Most developed societies. When you live by subsistence agriculture, having lots of kids is a benefit as they are free labor. When a culture urbanizes and industrializes, they are less of a benefit.
It's most societies, period. Of the 237 nations and territories recognized by UN, only 42% had a sustainable fertility rate. [1]
It seems reasonable to assume kids having more positive economic power in the past played a significant role, but there's counter-examples abounds in modern times. Nigeria is heavily urbanized yet has one of the highest fertility rates in the world. Or on the other end of the spectrum, Thailand has a very large chunk of people living off farming, yet has one of the lowest fertility rates in the world.
What's a "sustainable" fertility rate to you? Given that the current human biomass is completely unprecedented in all of history and is only made possible by artificial energy inputs (fossil fuels and ammonia/fertilizer), I posit that the only sustainable fertility rate is well below 2.0
When discussing fertility rates "sustainable" usually means "ratio of births to deaths in order to have a stable population". In developed economies that's around 2.1 children per woman. Anything under that and the population declines without immigration.
Not only does it decline, but it declines exponentially. This is why fertility changes will have a far bigger impact on societies than most people appreciate. If a country has a fertility rate of 1 (and places like South Korea are now well below that even) then that means each successive generation is 50% smaller. And this change is compounding. So after 5 generations, for instance, your generational size will be 97% smaller (1 - 0.5^5).
And all the while this is happening your economy is getting wrecked not only because suddenly you have far fewer people, but because your society turns into a giant retirement home, as fertility also determines the age ratios within a society. You can see this by working backwards. Again taking a fertility rate of 1 as an example, if we have 1 newborn then we have 2 20-40 year olds, 4 40-60 year olds, 8 60-80 year olds, and so on. So you have a shrinking economy paired alongside an even smaller ratio of workers, and much greater social strains on that economy as well.
This is also why immigration isn't anything more than a very short-term bandaid. When you're speaking of population declines near 100% over such small windows of time, you end up needing to basically replace the entire population of a region. Doing this with immigration, let alone as this is increasingly a global crisis, just isn't realistic. And this is completely ignoring the social consequences of such actions.
