We identify externalities in human capital production function arising from sibling spillovers. Using regression discontinuity design generated by school-entry cutoffs and school records from one district in Florida, we find positive spillover effects from an older to a younger child in less affluent families and negative spillover effects from a younger to an older child in more affluent families. These results are consistent with direct spillovers dominating in economically disadvantaged families and with parental reinforcement in more affluent families.