--- - |2 Abstract Recent work has documented that despite preschool‐aged children's understanding of social norms surrounding sharing, they fail to share their resources equally in many contexts. Here we explored two hypotheses for this failure: an insufficient motivation hypothesis and an insufficient cognitive resources hypothesis. With respect to the latter, we specifically explored whether children's numerical cognition—their understanding of the cardinal principle—might underpin their abilities to share equally. In Experiment 1, preschoolers’ numerical cognition fully mediated age‐related changes in children's fair sharing. We found little support for the insufficient motivation hypothesis—children stated that they had shared fairly, and failures in sharing fairly were a reflection of their number knowledge. Numerical cognition did not relate to children's knowledge of the norms of equality (Experiment 2). Results suggest that the knowledge–behavior gap in fairness may be partly explained by the differences in cognitive skills required for conceptual and behavioral equality. - Developmental Science, EarlyView.