All the parents of multiple children I know admit that they stress less about - and do less for - the later arrivals: "We went crazy baby-proofing when we had our first baby; but by the time his sister came along, who had the energy?" Does anyone know if there is any data showing that reduced parental safety input for later-borns has any effect on safety output? Within a family, do later-borns have higher rates of death, accident, sickness, etc?
Note: As in other birth-order studies, you can't just compare big and small families, because big families usually have lower socio-economic status. The best approach is to compare safety within the family (or barring that, look at safety controlling for SES).