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Human cooperation varies both across and within societies, and developmental studies can inform our understanding of the sources of both kinds of variation. One key candidate for explaining within-society variation in cooperative behaviour is gender, but we know little about whether gender differences in cooperation take root early in ontogeny or emerge similarly across diverse societies. Here, we explore two existing cross-cultural datasets of 4- to 15-year-old children's preferences for equality in experimental tasks measuring prosociality (14 societies) and fairness (seven societies), and we look for evidence of (i) widespread gender differences in the development of cooperation, and (ii) substantial societal variation in gender differences. This cross-cultural approach is crucial for revealing universal human gender differences in the development of cooperation, and it helps answer recent calls for greater cultural diversity in the study of human development. We find that gender has little impact on the development of prosociality and fairness within these datasets, and we do not find much evidence for substantial societal variation in gender differences. We discuss the implications of these findings for our knowledge about the nature and origin of gender differences in cooperation, and for future research attempting to study human development using diverse cultural samples. This article is part of the theme issue ‘Cooperation among women: evolutionary and cross-cultural perspectives’.

Recent work suggests that an important cognitive mechanism promoting coordination is common knowledge—a heuristic for representing recursive mental states. Yet, we know little about how common knowledge promotes coordination. We propose that common knowledge increases coordination by reducing uncertainty about others’ cooperative behavior. We examine how common knowledge increases cooperation in the context of a threshold public goods game, a public good game in which a minimum level of contribution—a threshold—is required. Across three preregistered studies (N = 5,580), we explored how varying (1) the information participants had regarding what their group members knew about the threshold and (2) the threshold level affected contributions. We found that participants were more likely to contribute to the public good when there was common knowledge of the threshold than private knowledge. Participants’ predictions about the number of group members contributing to the public good and their certainty ratings of those predictions mediated the effect of information condition on contributions. Our results suggest that common knowledge of the threshold increases public good contributions by reducing uncertainty around other people’s cooperative behavior. These findings point to the influential role of common knowledge in helping to solve large-scale cooperation problems.

Judgments surrounding resource acquisition and valuation are ubiquitous in daily life. How do humans decide what something is worth to themselves or someone else? One important cue to value is that of resource quantity. As described by economists, the principle of diminishing marginal utility (DMU) holds that as resource abundance increases, the value placed on each unit decreases; likewise, when resources become more scarce, the value placed on each unit rises. While prior research suggests that adults make judgments that align with this concept, it is unclear whether children do so. In Study 1 (n = 104), children (ages 5 through 8) were presented with scenarios involving losses or gains to others’ resources and predicted the actions and emotions of the individuals involved. Participants made decisions that aligned with DMU, e.g., expecting individuals with fewer resources to expend more effort for an additional resource than individuals with greater resources. In Study 2 (n = 104), children were capable of abandoning a quantity-based approach to resource valuation when it was appropriate to do so, showing the capacity to incorporate information about preferences when inferring others’ resource valuations. Our results indicate the early emergence of an intuitive economic theory that aligns with an important economic principle. Long before formal learning on this topic, children integrate quantity and preference information to sensibly predict others’ resource valuations, with implications for economic decision-making, social preferences, and judgments of partner quality across the lifespan.

Interpersonal trust is a key component of cooperation, helping support the complex social networks found across societies. Trust typically involves two parties, one who trusts by taking on risk through investment in a second party, who can be trustworthy and produce mutual benefits. To date, the developmental literature has focused primarily on the trustor, meaning we know little about the ontogeny of trustworthiness. Whereas trusting can be motivated by self-interest, one-shot trustworthiness is more squarely situated in the prosocial domain, involving a direct tradeoff between self-interest and others' interests. However, this raises the question of whether trustworthiness is distinct from generosity. In this preregistered study, we examine the origins of trustworthiness using an intuitive version of the Trust Game, in which a first party invests resources in a second party who can split the gains. We recruited N = 118 5-to-8 year-old American children (Mage = 6.94, n = 59 girls, 57% White, 88% of parents with bachelor's degree or higher), split between the Trustworthiness condition, where another party's investment is instrumental for obtaining greater resources, and the Generosity condition, where the other party is a passive recipient. We found that children in the Trustworthiness condition shared significantly more resources than those in the Generosity condition. Further, children in the Trustworthiness condition predicted that the first party expected them to share a greater number of resources. Overall, these results demonstrate that trustworthiness is distinct from generosity in childhood and suggest that children spontaneously grasp and engage in a key aspect of cooperation. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

In the United States, there is an unfortunate yet pervasive gender gap in wages: Women tend to make less than men for doing the same work. One prominent account for why this wage gap exists is that women and men negotiate differently. However, we currently do not know whether differences in negotiation are a product of extensive experience or are deeply rooted in development. Here, we brought data from children to bear on this important question. We gave 240 children between the ages of 4 and 9 years old a chance to negotiate for a bonus with a female or a male evaluator. Boys asked for the same bonus from a male and a female evaluator. Older girls, in contrast, asked for a smaller bonus from a male than a female evaluator. Our findings suggest that a gender gap in negotiation emerges surprisingly early in development, highlighting childhood as a key period for interventions.

There are two broad functional explanations for second-party punishment: fitness-leveling and deterrence. The former suggests that people punish to reduce fitness differences, while the latter suggests that people punish in order to reciprocate losses and deter others from inflicting losses on them in the future. We explore the relative roles of these motivations using a pre-registered, two-player experiment with 2426 US participants from Amazon Mechanical Turk. Participants played as the “responder” and were assigned to either a Take or Augment condition. In the Take condition, the “partner” could steal money from the responder's bonus or do nothing. In the Augment condition, the partner could augment the responder's bonus by giving them money at no cost to themselves or do nothing. We also manipulated the responders' starting endowments, such that after the partner's decision, responders experienced different payoff outcomes: advantageous inequity, equality, or varying degrees of disadvantageous inequity. Responders then decided whether to pay a cost to punish the partner. Punishment was clearly influenced by theft and was most frequent when theft resulted in disadvantageous inequity. However, people also punished in the absence of theft, particularly when confronted with disadvantageous inequity. While the effect of inequity on punishment was small, our results suggest that punishment is motivated by more than just the desire to reciprocate losses. These findings highlight the multiple motivations undergirding punishment and bear directly on functional explanations for the existence of punishment in human societies.

Human adults use a range of social cues to obtain information about potential partners in cooperative contexts: we prefer partners who are competent, wealthy and generous, and those who abide by moral and social rules. One factor that carries particular weight is whether a prospective partner is fair. Here we ask whether children share this preference for fair partners and, if so, whether a prospective partner’s past fair behaviour influences children’s behaviour in a cooperative dilemma. Six- to nine-year-olds chose between partners who accepted or rejected resource allocations that were either strongly advantageously unequal, strongly disadvantageously unequal, or equal. They then played a one-shot Prisoner’s Dilemma Game with their chosen partner. Children overwhelmingly preferred to play with the partner who accepted rather than rejected allocations. Regardless of their partner choice decisions, children tended to defect in the Prisoner’s Dilemma Game, yet expected that their partners would be relatively more cooperative. Finally, children were more likely to cooperate with those they believed would cooperate. Together, these findings shed new light on the links between partner choice, fairness and cooperation in child development.

Risk and time preferences have often been viewed as reflecting inherent traits such as impatience and self-control. Here, we offer an alternative perspective, arguing that they are flexible and environmentally informed. In Study 1, we investigated risk and time preferences among children in the United States, India, and Argentina, as well as forager-horticulturalist Shuar children in Amazonian Ecuador. We find striking cross-cultural differences in behavior: children in India, the United States, and Argentina are more risk-seeking and future-oriented, whereas Shuar children are more risk-averse and exhibit more heterogeneous time preferences, on average preferring more today choices. To explore 1 of the socioecological forces that may be shaping these preferences, in Study 2, we compared the behavior of more and less market-integrated Shuar children, finding that those in market-integrated regions are more future-oriented and risk-seeking. These findings indicate that cross-cultural differences in risk and time preferences can be traced into childhood and may be influenced by the local environment. More broadly, our results contribute to a growing understanding of plasticity and variation in the development of behavior. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

The coronavirus pandemic has had a significant influence on social interactions, introducing novel social norms such as mask-wearing and social distancing to protect people's health. Because these norms and associated practices are completely novel, it is unknown how children assess what kinds of interventions are appropriate under what circumstances and what principles they draw on in their decisions. We investigated children's reasoning about interventions against individuals who failed to adhere to COVID-19 norms. In this pre-registered study (N = 128), 4- to 7-year-olds heard stories about a norm violator, that is, a person who refuses to wear a mask in class (COVID condition) or wear indoor shoes in class when his or her shoes are muddy (Muddy Shoes condition). Children evaluated four different interventions-giving a mask/indoor shoes (Giving), preventing the person from entering (Exclusion), throwing a paper ball at the person (Throwing), and not intervening (Doing Nothing)-in terms of their rightness, niceness, and effectiveness. We found that across measures children evaluated Giving most positively, whereas they viewed Throwing most negatively. Doing Nothing and Exclusion received mixed evaluations across measures, revealing nuanced judgments of these interventions in children. In most measures, there was no difference between the COVID and Muddy Shoes conditions, suggesting that children's evaluations are not specific to the novel COVID-19 context. Together, our results show that children dynamically evaluate each intervention, taking multiple factors into account. The current study has implications for the development of interventions against norm violations.

Can children exploit knowledge asymmetries to get away with selfishness? This question was addressed by testing 6- to 9-year-old children (N = 164; 81 girls) from the Northeastern United States in a modified Ultimatum Game. Children were assigned to the roles of proposers (who offered some proportion of an endowment) and responders (who could accept or reject offers). Both players in the Informed condition knew the endowment quantity in each trial. However, in the Uninformed condition, only proposers knew this information. In this condition, many proposers made "strategically selfish" offers that seemed fair based on the responders' incomplete knowledge but were actually highly selfish. These results indicate that even young children possess the ability to deceive others about their selfishness.

Early in human development, children react negatively to receiving less than others, and only later do they show a similar aversion to receiving more. We tested whether theory of mind (ToM) can account for this developmental shift we see in middle childhood. We conducted a face-to-face fairness task that involved a ToM manipulation, measured individual differences in ToM, and collected parent-ratings of children’s empathy, a construct related to ToM. We find that greater ToM capacities lead to more rejections of unequal offers, regardless of the direction of inequality, demonstrating that children with greater ToM are more likely to engage in costly compliance with fairness norms. Moreover, drawing attention to mental states sufficiently elicits aversion to advantageous inequity in younger children. These findings contribute to our growing understanding that people’s concerns for fairness rely not just on their own thoughts and beliefs but on the thoughts, beliefs, and expectations of others.

In the context of economic games, adults sacrifice money to avoid unequal outcomes, showing so-called inequity aversion. Child-friendly adaptations of these games have shown that children, too, show inequity aversion. Moreover, inequity aversion shows a clear developmental trajectory, with young children rejecting only disadvantageously unequal distributions and older children rejecting both disadvantageously and advantageously unequal distributions. However, based on existing work, it is difficult to compare adult and child responses to inequity because (1) adapting economic games to make them child-friendly may importantly alter the dynamics of the fairness interaction and (2) adult work typically uses abstract rewards such as money while work with children typically uses more concrete rewards like candy, stickers or toys. Here we adapted the Inequity Game—a paradigm designed to study children’s responses to inequality in isolation from other concerns—to test inequity aversion in adults (N = 104 pairs). We manipulated whether participants made decisions about concrete rewards (candy) or abstract rewards (tokens that could be traded in for money). We found that, like children, adults rejected unequal payoffs in this task. Additionally, we found that reward type mattered: adults rejected disadvantageous—but not advantageous—monetary distributions, yet rejected both disadvantageous and advantageous candy distributions. These findings allow us to draw clearer comparisons across child and adult responses to unfairness and help paint a fuller picture of inequity aversion in humans.

Why do people punish selfish behavior? Are they motivated to punish perpetrators of selfishness (retribution) or to compensate the victims of selfishness (restoration)? Developmental data can provide important insight into these questions by revealing whether punishment of selfishness is more retributive or restorative when it first emerges. Across two studies, we examined costly third-party intervention in 6- to 9-year-olds. In Study 1, children learned about a selfish actor who refused to share with a recipient. Children then chose to (1) punish the selfish actor by rejecting their payoff (retribution); (2) compensate the victim of selfishness by equalizing payoffs between the perpetrator and victim (restoration); or (3) do nothing. We found that children were more likely to punish than compensate in response to selfishness, suggesting that intervention in this context is more retributive than restorative. In Study 2, we tested third-party intervention in the face of generosity which, like selfishness, can lead to unequal outcomes. As in Study 1, children in this context could reject unequal payoffs, thereby depriving the recipient of the advantageous payoff but having no effect on the actor. Children could also use compensation in this context, equalizing the payoffs between actor and recipient. We found that children did not punish inequality that stemmed from generosity, suggesting that the retributive punishment in Study 1 was specifically targeting selfishness rather than inequality more generally. These results contribute to the debate on the function of third-party punishment in humans, suggesting that retributive motives toward selfish transgressors are privileged during ontogeny.

We explore the developmental origins of intervention against gender-based pay inequality in 4- to 9-year-old children in the United States (N = 123; Study 1) and Peru (N = 115; Study 2), two countries characterized by different norms surrounding gender pay equity. We presented children with scenarios that featured gender-based pay inequality, and they could intervene at a cost to redistribute the earnings. We examined whether children favor equality or show gender bias in intervention depending on the direction of gender pay inequality. Across both societies, both girls and boys intervened against gender inequality regardless of its direction, a tendency that grew stronger with age. These findings suggest that despite developing in societies with existing gender pay inequalities, children strongly privilege equality.

Much of human experience is informed by our ability to attribute mental states to others, a capacity known as theory of mind. While evidence for theory of mind in animals to date has largely been restricted to primates and other large-brained species, the use of ecologically-valid competitive contexts hints that ecological pressures for strategic deception may give rise to components of theory of mind abilities in distantly-related taxonomic groups. In line with this hypothesis, we show that cleaner wrasse (Labroides dimidiatus) exhibit theory of mind capacities akin to those observed in primates in the context of their cooperative cleaning mutualism. These results suggest that ecological pressures for strategic deception can drive human-like cognitive abilities even in very distantly related species., McAuliffe et al. demonstrate that in the context of cooperative cleaning, wild-caught female cleaner wrasse are more likely to cheat when their partners are out of view. This provides evidence that cleaner wrasse possess a building block of theory of mind: sensitivity to what others can and cannot see.

Human punishment is biased by ingroup favoritism: we tend to punish outgroup members more harshly than ingroup members. Such preferential punishment of outgroup members could reflect an influence of group membership specifically on punishment, or may reflect a general tendency to respond to outgroup members’ behavior more harshly, regardless of the type of response used. To investigate this question, we contrasted punishment with the decision to reject a partner, often termed partner choice. In two studies, participants interacted with other players in an incentivized economic game. We assigned participants to groups using a “minimal” groups paradigm (Study 1) or a consequential political position (Study 2). Across both studies, when participants could respond to their partner’s behavior with punishment, they punished outgroup members more harshly than ingroup members, replicating past work. We also extend prior work by showing that this difference principally reflects outgroup derogation rather than ingroup love, through the inclusion of neutral individuals in Study 2. In contrast, when participants could respond by either continuing to interact with their current partner or instead reject them and be paired with a new player, participants’ decisions were almost completely unaffected by group membership. Thus, group membership has a strong influence on how we punish others, but almost no influence on how we make partner choice decisions. These results shed light on the breadth of influence group membership can have, especially on how we respond to transgressions, and provide insight into the unique psychological processes supporting punishment and partner choice decisions.

Humans have a deeply rooted sense of fairness, but its emotional foundation in early ontogeny remains poorly understood. Here, we asked if and when 4- to 10-year-old children show negative social emotions, such as shame or guilt, in response to advantageous unfairness expressed through a lowered body posture (measured using a Kinect depth sensor imaging camera). We found that older, but not younger children, showed more negative emotions, i.e. a reduced upper body posture, after unintentionally disadvantaging a peer on (4,1) trials than in response to fair (1,1) outcomes between themselves and others. Younger children, in contrast, expressed more negative emotions in response to the fair (1,1) split than in response to advantageous inequity. No systematic pattern of children's emotional responses was found in a non-social context, in which children divided resources between themselves and a non-social container. Supporting individual difference analyses showed that older children in the social context expressed negative emotions in response to advantageous inequity without directly acting on this negative emotional response by rejecting an advantageously unfair offer proposed by an experimenter at the end of the study. These findings shed new light on the emotional foundation of the human sense of fairness and suggest that children's negative emotional response to advantageous unfairness developmentally precedes their rejection of advantageously unfair resource distributions.

Forgiveness is a powerful feature of human social life, allowing for the restoration of positive cooperative relationships. Despite its importance, we know relatively little about how forgiveness develops during early life and the features that shape forgiveness decisions. Here, we investigated forgiveness behavior in children aged 5–10 years (N = 257) from the United States, varying transgressor intent and remorse in a behavioral task that pitted punishment against forgiveness. We found that baseline levels of forgiveness are high, suggesting that children assume the best of transgressors in the absence of information about intent and remorse. We also found age-related increases in sensitivity to intent but not remorse, such that older children are more likely to forgive accidental transgressions. Because forgiveness is an important tool in the human social toolkit, exploring the ways in which this ability develops across age can help us to better understand the early roots of human cooperation.

Like psychology more broadly, developmental psychology has long suffered from a narrow focus on children from WEIRD societies—or those that are Western, Educated, Industrialized, Rich, and Democratic. In this review, we discuss how developmental scientists have sought to correct this bias through two complementary approaches: one centered on detailed, ethnographic investigations of child development within populations (increasing the depth of our understanding) and one focused on larger, multi-site studies that test children on standardized tasks across populations (increasing breadth) . We review key papers from each of these approaches, describe how they are currently practiced, and discuss their strengths and weaknesses. Next, we highlight exemplary papers from the adult literature that offer useful insights, namely the importance of formal modeling and a greater focus on studying variation at multiple levels of analysis. We end by outlining best practices for future waves of cross-cultural, developmental science. Overall, we argue that a more integrated perspective, combining the strengths of the breadth & depth approaches, can help better elucidate the developmental origins of human behavioral diversity.

Recent research has shown that children’s sense of fairness is shaped in part by cultural practices, values, and norms. However, the specific social factors that motivate children’s fairness decisions remain poorly understood. The current study combined an ethnographic approach with experimental tests of fairness (the Inequity Game) in two Chinese schools with qualitatively different practices and norms. In the “University school,” children received explicit moral instruction on fairness reinforced by adults when supervising children’s activities. By contrast, in the “Community school,” children received less formal moral education and little adult supervision during play time, but norms of cooperation and fairness emerged through informal interactions with peers and other members of the community. Contrary to our predictions, children in both schools (N = 66) rejected both disadvantageous and advantageous allocations of resources in the test trials. However, in the very first practice trials, children from the Community school tended to reject all inequalities, whereas children from the University school tended to accept inequalities. We draw on the ethnographies of the schools to interpret these results, concluding that, despite the similarities in the experimental results, different motivations and social factors likely underlie the rejection of inequality in the two schools.

Ingroup favoritism influences third-party norm enforcement: Third-party punishers are more lenient when an ingroup member has been unfair. By contrast, in 2-party contexts, where punishers are the victims of unfairness, group bias effects are absent or inconsistent. Thus, group bias appears to be particularly influential when enforcing fairness among others, but less so when protecting oneself from unfairness. This would have implications for theories of how cooperation and intergroup cognition interact, but a more direct empirical test is lacking. To this end, developmental data are particularly useful as they can tell us whether and if so how group bias and fairness norm enforcement are related from their first emergence. Using a minimal groups manipulation, we induced ingroup bias in 6- to 9-year-olds and tested their willingness to reject disadvantageous (more for you) and advantageous (more for me) resource allocations when paired with in- and outgroup members. Group bias did not affect children's rejections of unfair allocations, although they reported that it was worse when an outgroup member had more. Our findings suggest that ingroup bias does not influence children's costly endorsement of equality, indicating that children perceive the equality norm to be indiscriminate and enforceable across group boundaries. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Children across diverse societies reject resource allocations that place them at a disadvantage (disadvantageous inequity aversion; DI). In certain societies, older children also reject advantageous allocations (advantageous inequity aversion; AI). Other work demonstrates that after collaboration, children reduce inequity by sharing. However, it is unknown whether collaboration leads to greater sharing because it encourages prosociality (Generosity Hypothesis) or because collaboration elicits stronger equitable tendencies (Equity Hypothesis). Here we use measures of inequity aversion that can disambiguate between these hypotheses. We tested 7- to 13-year-old children from rural India, a population that has shown DI but not AI, and 4- to 10-year-old children from rural Canada, a population that shows both AI and DI. Pairs of children worked either collaboratively or in parallel obtaining candy that was then used in a test of DI and AI. Results showed that in both societies collaboration did not encourage children to accept DI offers, providing evidence against the Generosity Hypothesis. However, in both societies older children demonstrated AI after collaboration but not after parallel work. For children in India AI emerged in ages where it had not been previously observed and children in Canada showed AI during early middle childhood. This suggests that collaboration can induce a willingness to sacrifice an advantage to achieve equity, consistent with the Equity Hypothesis.

Among the many factors that influence our moral judgments, two are especially important: whether the person caused a bad outcome and whether they intended for it to happen. Notably, the weight accorded to these factors in adulthood varies by the type of judgment being made. For punishment decisions, intentions and outcomes carry relatively equal weight; for partner choice decisions (i.e., deciding whether or not to interact with someone again), intentions are weighted much more heavily. These behavioral differences in punishment and partner choice judgments may also reflect more fundamental differences in the cognitive processes supporting these decisions. Exploring how punishment and partner choice emerge in development provides important and unique insight into these processes as they emerge and mature. Here, we explore the developmental emergence of punishment and partner choice decisions in 4- to 9-year-old children. Given the importance of intentions for partner choice decisions-from both theoretical and empirical perspectives-we targeted the sensitivity of these two responses to others' intentions as well as outcomes caused. Our punishment results replicate past work: Young children are more focused on outcomes caused and become increasingly sensitive to intentions with age. In contrast, partner choice judgments exhibit sensitivity to intentions at an earlier age than punishment judgments, manifesting as earlier partner choice in cases of attempted violations. These results reveal distinct developmental trajectories for punishment and partner choice judgments, with implications for our understanding of the processes underlying these two responses as well as the development of moral judgment more broadly. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Partner choice captures the idea that individuals exist in a biological market of potential partners, and we can therefore choose or reject our social partners. While prior work has principally explored the functional basis of partner choice, here we focus on its mechanistic basis, motivated by a surge of recent work exploring the psychology underlying partner choice decisions. This work demonstrates that partner choice is predictably sensitive to a number of factors, including 1) a potential partner’s generosity and fairness, 2) cooperative disposition, 3) moral decision-making, and 4) intentions. We then broaden our scope, first reviewing work suggesting that, in some cases, the psychology underlying partner choice may be distinct from other responses to a partner’s behavior. We then discuss work demonstrating the sensitivity of partner choice decisions to market characteristics as well as work that illuminates the neural, ontogenetic and phylogenetic basis of partner choice. We conclude by highlighting outstanding questions and suggest directions for future research.

The cooperative breeding hypothesis (CBH) states that cooperative breeding, a social system in which group members help to rear offspring that are not their own, has important socio‐cognitive consequences. Thornton & McAuliffe (2015; henceforth T&M) critiqued this idea on both conceptual and empirical grounds, arguing that there is no reason to predict that cooperative breeding should favour the evolution of enhanced social cognition or larger brains, nor any clear evidence that it does. In response to this critique, Burkart & van Schaik (2016 henceforth B&vS) attempt to clarify the causal logic of the CBH, revisit the data and raise the possibility that the hypothesis may only apply to primates. They concede that cooperative breeding is unlikely to generate selection pressures for enhanced socio‐cognitive abilities, but argue instead that the CBH operates purely through cooperative breeding reducing social or energetic constraints. Here, we argue that this revised hypothesis is also untenable because: (1) it cannot explain why resources so released would be allocated to cognitive traits per se rather than any other fitness‐related traits, (2) key assumptions are inconsistent with available evidence and (3) ambiguity regarding the predictions leaves it unclear what evidence would be required to falsify it. Ultimately, the absence of any compelling evidence that cooperative breeding is associated with elevated cognitive ability or large brains (indeed data suggest the opposite is true in non‐human primates) also casts doubt on the capacity of the CBH to explain variation in cognitive traits.

Communication is a powerful tool for promoting cooperation in adults and is considered one of the most important solutions to social dilemmas. One feature that makes communication particularly useful in cooperative contexts is that it allows people to advertise their intentions to partners. Some work suggests that adults cooperate more after making nonbinding commitments to cooperate (i.e., commitments they do not need to uphold) than when they are not allowed to communicate their intentions to their partners. However, we know little about whether nonbinding commitments play a similar role in children. We addressed this gap by testing 6- to 9-year-old children in a simultaneous version of the iterated prisoner's dilemma game. In the communication condition, children could communicate their intended decision prior to their actual choice, whereas in the silent condition, they could not communicate. Overall, children in the communication condition were no more likely to cooperate than children in the silent condition. However, in the communication condition, but not in the silent condition, children's behavior was contingent on their previous decision; they were more likely to cooperate or defect when they had previously cooperated or defected, respectively. In addition, they rarely reversed their intended decisions in the game. Our findings suggest that, although nonbinding commitment does not promote children's cooperation in general, it may encourage children to stick to their chosen strategy, perhaps for the sake of appearing consistent. More broadly, these results contribute to our emerging understanding of the ways in which children solve social dilemmas.

Social evaluative abilities emerge in human infancy, highlighting their importance in shaping our species' early understanding of the social world. Remarkably, infants show social evaluation in relatively abstract contexts: for instance, preferring a wooden shape that helps another shape in a puppet show over a shape that hinders another character (Hamlin et al., 2007). Here we ask whether these abstract social evaluative abilities are shared with other species. Domestic dogs provide an ideal animal species in which to address this question because this species cooperates extensively with conspecifics and humans and may thus benefit from a more general ability to socially evaluate prospective partners. We tested dogs on a social evaluation puppet show task originally used with human infants. Subjects watched a helpful shape aid an agent in achieving its goal and a hinderer shape prevent an agent from achieving its goal. We examined (1) whether dogs showed a preference for the helpful or hinderer shape, (2) whether dogs exhibited longer exploration of the helpful or hinderer shape, and (3) whether dogs were more likely to engage with their handlers during the helper or hinderer events. In contrast to human infants, dogs showed no preference for either the helper or the hinderer, nor were they more likely to engage with their handlers during helper or hinderer events. Dogs did spend more time exploring the hindering shape, perhaps indicating that they were puzzled by the agent's unhelpful behavior. However, this preference was moderated by a preference for one of the two shapes, regardless of role. These findings suggest that, relative to infants, dogs show weak or absent social evaluative abilities when presented with abstract events and point to constraints on dogs' abilities to evaluate others' behavior.

An analysis of when children develop a sense of fairness (receiving less or more than a peer) is compared across seven different societies; aversion to receiving less emerges early in childhood in all societies, whereas aversion to receiving more emerges later in childhood and only in three of the seven societies studied. Previous research has shown that children develop a sense of fairness during childhood, but it is not known how the two components of this — aversion to being unfairly treated and aversion to seeing others unfairly treated — vary across cultures. These authors conducted experiments in seven cultures and find that aversion to being unfairly treated emerges early in childhood and consistently across cultures. Aversion to seeing others unfairly treated emerges later in childhood and is only seen in the experiment in three of the seven cultures studied. A sense of fairness plays a critical role in supporting human cooperation1,2,3. Adult norms of fair resource sharing vary widely across societies, suggesting that culture shapes the acquisition of fairness behaviour during childhood4,5. Here we examine how fairness behaviour develops in children from seven diverse societies, testing children from 4 to 15 years of age (n = 866 pairs) in a standardized resource decision task6,7. We measured two key aspects of fairness decisions: disadvantageous inequity aversion (peer receives more than self) and advantageous inequity aversion (self receives more than a peer). We show that disadvantageous inequity aversion emerged across all populations by middle childhood. By contrast, advantageous inequity aversion was more variable, emerging in three populations and only later in development. We discuss these findings in relation to questions about the universality and cultural specificity of human fairness.