The presence of ADHD: Spillovers between siblings

•This paper studies potential spillover effects from having a sibling with ADHD.•This paper employs OLS and cousin fixed effects using Danish register-data.•Children are negatively affected by the presence of younger siblings with ADHD.•The results are robust across specifications.

This paper uses high quality register-data to study the spillover effects on firstborns from having a younger sibling suffering from ADHD. Using OLS and cousin fixed effects analyses it is found that the educational outcomes of healthy firstborn children are significantly reduced by the presence of a disordered sibling.