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.

We investigate the effects of early-life medical treatments on the treated children and their families. We use a regression discontinuity design that exploits changes in medical treatments across the very low birth weight (VLBW) cutoff. Using administrative data from Denmark, we establish that VLBW children have better health and higher test scores. We find that these benefits spill over to other family members: mothers enjoy better mental health, and siblings have higher test scores. Maternal mental health improvements seem to be driven by better focal child health and sibling spillovers by improved interactions within the family and parental compensating behavior.