Fairness is a cornerstone of human cooperation, yet the norms that govern what constitutes a fair act vary across societies. Children are faced with the challenge of acquiring and conforming to these local norms. Even more challenging, conforming to norms of fairness oftentimes requires that children act against self-interest. My talk explores how norms influence fairness behavior in children across societies. I will focus on advantageous inequity aversion—refusing an advantageously unfair allocation of resources—as a useful index of children’s costly fairness behavior. In the first part of my talk, I will (briefly!) review older data showing that this form of fairness behavior emerges relatively late in childhood in the USA and shows cross-cultural variation. I will then turn to data from several new, unpublished studies exploring: (1) whether a simple fairness prompt can encourage costly fairness decisions, (2) the gap between children’s fairness behavior and their understanding of fairness norms, (3) the relative influence of adults versus peers in shifting children’s decisions toward or away from fairness and, finally, and only if time allows, (4) descriptive norms and their influence on children’s sense of what one should do.