Firstborns, on average, complete more education than laterborns. We study whether individuals’ endowments measured by genetic information amplify this effect. Our familyfixed effects approach allows exploiting exogenous variation in birth order and genetic endowments among 14,850 siblings in the UK Biobank. We find that those with higher genetic endowments benefit disproportionately more from being firstborn compared to those with lower endowments, providing a clean example of how nature and nurture interact in producing human capital. Since parental investments are a dominant channel driving birth order effects, our results are consistent with complementarity between endowments and investments in human capital formation.

We discuss how to estimate the interplay between genes (nature) and environments (nurture), with an empirical illustration of the moderating effect of school starting age on one’s genetic predisposition towards educational attainment. We argue that gene–environment (G×E) studies can be instrumental for (i) assessing treatment effect heterogeneity, (ii) testing theoretical predictions, and (iii) uncovering mechanisms, thereby improving understanding of how (policy) interventions affect population subgroups. Empirically, we find that being old-for-grade and having a higher genetic propensity for education benefits children on assessment tests as they progress through school. In this setting, families appear to increase genetic inequalities while schools seem to reduce them.