Do children use environmental base rates to infer unobservable characteristics of their parents? We examined whether children who lack transmitted knowledge of their parents’ country of birth fill this gap using base-rate-driven assumptions. Using data from over 200,000 children across 45 countries (TIMSS 2015), we compared the accuracy of children’s reports about parental nativity when parents were native-born versus foreign-born. Children were substantially (~17 percentage points) more accurate when parents were born in the country than when parents were born abroad, corresponding to meta-analytic odds ratios exceeding 3. Critically, this accuracy gap was almost entirely explained by children giving wrong answers rather than acknowledging uncertainty, consistent with confident inference from base rates rather than mere ignorance. Supporting this interpretation, the magnitude of the accuracy gap correlated strongly with the proportion of native-born parents in each country (rS = .80). Importantly, higher base rates were both associated with greater accuracy for children of natives and lower accuracy for children of immigrants. The same pattern emerged at the school level: the accuracy gap shrank in schools with higher proportions of immigrant families, demonstrating that children’s inferences track local base rates in their immediate social environment. These results demonstrate base-rate inference operating spontaneously in a naturalistic domain, extending laboratory findings to show how environmental statistics shape everyday judgments about family characteristics.