We make use of a new data resource-merged birth and school records for all children born in Florida from 1992 to 2002-to study the relationship between birth weight and cognitive development. Using singletons as well as twin and sibling fixed effects models, we find that the effects of early health on cognitive development are essentially constant through the school career; that these effects are similar across a wide range of family backgrounds; and that they are invariant to measures of school quality. We conclude that the effects of early health on adult outcomes are therefore set very early. (JEL I12, J13, J24) A large literature documents the effects of neonatal health (commonly proxied by birth weight) on a wide range of adult outcomes such as wages, disability, adult chronic conditions, and human capital accumulation. A series of studies, conducted in a variety of countries including Canada, Chile, China, Norway, and the United States, have made use of twin comparisons to show that the heavier twin of the pair is more likely to have better adult outcomes measured in various ways. 1

Boys born to disadvantaged families have higher rates of disciplinary problems, lower achievement scores, and fewer high school completions than girls from comparable backgrounds. Using birth certificates matched to schooling records for Florida children born 1992–2002, we find that family disadvantage disproportionately impedes the pre-market development of boys. The differential effect of family disadvantage on boys is robust to specifications within schools and neighborhoods as well as across siblings within families. Evidence supports that this is the effect of the postnatal environment; family disadvantage is unrelated to the gender gap in neonatal health. We conclude that the gender gap among black children is larger than among white children in substantial part because black children are raised in more disadvantaged families. (JEL D91, I24, I32, J13, J15, J16)

Accurate understanding of environmental moderation of genetic influences is vital to advancing the science of cognitive development as well as for designing interventions. One widely reported idea is increasing genetic influence on cognition for children raised in higher socioeconomic status (SES) families, including recent proposals that the pattern is a particularly US phenomenon. We used matched birth and school records from Florida siblings and twins born in 1994-2002 to provide the largest, most population-diverse consideration of this hypothesis to date. We found no evidence of SES moderation of genetic influence on test scores, suggesting that articulating gene-environment interactions for cognition is more complex and elusive than previously supposed.

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