Infants who are underweight at birth earn less, score lower on tests, and become less educated as adults. Previous studies have found mixed evidence that socioeconomic status mitigates this effect. In this paper, we reconcile these findings using a unique dataset in which adoptees were quasi-randomly assigned to families. We find that the average income within a zip code mitigates the effects of low birth weight, as in Currie and Morreti (2007) , whereas other family characteristics do not, as

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... n Currie and Hyson (1999) . These results cannot be explained by differences in genetics, prenatal healthcare or neonatal healthcare. JEL Classification: I1, J1