One thing to understand about the post-2008 rich-world fertility decline is it happened to the very-conservative formerly-high-fertility subgroups too: 
Here's the Amish. Amish TFR is down from about 7 in 2008 to under 5 in 2020.
The Amish!
The Amish!
And tbh the Amish decline looks like it began in the 1970s and has just been slowly falling every since
As an aside, Finland's long-term birth data is always really interesting. Here's share of babies in Finland born in wedlock. You can see a slow rise from 1760s to mid-19th century, but then WOW the post-war period!
You get a significant pivot away from unwed motherhood immediately postwar, but then a huge increase in the 1970s.
And here's Finland's fertility rate since 1776, in raw terms and also adjusted for how many of those kids likely survived to puberty.
One thing I like about mortality-adjustments like this is they help us understand a key curiosity: if human survival was predicated on 5 or 6 or 7 kids each, why don't we have more deeply-rooted and hard-to-change preferences for 5 or 6 or 7 kids?
My answer is that what we actually have is a very historically deeply-rooted average norm of 2-4 kids, that that was the actual "efficient family size" for almost all of human history, but that high mortality rates drove the high TFRs we observe.
So "growing up with 7 surviving siblings" was probably *never* the norm except for basically 2 generations around early modernity.
Of course now we're way below even that 2 level, but still.
Of course now we're way below even that 2 level, but still.
The 1950s is 200 years after the beginning of demographic modernity.
https://twitter.com/DarcysMick/status/1493616188433305604
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