Because eduction and prosperity reduce the brith rate. You don't need that many children to care for you when you're old, if you can expect to get healthcare. You don't shouldn't get accidental children if you're educated about sex and protection. In an educated society you will have fewer children later in your life, because perhaps after school you will also go to university, delaying the moment where you feel stable and settled enough to start a family. The more educated you are, the higher the chance that having children is a deliberate, careful choice, and not just something that happens. And last but not least, politics play a huge role in what incentives you have. Especially for this point, I recommend the Kurzgesagt videos about South Korea and Germany.

• https://www.youtube.com/watch?v=Ufmu1WD2TSk
• https://www.youtube.com/watch?v=n-gYFcVx-8Y

Let's not forget that it's also increasingly expensive to raise kids, so that deliberate choice gets kicked down the road. Later first kid necessarily means less time for more.

Poor people have more kids because their mortality rates are higher, etc.

More importantly is there any citation for this supposed "fact"?

Lotta things here but poor families are often religious which tells them to have as many babies as possible, lower to no access to birth control and health care, and low paying jobs mean more hands working can provide a little more. People with means and availability typically have less children.

But also in major capitalist countries people work so god damned much they dont want or have time for kids

when women have a choice (income, a creer, birth control) they mostly choose 0, 1 or 2 kids.

For every woman who then chooses 0, you need another who chooses,5 just to maintain the status quo.

Poorer, underdeveloped countries tend to have a higher % of rural population, and they're incentivized to raise more kids as they'll later be able to help with the farm work.

Edit: Wtf are these answers

I saw an interesting thing a few years back (which I can't now find!). It was a plot of energy usage Vs reproductive rates of various animals. The graph produced quite a linear line (more energy expended==>lower reproductive speed). Humans were wildly off that line. At least until you accounted for external energy usage (burnt fuel, electricity etc). That jumped us back to the line.

I'm still not sure if this was just a coincidental result, or pointed to a deeper correlation.

Nuclear anxiety might have also played a role.

Exactly. Thank you for mentioning it explicitly, I shoved into the "politics" remark.

https://www.gapminder.org/tools/?from=world#%24ui%24chart%24trails%3Afalse%3B%3B=&chart-type=bubbles&url=v2

Religion has essentially no bearingon births per woman. Religious societies like Ukraine have some of the lowest birth rates, while secular countries like Finland have higher rates.

It's almost a perfect inverse linear correlation between GDP/capita and births/woman. The only outlier is Israel, which is a highly polarized society with high levels of child poverty.

When child mortality goes down, births go down too. That means that basic nutrition‌, vaccines, and basic neonatal health care are responsible for the bulk of nativity reduction. The rest can be attributed to access to contraceptives, compulsary education and women in the workforce.

They are dropping in poor countries too. Poor countries tend to have higher birth rates, but they are dropping.

Religion has essentially no bearingon births per woman

There are millions of catholic people who think birth control is evil....

And yet, Catholic countries have some of the lowest birth rates. Go figure.