I suppose people in rich countries have more choices - do they want to focus on a career? Do they want to spend a few more years in university? Do they want to save more money early on in life? All those decisions significantly delay the decision about having children, especially for women.

And women can't have children past a certain age - so even if they eventually decide on maternity, they might have fewer children. This can be clearly seen in statistics - the age of women getting married and having their first child is rising over the last years.

With high wealth also come higher standards - it's hard to imagine having a lot of children in a small apartment, or living in the middle of nowhere without a toilet and electricity. In rich countries children have to be educated, cannot be used as cheap labor, and parents deeply care about their well-being, which raises the standards even more.

The culture changes also might stem from wealth and technology. We have cheap and efficient contraception methods, social media that in many ways make finding a partner more difficult, and simply more knowledge about what it takes to raise children. I also think that with capitalism comes individualism - fewer people care about traditions, family bonds or religions.

Now the paradox is most of those things - freedom of choice, higher standards, more knowledge - are generally very positive. But they don't benefit the demographics, and with how our economics are built, this will end in disaster - especially in retirement pensions and healthcare.