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, the authors 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. The authors 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.