Little is known about the role birth order plays in delinquency and adult crime outcomes that carry significant externalities. We use rich data sets from Denmark and Florida to examine these outcomes and explore potential mechanisms. Despite large environmental differences between the areas, we find remarkably consistent results: in families with two or more children, secondborn boys are 20%–40% more likely to be disciplined in school and enter the criminal justice system than are their firstborn male siblings. We rule out health at birth and school quality as mechanisms but find evidence for the role of parental time investment.