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:

But what, you might ask, about heritability (intergenerational transmission of high-fertility cultural practices)? 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. We have addressed this question at more length in Arenberg (2022). In the very long run (i.e., potentially after the coming few centuries of decline), two facts would have to be true for heritability to be a solution: 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. Where researchers have studied the empirical magnitude of these intergenerational correlations as they have played out in actual practice, they have found them to be positive, but small—too small, in fact, for the high fertility group to make much of a dent in overall population. It turns out your kids might choose not to inherit your cultural practices and beliefs.

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.

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If the population falls the circumstances that created the low birth rate will change. This seems like the equivalent of an economist extrapolating a high inflation situation into the future and determining that only billionaires will be able to afford tomatoes.

How will falling population change the high birth rate? I hadn't heard an argument that people aren't having kids because there are too many people. I'd heard that birth rates fall when women get to decide whether to have kids. I'd assumed that's because having kids is really hard, and that labor has just been dumped on women without their consent in the past.

If this is more or less true, it is subject to change in a wealthier society. I would've had kids if I could have been a stay-at-home dad, well-supported by one income. If both parents combined only had to work 20 hours or less, I think people would have a lot more kids. If there were more societal support for having kids (better schools and childcare), even more people would have more kids.

1.66 children per woman in the US

I want to stress that this is the total fertility rate (TFR), and not the completed cohort fertility (CCF), and therefore it is not a very good proxy for what you want to measure, especially since women are having children later. I wrote a post about it a while back, although it is far from perfect. You can also look it up on wikipedia or something similar.

[-]p.b.194

I think one salient point is the fact that we live in a world where the number of children you have is pretty much directly equivalent to your evolutionary fitness. In the past your evolutionary fitness was bottlenecked by whether you survive childhood, whether your children survive childhood, whether you are able to feed your children, etc - all in a malthusian environment. 

This means that the selection pressure for genes that increase your fertility is extremely strong. Much stronger than any selection pressure on any single trait that has been selected for in the past, say light skin or lactase persistence in Europeans. 

I don't think the claim "if human population is only 560M a minor disaster could be enough to wipe out humanity" stands under scrutiny. There can be all kind of different arangement that these number of people live, wth different levels of fragility.

Imagine a universe with only 560M humans spreaded through the galaxy. Say there are only about a hundred people on every inhabited planet. However there are also bilions of aligned robots ensuring the wellbeing of these humans and capable to repopulate the humans if they die out.

Such arangement seems more protected from disasters than tens of billions of humans living on Earth in high density.

I agree. Note that 560M is more than the estimated world population around year 1350, and not even the Plague managed to wipe out humanity at that point (despite being a terrible disease with huge mortality rate in a world basically without real medical knowledge). I don't understand what crazy kind of "minor disaster" they are thinking about...

Any population argument that stretches centuries in the future needs to contend with whether humans will attain clinical, biological, or some other form of immortality or amortality in that time. If we increase the lifespan to 300 or 1000 or 10000 years of healthy life, that both forestalls population decline and enables those who do want children to have more of them over the course of their lives.

Arguments that stretch significantly less far into the future still need to contend with that. 

True, but both the size of the impact and the probability of us having that capability increase with time, and I'd rather not get into a discussion of the timeline for attaining immortality, so I hedged my claim a bit. 

Fair enough.

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 starve to death, but exponent < 1 means we go extinct, and exponent = 1 (with infinitely many decimal places) is statistically unlikely. So by that logic we are doomed no matter what happens.

[-]Foyle1110

Ruminations on this topic are fairly pointless because so many of the underpinning drivers are clearly subject to imminent enormous change.  Within 1-2 generations technology like life extension, fertility extension, artificial uteruses, superintelligent AI, AI teachers and nannies, trans-humanism and will render meaningless today's concerns that currently seem dominating and important (if Humans survive that long).  Existential risk and impacts of AI are really the only issues that matter.  Though I am starting to think that the likely inevitable next generation of MAD - space based nukes hanging like an undetectable sword of Damocles over our heads is scary as hell too - coordinated global 1st strikes with only 2-3 seconds between detectability and detonation of stealth bombs re-entering from far distant orbits at >10km/s.

But even in their absence if there was close to a technology level freeze starting now evolution would move in to fill the gap - within a few generations individuals and cultures whose psychology or rules moved them to have more kids - eg religious fundamentalists, highly impulsive, strong maternal or paternal drives and strongly patriarchal cultures (Looking around world places that treat women like shit seems to have higher fertility) would steadily grow to dominate the gene pool - seeing a greater number of kids being born. 

[-]dr_s78

Honestly this worry in general, at the current stage, seems a bundle of nonsense to me. There is obviously a real and relatively pressing issue about an inverted demographic pyramid and its sustainability - that's a different matter, as it starts being felt pretty quickly. But "people will keep reproducing below replacement rates for centuries until humanity basically goes extinct" is so absurd I can't even begin to take it seriously. The current rates of reproduction are a product of culture as well as socio-economic factors. None of those factors would survive impact with even just a halving of the current population! I can imagine a dozen ways in which the assumptions would break, from "lifestyle has worsened, thus people have a lower bar for what counts as a good situation to have kids in, rolling back roughly to the 1960s" to "a shrinking and aging population means way more opportunities for young people or working age, which raises the relative well-being of such people, which makes it easier to decide to have children". A world in which population crashes is a world in which housing and jobs are desperately seeking for people to fill them, not the other way around - incentives change wildly, and so would people's priorities. It's honestly absurd to even try to extrapolate that way. The one scenario in which I think this wouldn't happen is if somehow we all had perfect friendly AGI-powered automation, with robots taking care of all our needs and entertaining us, possibly even replacing human connections. It's up to anyone to decide whether that's a good or terrible future, or how likely it is, but it's certainly much more than just a simple extrapolation from current trends.

[-]Viliam6-3

I can imagine humanity going voluntary extinct in the future. Making children is just one possible activity among many, and the number of possible activities and their attractiveness is probably going to grow, so at some moment most people may go "meh, too much effort". Or there can can be some weird economical effect where most people won't be able to afford children -- not because of meaningful resources such as food, but because of some bullshit reason (something that becomes infinitely expensive for some stupid reason, such as university education, will be considered a basic human right and you are not allowed to have kids if you can't afford to buy it for them). Or there could be a passively-aggressively unfriendly AI which can't hurt or defy humans openly, but may subtly discourage them from reproducing, because it is allowed to destroy humanity as long as it happens voluntarily and nonviolently. All of this seems possible to me, and yet the article seems needlessly alarmist.

The combination of technological progress and population decline could make children much cheaper. Just ask people how many children they would have, if they had universal basic income, affordable housing, affordable robotic nanny, and affordable robotic tutors. (As opposed to today, when often both parents need a job, stuff is expensive, and yet most people choose to have kids.)

I am curious about the specific details of "minor disasters" that can wipe out a population of 500M, but cannot wipe out a population of 8000M. Fewer people would probably still be distributed across the planet. For example, lower population density and more time spent online would reduce the risk of pandemics.

The attractiveness of "making children" can also grow. Imagine an automated robotic baby changing table. 

When it's robots that do all the boring or messy bits, leaving parents with only the fun bits, then parenting becomes more attractive. But sure, maybe video games are winning out in the attractiveness race. 

Although a population decline requires people die of something. So this scenario makes sense if we invent super video games, but not immortality.

So this scenario makes sense if we invent super video games, but not immortality.

Looking at what we have now -- there are 3D video games, 3D porn, but no 150 years old people (and few actually live to 100) -- unfortunately, the invention of video games is much easier task.

[-][anonymous]51

Well what about (wealthy) governments evolving a solution to fertility? So far governments have found it easier to import foreigners, where all the costs to raise them to educated adults have been paid for and they already have a professional job offer. This "brain drain" strategy only works until there is nowhere to drain from. (because the most populous countries that provide educated individuals are both becoming relatively richer and experiencing their own population declines)

As far as I understand it wealthy governments have barely tried seriously at all, perhaps from various Overton windows about how parents are supposed to be responsible to care for their own children. (Yet the education and career system withholds the funds to do so until parents are near the upper end of their prime breeding years. In addition housing shortages created almost entirely by the government, as well as medicine and education costs ever rising, mainly as a consequence of government action, cause the obvious consequences)

The government could afford to pay a lot more to young adults for the burden of having children. Each child is millions in lifetime income to the government if they succeed.

The government could do a lot of things to maximize the return on its investment.

One of the big problems with creche parents is statistically strangers care for others children less than their own and commit abuse. But with ubiquitous surveillance - lots of cheap cameras streaming to the cloud with basic AI to transcribe and look over the footage - abuse would be nearly impossible, and professional adults who's salaries are mainly paid by the government could take care of the young children, freeing up the parents to finish school/early career/socialize and have additional children.

Combine things like this together and you could settle on 4+ children per mother - where some have 10 or more as it is profitable to do so - and essentially eliminate your problems.

IMO I think the most important takeaways from this work are the following:

  1. Assuming you both have a view where certain humans are positive EV in your book and you think the long term matters, extinction risk reduction may look far less valuable, and in particular we can no longer assume that mere global catastrophes are survivable for the human race, like nuclear war or bioweapons, since we can no longer assume that humans will breed back from a mere catastrophe. Thus, extinction risk no longer looks that special in the grand scheme of things, and global catastrophic risks look more valuable for prevention.

  2. It imposes a deadline on long-termism, and in particular means that we can no longer appeal to very long-term projects. This type of reasoning was implicit in some arguments against making advanced AI like Geoffrey Miller's arguments, and while I think it could be salvaged somewhat, I do think that most reasoning related to hundreds to thousands of years long feedback loops or more needs to address the paper seriously, or it has 0 expected value in my book.

  3. For those who believe that certain humans have positive EV in your book, then becoming grabby soon (as in the next few centuries) is very important, and that it's plausible that the actions taken in the next few centuries determine pretty much the whole expected value of the future, and depending on whether infinities are supported, may be the difference between a future where the expected value is 0, and a future where the expected value is +infinity or -infinity.

  4. While I don't believe that this model will always be robust in all situations, I do think it's central conclusions are probably going to hold up assuming a non-grabby humanity in the next few centuries, and a humanity that doesn't get rocked by any global catastrophes, because of the essential uniformity of fertility and the inability of cultures to get out of the low fertility state once they get into it, combined with poor at best transmission of high fertility genetics.

I think the whole thing is a load of nonsense. There are lots of things that are likely to impact fertility rates.

 Firstly, it doesn't at all account for people trying to do something, like generous child tax credits or similar. It requires large numbers of humans to see that the population has declined for centuries, and do nothing to fix this. 

Then there is the more high tech stuff, artificial wombs and robot childcare, life extention anti aging tech, or a full tech singularity. 

Then there are random economic shifts. A rise in remote work sees many people leaving cities, they move to large countryside houses with room for many children. (And the ability to look after kids while working) 

Then there is evolution, biological and cultural.

Then the economic conditions that created the fall in fertility require a level of tech and wealth. What is the proposed model of the economy doing here. If we are reverting to medieval serfdom, fertility will rise. If we are looking at a high tech and wealthy society, why have they not invented any of the techs that would change the game? Is biological immortality really that hard? And more to the point, what disaster would kill 500 million wealthy, high tech people spread across the world that wouldn't also kill 8 billion people? 

I am really struggling to imagine any model of the future that fits their graph. As far as I can tell, their model was constructed by looking at some fertility data, and then pretending that, apart from the changes in fertility and population, nothing else would ever happen. 

Would you be up for expanding more on your last point? What's the reason for thinking the genetic heritability of fertility is "poor at best"?

I might want to mostly change the genetics to cultural transmission of fertility, but the biggest issue IMO is 2 issues:

  1. Even the high-fertility cultures are declining in fertility, and if the highest fertility culture is essentially 2.0 or lower, which demographers predict, then nothing can really save you over the long run, except evolution, and the issue will be discussed below.

  2. Admittedly, this is a cached thought I might have, but the basic issue is one of time. If it was happening in 10,000 years or more, I wouldn't be worried about it too much, but the big issue is that the time scale is probably too fast for evolution to catch up by default. This will happen in centuries, not millennia, and if I remember correctly, only bacteria or very small life can evolve non-trivial traits on the necessary time-scale. Maybe it's possible, but I currently suspect that this will be a tall order to select for higher fertility fast enough, and I think the selection effects are probably not strong enough to work.

Evolution can do some things in centuries, if the selection pressure is huge, which it is, and the change is simple, just adjusting a few parameters, which it is. 

More to the point, most of the reasons why this model is bunk are technological or cultural changes, not evolution. 

[-]dr_s20

the big issue is that the time scale is probably too fast for evolution to catch up by default

This isn't growing wings, it's some very simple changes. If the problem is literal fertility (too few sperms, women having difficulties getting embryos to implant, etc) then it's probably exactly the kind of thing that evolution can select for in a handful of generations. If the problem is a more general cognitive one (given the existing hyperstimuli and/or cultural context that make people less willing to have children, evolve people whose values are geared so that they have a stronger drive to have children even in these circumstances), that might be a lot more complex, if possible at all. But honestly anyway I doubt biology will play any major role in this either way. It's a matter of culture and economic incentives, mostly.

Yes, this will definitely be a problem starting in a few decades. Developing technology that can help people have kids (in vitro gametogenesis, and especially artificial wombs) is going to be important. Artificial wombs are harder, but probably more impactful for overall fertility rates in vitro gametogenesis.

 

Will Chertman wrote a very nice whitepaper on fertility last year, but I can't seem to find it now. (edit: here it is: https://docs.google.com/document/d/e/2PACX-1vT1hgf95phUa4e1nqf2VTfjFmUeSBwb7mihE86cU6bE1EI-EZteWm6UmeC18PJSs3Laog4rP7qKtT1j/pub)

I don't see any reason how the population can stabilize at the arbitrary level of 560 million. Either it will start to increase again at some point (due to cultural shift, natural selection or some other reason), or it will decline until the collapse of the industrial civilization. But in that case, with no universal education anymore, the demographic transition will reverse, and the population starts growing again to limits imposed by pre-industrial agriculture (which is somewhere around 2 to 3 billion, not half a billion).

Where do you see the paper predicting stabilization at 560M?

560M is not from the paper, it is from the post. The paper has graph with births per year stabilizing at around 5M, which can correspond to different population sizes depending on child mortality, but all of them are unrealistically low.

This would leave us with a global population around 560M.

Sorry, I was just trying to say that the paper says that in 300-600 years we'd have ~560M people, not that it says that we'd get to 560M and level out.