no paywall: https://archive.is/kT51K

> The Danish data suggest that illness could account for roughly half of the 1.9% wage gap between first- and second-born siblings.

If this is true, we would expect to see the effect blunted during COVID lockdowns, when illnesses were in general at very low levels. If older siblings weren't going to preschool/school, they wouldn't have been bringing home illnesses that would have infected their in utero / baby siblings.