Cooperation confounds us: Humans are the only members of the animal kingdom to display this tendency to the extent that we do, and it's an expensive endeavor with no guarantee of reciprocal rewards. While we continue to look for answers about how and why cooperation may have emerged in human social and cultural evolution, we are beginning to trace the developmental roots of prosocial behaviors.
A recent PLoS paper presents evidence that children as young as 15 months old may have a rudimentary sense of fairness. Adults regularly appear to include fairness measures when making decisions. You might not believe it's true, but there's proof: Neuroimaging highlights increased activity in the reward regions of our brains when we consider fair offers and increased activity in the amygdala when we're faced with unfair options. To be able to weigh fairness and unfairness, we have to have a sense of of the psychological and social state of other. Such "other-regarding" behavior is believed to be a human tendency, although it remains to be confirmed whether it is specifically a human trait. However, it was commonly held that other-regarding behavior emerged late in development—as something we learned. But a growing body of research keeps pushing back the age at which these tendencies are exhibited.
For example, children as young as 3.5 years will distribute resources fairly, and at 25 months, toddlers will share resources with adults. Researchers Schmidt and Sommerville found that 15 month old infants can identify events that deviate from expectations with regard to fairness and are likely to share resources requested by an adult. In a violation of expectations task (VOE), infants looked longer at events that deviated from the norm—in this case, when milk or crackers were unevenly distributed. They were also more likely to share a toy with an adult who requested the item.
Cooperative behaviors may play an important role in the development of cohesive social groups, allowing larger and larger groups of genetically unrelated individuals to establish and abide by shared norms. The earlier we can trace this behavior, the more we may come to understand about how unique this tendency may be to humans.
If this behavior emerges early in children, what happens as we get older? Adults may weigh fairness and unfairness, but they do not always act in accordance with their judgments. Children, for their part, also do not always take the more altruistic path. For example, in the sharing task discussed above, the infants had two toys, a preferred one which they favored and one they did not. They were just as likely to share the favored toy (altruistic sharing) as the unpreferred one (selfish sharing), which might suggest a spectrum along which altruistic decisions are made. How do we develop this scale? Is it the outcome of socialization? Or are we wired to map this ourselves?
Schmidt, M., & Sommerville, J. (2011). Fairness Expectations and Altruistic Sharing in 15-Month-Old Human Infants PLoS ONE, 6 (10) DOI: 10.1371/journal.pone.0023223