Fertility is actually falling all over the world pretty rapidly. If anything it’s falling too fast for current political/economic systems to cope.
Not just in rich countries either. It has been understood for a while that post-industrial economies tend to see a reduction in fertility rate as women enter the workforce and delay having children. Also, less economic need outside of subsistence farming societies because extra children don’t contribute positive marginal labor when they are expected to be in school. Also as infant mortality falls, you need fewer children born as more of them will survive to reproductive age. All that is expected.
What is unexpected is that it is falling almost as rapidly in industrializing (think India, Brazil) and pre-industrial economies too (think most of Africa). Read that again. Fertility is falling all over the world, and the social scientists don’t know why. From a purely material perspective, well managed the world could support probably 12-15 BN people. As things are, we are on track to peak somewhere around 10-12BN late this century before heading into a decline, unless something changes fast.
There are classic economic schools (Malthus springs to mind) that posit that the population would hit a wall only when it ran out of ways to feed and medicate itself. And most economists accepted some form of that argument, with the arguments really being about what that number would be and why.
Now, it seems some force we haven’t accounted for is encouraging us as a species to stop expanding sooner. It’s actually a really interesting problem with no simple single solution. Or even broad agreement that it is in fact a problem.
Personally, I think when the whole world stops having kids at the same time, that tells you something isn’t right. My guess is that for various reasons around the world there’s just a lot of pessimism about the future right now (and it’s not unjustified). My theory is that people broadly will only have children if they feel that the children will be at least as well off as them, and right now around the world people don’t. Maybe that’s a new thing, we don’t really know. Looking at history, we don’t have hard data for the bronze age collapse or the fall of Rome, and the bubonic plague did decrease the world population slightly but that was due to illness, not a reduction in population growth. All other cases where we’re talking about population decline of anywhere near this magnitude are wars, and without exception they are more localized. So this is such a novel thing, the idea that the world population will be in a sustained decline for the first time in human history, that we don’t really know what is currently causing it.
I can say, it’s not necessarily a problem (though I think it IS a symptom), except for the fact that pension systems the world over were built with the assumption that population would continue to grow until it leveled off around the early 2100s. The fact that we may be shrinking before then is a financial/economic problem that will have to be addressed.