This 2004 paper examining the difference in infant mortality in The Netherlands[1] concludes that it's basically due to immigration, parents being older (father

This 2004 paper examining the difference in infant mortality in The Netherlands[1] concludes that it's basically due to immigration, parents being older (father's age is also a factor), and fertility treatments. Some ethnic groups in The Netherlands have 2x higher infant mortality after birth than others, people are older when they're having kids, and having twins (or more) is now 2x as common as in the 1950s due to the availability of fertility treatment. I wonder how up-to-date the results of that older paper are, the other ones discussed here seem to address much narrower questions. 1. https://www.demographic-research.org/volumes/vol11/13/11-13.pdf https://www.demographic-research.org/volumes/vol11/13/11-13....