Preschoolers value those who sanction non-cooperators.

Large-scale human cooperation among unrelated individuals requires the enforcement of social norms. However, such enforcement poses a problem because non-enforcers can free ride on others' costly and risky enforcement. One solution is that enforcers receive benefits relative to non-enforcers. Here we show that this solution becomes functional during the preschool years: 5-year-old (but not 4-year-old) children judged enforcers of norms more positively, preferred enforcers, and distributed more resources to enforcers than to non-enforcers. The ability to sustain not only first-order but also second-order cooperation thus emerges quite early in human ontogeny, providing a viable solution to the problem of higher-order cooperation.