In terms of mechanisms, we can rule out large classes of explanations. These include worse health at birth (second-born children appear healthier) or in childhood (second-born children have fewer disabilities), schooling decisions including the age of entry and the quality of schools chosen (second-born children attend no worse schools and are more likely to attend pre-kindergarten and daycare) as well as maternal employment (measured by maternity leave) in the first year of life. We do find that maternal employment and the use of daycare is higher for second-borns in years 2-4 compared to older siblings. While it is well known that first-borns have undivided attention until the arrival of the second-born, these results show that the arrival of the second-born child has the potential to extend the early-childhood parental investment in the first-born child and a concomitant bifurcation of parental attention between first- and second-born children.
--Sanni Breining, Joseph Doyle, David Figlio, Krzysztof Karbownik, and Jeffrey Roth, NBER Working Paper 23038, on the importance of parental investment