Human cooperation is a key driving force behind the evolutionary success of our hominin lineage. At the proximate level, biologists and social scientists have identified other-regarding preferences – such as fairness based on egalitarian motives, and altruism – as likely candidates for fostering large-scale cooperation. A critical question concerns the ontogenetic origins of these constituents of cooperative behavior, as well as whether they emerge independently or in an interrelated fashion. The answer to this question will shed light on the interdisciplinary debate regarding the significance of such preferences for explaining how humans become such cooperative beings. We investigated 15-month-old infants' sensitivity to fairness, and their altruistic behavior, assessed via infants' reactions to a third-party resource distribution task, and via a sharing task. Our results challenge current models of the development of fairness and altruism in two ways. First, in contrast to past work suggesting that fairness and altruism may not emerge until early to mid-childhood, 15-month-old infants are sensitive to fairness and can engage in altruistic sharing. Second, infants' degree of sensitivity to fairness as a third-party observer was related to whether they shared toys altruistically or selfishly, indicating that moral evaluations and prosocial behavior are heavily interconnected from early in development. Our results present the first evidence that the roots of a basic sense of fairness and altruism can be found in infancy, and that these other-regarding preferences develop in a parallel and interwoven fashion. These findings support arguments for an evolutionary basis – most likely in dialectical manner including both biological and cultural mechanisms – of human egalitarianism given the rapidly developing nature of other-regarding preferences and their role in the evolution of human-specific forms of cooperation. Future work of this kind will help determine to what extent uniquely human sociality and morality depend on other-regarding preferences emerging early in life.

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Recent research has produced new insights into the early development of social cognition and social learning. Even very young children learn and understand social activities as governed by conventional norms that (a) are arbitrary and shared by the community, (b) have normative force and apply to all participants, and (c) are valid in context‐relative ways. Importantly, such understanding is revealed both in the fact that children themselves follow the norms, and in the fact that they actively enforce them toward third parties. Human social cognition thus has a fundamental normative dimension that begins early. This norm psychology plausibly evolved due to its role in stabilizing group coordination and cooperation, and is one of the foundations of what is uniquely human social learning and culture.

Recent work provides evidence that expectations regarding a fair (i.e., equal) distribution of goods and resources arise sometime in the second year of life. To investigate the developmental trajectory of fairness expectations, and their potential relation to prosocial behavior, infants participated in a violation‐of‐expectancy (VOE) paradigm designed to assess expectations regarding how resources are typically distributed, and in a sharing task, an informational helping task, and an instrumental helping task. Infants’ expectations regarding resource distribution showed age‐related changes between 12 and 15 months, with only 15‐month‐old infants showing greater attention to unfair (unequal) over fair (equal) outcomes in the VOE. Individual differences in infants’ sensitivity to unfair outcomes were related to infants’ willingness to share a preferred toy. In contrast, helping behavior was unrelated to infants’ sensitivity to unfair outcomes and did not vary according to whether infants shared a preferred or non‐preferred toy during the sharing task. Our findings suggest a developmental transition in expectations regarding how resources are distributed from 12 to 15 months of age, linked to infants’ sharing behavior, suggesting that such expectations are learned through experience. Our results also contribute to the ongoing discussion regarding how best to assess the construct of prosociality in infancy.