It's a very fuzzy line isn't it? There are quite a few axes you could measure to decide what is developed vs undeveloped, and even the richest countries of the

It's a very fuzzy line isn't it? There are quite a few axes you could measure to decide what is developed vs undeveloped, and even the richest countries of the early 20th century would look undeveloped in the modern world. But in the context of birth rates, I would say the three factors that matter the most is the infant mortality rate, the state of retirement, and the existence of child labour laws. High infant mortality increases birth rates because parents have more "backup" children in case some die. A good retirement system reduces the number of children because people are less likely to have children just to look after them in old age, and child labour laws reduce birth rates because it turns children from an economic benefit to an economic burden. Alternatively you could consider a country "more developed" if it has better support for parents through parental leave and baby bonuses. So ultimately more developed = less children is a bit of a simplification, but I think it's _less_ of a simplification than more money = less children which is what I was arguing against.