I take issue with the authors here:
Won't the Amish or some other high-fertility, perhaps religious, sub-population expand to be as many as we need? For several reasons, no.
...
First, fertility in a high-fertility sub-group would have to be high enough (certainly above two, for example). We've already seen above that the "high fertility" of high fertility subgroups has been declining over the decades. High fertility used to mean 6 children per woman. Now it means 2.5. Before long, it may mean 1.8. Second, the children of high-fertility parents would have to be very likely to remain in their high-fertility cultural group.
I grew up in a small-town homeschooling community where large families were fairly normal, but many of the children did not go on to have many children, or homeschool their own children.
In contrast to that, I am now a member of a church where 4 children is considered a small family, and 12 children is considered a large family.
Many of the families are second-generation, and one of the topics of conversation is how to pass on a worldview that involves passing on that worldview recursively through the generations.
We're not Amish, but the Amish do seem to be doing a good job of this – per https://en.wikipedia.org/wiki/Amish#Population_and_distribution their numbers have been impressively consistent over the years. I don't know why the authors think that their next 100 years of growth would change from the last 100.
Now, it's going to take a while for their numbers to affect the global population. But it doesn't seem crazy to me that in a few centuries the world's population will be comparably large, and even more dominated by a few Islamic and Christian sects than it is now.
Thanks for the information! I was thinking in a similar direction:
Just like the Amish culture has emerged from the standard population, a new "Amish-like" culture may emerge again, or perhaps a new "super-Amish" culture may emerge from the Amish. There is no law of nature saying that there will be exactly one Amish-like culture ever, and once its fertility declines, it is all over.
And generally, I think that the exponential curve drives people to hysterical predictions, because it ends with a disaster either way -- exponent > 1 means we overpopulate and... (read more)
In With a Whimper: Depopulation and Longtermism, Geruso and Spears give the following argument for why most people who'll ever live may have already died:
People are generally having children below replacement rate: 1.66 children per woman in the US, and total global annual births peaked in 2014.
If you project this forward 300-600 years, annual births fall below ~10M.
This would leave us with a global population around 560M.
Only a minor disaster could be enough to wipe out humanity once our population is so low.
They include a pretty bold chart:
To be fair, pretty much any continuation of that chart into the future is wild, but the one they've ended up with seems especially so!
I don't find this argument very convincing for several reasons, but I want to focus on a specific one: even granting all their assumptions I think we'd see evolution for higher fertility long before we got down to 10M annual births.
The paper says:
If cultural evolution isn't enough, what about genetic evolution? They cite their 2022 research note Intergenerational Transmission Is Not Sufficient for Positive Long-Term Population Growth which does consider both cultural and genetic fertility, but their model of the latter doesn't seem to me to address the main ways I'd expect genetic evolution to reverse global population decline. Let me give a sketch:
Humans vary a lot in the strength and timing of their desire to reproduce. Some people never have any interest in children, others feel deeply drawn to parenthood from a young age, and for others it varies. One common way that it varies is that many non-parents, as they get older, find themselves suddenly seriously drawn to having children. This suggests, though doesn't demonstrate on its own, that desire to have children can be and sometimes is strongly influenced by our biology.
If a strong biologically driven desire to reproduce were possible, though, why wouldn't we already have one? Historically, having such a desire probably didn't have much of an effect on how many kids you had. Given the lack of cheap reliable birth control and much lower levels of sexual education, evolution mostly gave us a strong desire to copulate. Combined with cultural taboos on finding alternative outlets for this desire, there wouldn't have been much selection on desire specifically to reproduce. With those conditions changed, however, I'd predict we're already seeing strong evolutionary selection for this desire among humans.
Why doesn't this show up in any of the data Geruso and Spears consider in their article? I think the problem is that they look at many groups and see declining fertility in all of them, but all of these groups are large enough that they include people with a wide range of genetically-driven desires for children. I'd predict that within each group that share could be increasing, but it's not visible in the overall numbers for any group because other factors are stronger for now. Maybe research on the heritability of family size (comparing twins, or checking whether some families are consistently larger or smaller than otherwise similar ones across generations) would help check this?
This is a pretty specific story, and I'm not an expert in the areas I'm talking about. If anyone could point me to more analysis on these questions I'd like to see it! For example, it could turn out that there isn't very much biological influence on family size, biological drives are mostly only able to push people from zero to one children but not higher, or a few dozen generations wouldn't be enough to have much of an effect.
Comment via: facebook, mastodon