Young children share fairly and expect others to do the same.Yet little is known about the underlying cognitivemechanisms that support fairness. Across two experiments,we investigated whether children’s numerical competenciesare linked with their sharing behavior. Preschoolers (aged2.5-5.5) participated in either third-party (Experiment 1) orfirst-party (Experiment 2) resource allocation tasks.Children’s numerical competence was then assessed using theGive-N-Task (Sarnecka & Carey, 2008; Wynn, 1990).Numerical competence – specifically knowledge of thecardinal principle explained age-related changes in fairsharing in both the third- and first-party contexts. Theseresults suggest that an understanding of the cardinal principleserves as an important mechanism for fair sharing behavior.