others to become intuitively compelling, and, as a consequence, exert a crucial influence on political choices.

Evidence for strategic cooperation in humans

Humans may cooperate strategically, cooperating at higher levels than expected from their short-term interests, to try and stimulate others to cooperate. To test this hypothesis, we experimentally manipulated the extent an individual's behaviour is known to others, and hence whether or not strategic cooperation is possible. In contrast with many previous studies, we avoided confounding factors by preventing individuals from learning during the game about either pay-offs or about how other individuals behave. We found clear evidence for strategic cooperators-just telling some individuals that their groupmates would be informed about their behaviour led to them tripling their initial level of cooperation, from 17 to 50%. We also found that many individuals play as if they do not understand the game, and their presence obscures the detection of strategic cooperation. Identifying such players allowed us to detect and study strategic motives for cooperation in novel, more powerful, ways.

Attractiveness biases are the tip of the iceberg in biological markets

Physical attractiveness affects how one gets treated, but it is just a single component of one's overall "market value." One's treatment depends on other markers of market value, including social status, competence, warmth, and any other cues of one's ability or willingness to confer benefits on partners. To completely understand biased treatment, we must also incorporate these other factors.

Prosocial Orientation Alters Network Dynamics and Fosters Cooperation

Dynamic networks have been shown to increase cooperation, but prior findings are compatible with two different mechanisms for the evolution of cooperation. It may be that dynamic networks promote cooperation even in networks composed entirely of egoists, who strategically cooperate to attract and maintain profitable interaction partners. Alternatively, drawing on recent insights into heterogeneous social preferences, we expect that dynamic networks will increase cooperation only when nodes are occupied by persons with more prosocial preferences, who tend to attract and keep more cooperative partners relative to egoists. Our experiment used a standard procedure to classify participants a priori as egoistic or prosocial and then embedded them in homogeneous networks of all prosocials or all egoists, or in heterogeneous networks (50/50). Participants then interacted in repeated prisoner's dilemma games with alters in both static and dynamic networks. In both heterogeneous and homogeneous networks, we find dynamic networks only promote cooperation among prosocials. Resulting from their greater cooperation, prosocials’ relations are more stable, yielding substantially higher fitness compared to egoists in both heterogeneous and homogeneous dynamic networks. Our results suggest that a key to the evolution and stability of cooperation is the ability of those with prosocial preferences to alter their networks.

People Are More Generous to a Partner Who Pays Attention to Them

People use relatively low-cost signals to maintain close relationships, in which they engage in costlier exchanges of tangible support. Paying attention to a partner allows an individual to communicate his or her interest in the relationship with the partner. Previous studies have revealed that when Person A pays attention to Person B, B's feeling of intimacy toward A increases. If social attention strengthens the bond between A and B, it is predicted that A's attention will also increase B's generous behavior toward A. This study tested this prediction. Participants first engaged in a collaborative task using computers. In the task, the putative partner (a computer program) either paid or did not pay attention to participants (high attention condition vs. low attention condition). In the control condition, the partner could not choose when to pay attention to participants. They then played three rounds of the dictator game with the partner. Confirming the previous finding, perceived intimacy was highest in the high attention condition, in the middle in the control condition, and lowest in the low attention condition. More importantly, participants in the high attention condition decided to give more resources to their partner than those in the low attention condition (but the difference between the high attention condition and the control condition was not significant). In addition, self-reported intimacy was positively correlated with the resource allocated to the partner. The results of this study demonstrated that social attention fosters a partner's generosity.

Preferential interactions promote blind cooperation and informed defection

It is common sense that costs and benefits should be carefully weighed before deciding on a course of action. However, we often disapprove of people who do so, even when their actual decision benefits us. For example, we prefer people who directly agree to do us a favor over those who agree only after securing enough information to ensure that the favor will not be too costly. Why should we care about how people make their decisions, rather than just focus on the decisions themselves? Current models show that punishment of information gathering can be beneficial because it forces blind decisions, which under some circumstances enhances cooperation. Here we show that aversion to information gathering can be beneficial even in the absence of punishment, due to a different mechanism: preferential interactions with reliable partners. In a diverse population where different people have different-and unknown-preferences, those who seek additional information before agreeing to cooperate reveal that their preferences are close to the point where they would choose not to cooperate. Blind cooperators are therefore more likely to keep cooperating even if conditions change, and aversion to information gathering helps to interact preferentially with them. Conversely, blind defectors are more likely to keep defecting in the future, leading to a preference for informed defectors over blind ones. Both mechanisms-punishment to force blind decisions and preferential interactions-give qualitatively different predictions, which may enable experimental tests to disentangle them in real-world situations.

Exploring the trade-off between quality and fairness in human partner choice

Partner choice is an important force underpinning cooperation in humans and other animals. Nevertheless, the mechanisms individuals use to evaluate and discriminate among partners who vary across different dimensions are poorly understood. Generally, individuals are expected to prefer partners who are both able and willing to invest in cooperation but how do individuals prioritize the ability over willingness to invest when these characteristics are opposed to one another? We used a modified Dictator Game to tackle this question. Choosers evaluated partners varying in quality (proxied by wealth) and fairness, in conditions when wealth was relatively stable or liable to change. When both partners were equally fair (or unfair), choosers typically preferred the richer partner. Nevertheless, when asked to choose between a rich-stingy and a poor-fair partner, choosers prioritized fairness over wealth-with this preference being particularly pronounced when wealth was unstable. The implications of these findings for real-world partner choice are discussed.

Functional Interdependence Theory: An Evolutionary Account of Social Situations

Social interactions are characterized by distinct forms of interdependence, each of which has unique effects on how behavior unfolds within the interaction. Despite this, little is known about the psychological mechanisms that allow people to detect and respond to the nature of interdependence in any given interaction. We propose that interdependence theory provides clues regarding the structure of interdependence in the human ancestral past. In turn, evolutionary psychology offers a framework for understanding the types of information processing mechanisms that could have been shaped under these recurring conditions. We synthesize and extend these two perspectives to introduce a new theory: functional interdependence theory (FIT). FIT can generate testable hypotheses about the function and structure of the psychological mechanisms for inferring interdependence. This new perspective offers insight into how people initiate and maintain cooperative relationships, select social partners and allies, and identify opportunities to signal social motives.

Altruism predicts mating success in humans

In order for non-kin altruism to evolve, altruists must receive fitness benefits for their actions that outweigh the costs. Several researchers have suggested that altruism is a costly signal of desirable qualities, such that it could have evolved by sexual selection. In two studies, we show that altruism is broadly linked with mating success. In Study 1, participants who scored higher on a self-report altruism measure reported they were more desirable to the opposite sex, as well as reported having more sex partners, more casual sex partners, and having sex more often within relationships. Sex moderated some of these relationships, such that altruism mattered more for men's number of lifetime and casual sex partners. In Study 2, participants who were willing to donate potential monetary winnings (in a modified dictator dilemma) reported having more lifetime sex partners, more casual sex partners, and more sex partners over the past year. Men who were willing to donate also reported having more lifetime dating partners. Furthermore, these patterns persisted, even when controlling for narcissism, Big Five personality traits, and socially desirable responding. These results suggest that altruists have higher mating success than non-altruists and support the hypothesis that altruism is a sexually selected costly signal of difficult-to-observe qualities.

The Relative State Model: Integrating Need-Based and Ability-Based Pathways to Risk-Taking

Who takes risks, and why? Does risk-taking in one context predict risk-taking in other contexts? We seek to address these questions by considering two non-independent pathways to risk: need-based and ability-based. The need-based pathway suggests that risk-taking is a product of competitive disadvantage consistent with risk-sensitivity theory. The ability-based pathway suggests that people engage in risk-taking when they possess abilities or traits that increase the probability of successful risk-taking, the expected value of the risky behavior itself, and/or have signaling value. We provide a conceptual model of decision-making under risk-the relative state model-that integrates both pathways and explicates how situational and embodied factors influence the estimated costs and benefits of risk-taking in different contexts. This model may help to reconcile long-standing disagreements and issues regarding the etiology of risk-taking, such as the domain-generality versus domain-specificity of risk or differential engagement in antisocial and non-antisocial risk-taking.

Biological trade and markets

Cooperation between organisms can often be understood, like trade between merchants, as a mutually beneficial exchange of services, resources or other 'commodities'. Mutual benefits alone, however, are not sufficient to explain the evolution of trade-based cooperation. First, organisms may reject a particular trade if another partner offers a better deal. Second, while human trade often entails binding contracts, non-human trade requires unwritten 'terms of contract' that 'self-stabilize' trade and prevent cheating even if all traders strive to maximize fitness. Whenever trading partners can be chosen, market-like situations arise in nature that biologists studying cooperation need to account for. The mere possibility of exerting partner choice stabilizes many forms of otherwise cheatable trade, induces competition, facilitates the evolution of specialization and often leads to intricate forms of cooperation. We discuss selected examples to illustrate these general points and review basic conceptual approaches that are important in the theory of biological trade and markets. Comparing these approaches with theory in economics, it turns out that conventional models-often called 'Walrasian' markets-are of limited relevance to biology. In contrast, early approaches to trade and markets, as found in the works of Ricardo and Cournot, contain elements of thought that have inspired useful models in biology. For example, the concept of comparative advantage has biological applications in trade, signalling and ecological competition. We also see convergence between post-Walrasian economics and biological markets. For example, both economists and biologists are studying 'principal-agent' problems with principals offering jobs to agents without being sure that the agents will do a proper job. Finally, we show that mating markets have many peculiarities not shared with conventional economic markets. Ideas from economics are useful for biologists studying cooperation but need to be taken with caution.