Children's educational outcomes are strongly correlated with their parents' educational attainment. This finding is often attributed to the family environment-assuming, for instance, that parents' behavior and resources affect their children's educational outcomes. However, such inferences of a causal role of the family environment depend on the largely untested assumption that such relationships do not simply reflect genes shared between parent and child. We examine this assumption with an adoptee design in full-population cohorts from Danish administrative data. We test whether parental education predicts children's educational outcomes in both biological and adopted children, looking at four components of the child's educational development: (I) the child's conscientiousness during compulsory schooling, (II) academic performance in those same years, (III) enrollment in academically challenging high schools, and (IV) graduation success. Parental education was a substantial predictor of each of these child outcomes in the full population. However, little intergenerational correlation in education was observed in the absence of genetic similarity between parent and child-that is, among adoptees. Further analysis showed that what links adoptive parents' education did have with later-occurring components such as educational attainment (IV) and enrollment (III) appeared to be largely attributable to effects identifiable earlier in development, namely early academic performance (II). The primary nongenetic mechanisms by which education is transmitted across generations may thus have their effects on children early in their educational development, even as the consequences of those early effects persist throughout the child's educational development.