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A sense of fairness plays a critical role in supporting human cooperation. Adult norms of fair resource sharing vary widely across societies, suggesting that culture shapes the acquisition of fairness behaviour during childhood. 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 task. 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.

Despite the obvious benefits of directed mechanisms that facilitate the efficient transfer of skills, there is little critical evidence for teaching in nonhuman animals. Using observational and experimental data, we show that wild meerkats (Suricata suricatta) teach pups prey-handling skills by providing them with opportunities to interact with live prey. In response to changing pup begging calls, helpers alter their prey-provisioning methods as pups grow older, thus accelerating learning without the use of complex cognition. The lack of evidence for teaching in species other than humans may reflect problems in producing unequivocal support for the occurrence of teaching, rather than the absence of teaching.

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When enforcing norms for cooperative behavior, human adults sometimes exhibit in-group bias. For example, third-party observers punish selfish behaviors committed by out-group members more harshly than similar behaviors committed by in-group members. Although evidence suggests that children begin to systematically punish selfish behavior around the age of 6 y, the development of in-group bias in their punishment remains unknown. Do children start off enforcing fairness norms impartially, or is norm enforcement biased from its emergence? How does bias change over development? Here, we created novel social groups in the laboratory and gave 6-and 8-year-olds the opportunity to engage in costly third-party punishment of selfish sharing behavior. We found that by age 6, punishment was already biased: Selfish resource allocations received more punishment when they were proposed by out-group members and when they disadvantaged in-group members. We also found that although costly punishment increased between ages 6 and 8, bias in punishment partially decreased. Although 8-y-olds also punished selfish outgroup members more harshly, they were equally likely to punish on behalf of disadvantaged in-group and out-group members, perhaps reflecting efforts to enforce norms impartially. Taken together, our results suggest that norm enforcement is biased from its emergence, but that this bias can be partially overcome through developmental change.cooperation | ontogeny | equality

New behavioural and neuroscientific evidence on the development of fairness behaviours demonstrates that the signatures of human fairness can be traced into childhood. Children make sacrifices for fairness (1) when they have less than others, (2) when others have been unfair and (3) when they have more than others. The latter two responses mark a critical departure from what is observed in other species because they enable fairness to be upheld even when doing so goes against self-interest. This new work can be fruitfully combined with insights from cognitive neuroscience to understand the mechanisms of developmental change. Fairness is a hallmark of our species's ability to maintain cooperative relationships with large numbers of unrelated — and often unfamiliar — others. However, there is much debate over the psychological foundations of this sense of fairness. Which aspects of fairness are engrained in our biology and which depend on learned social norms? What are the psychological mechanisms and motivations that give rise to human fairness? Why does fairness appear to drive decisions in some contexts but not others? To answer these questions, it is critical to understand how fairness emerges in child development, as studies of adults alone do not allow us to differentiate between psychological processes that are acquired through socialization and those that have deeper biological roots. Indeed, a complete understanding of any behaviour requires a description of its developmental course and an examination of what causes behavioural change over the lifespan1,​2,​3. Studies of child development can provide unique insight into the psychology of fairness in important ways: first, developmental data can help to identify the aspects of fairness that are foundational and those that are more malleable. Second, charting how fairness behaviour changes across development can shed light on the specific cognitive mechanisms that enable its emergence and expression. Finally, combining insights from development with work examining how fairness is instantiated at the neural level can help us to understand the psychological foundations on which the human sense of fairness is built. In this Review, we appraise and integrate recent developmental and neuroscientific evidence on fairness. The resulting neurodevelopmental perspective provides novel insights into the foundations of fairness in our species. First, we argue that the signatures of our uniquely human sense of fairness are present in childhood. We then summarize the current state of knowledge for neural mechanisms that support fairness and highlight recent work on the developmental neuroscience of fairness. Last, we identify key neural processes that change during childhood and propose how the developmental integration of these systems can explain the temporal emergence of the human sense of fairness

Studies of mate choice in vertebrates have focused principally on birds, in which male ornaments are often highly developed, and have shown that females commonly select mates on the basis of particular phenotypic characteristics that may reflect their genetic quality. Studies of female mate choice in mammals are less highly developed and they have commonly focused on female mating preferences that are likely to be maintained by benefits to the female's own survival or breeding success. However, recent experimental studies of mate choice in mammals--especially rodents--provide increasing evidence of consistent female preferences that appear likely to generate benefits to the fitness of offspring. As yet, there is no compelling evidence that female mating preferences are less highly developed in female mammals than in female birds, although these preferences may more often be masked by the effects of male competition or of attempts by males to constrain female choice.

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