Cooperation often results in a final material resource that must be shared, but deciding how to distribute that resource is not straightforward. A distribution could count as fair if all members receive an equal reward ("egalitarian distributions"), or if each member's reward is proportional to their merit ("merit-based distributions"). Here, we propose that the acquisition of numerical concepts influences how we reason about fairness. We explore this possibility in the Tsimane', a farming-foraging group who live in the Bolivian rainforest. The Tsimane' learn to count in the same way children from industrialized countries do, but at a delayed and more variable timeline, allowing us to de-confound number knowledge from age and years in school. We find that Tsimane' children who can count produce merit-based distributions, while children who cannot count produce both merit-based and egalitarian distributions. Our findings establish that the ability to count--a non-universal, language-dependent, cultural invention--can influence social cognition.