I've done genealogy lately, and I've found out something that surprised me (but in retrospect, shouldn't have):<p>It works the other way too. It's not just wealthier people having less kids, it's families staying wealthier <i>because</i> they have few kids. Most of my ancestors were fairly poor, but it made a big difference whether they shared their inheritance with one sibling or with eight. It seems to have had a snowball effect too: it wasn't just that they inherited money, it was that they could use that money to get more money - and not necessarily even by working. There was plenty of rent-extraction in the 19th century which you didn't need to be a factory owner to do.<p>And maybe there we are at the heart of the fertility issue: when owning stuff makes a larger difference to your kids' wealth than their own ability to work, you are effectively hurting the kids you already have when you decide to have one more kid. And it starts long before they get an inheritance.