While Chetty’s sibling fixed-effects design reduces shared family confounding, it does not address child-specific genetic heterogeneity or broader regional gene–environment correlations created by selective migration. If genetically influenced traits affect both parental migration timing and children’s long-run earnings, then quasi-random exposure to neighborhoods may still reflect underlying gene–environment correlation rather than purely environmental causation. Without genetic data, the design cannot fully disentangle environmental effects from dynamic sorting processes intensified by assortative mating and increasing geographic stratification.