Daniel and Kelly’s Extraordinary Universe - Should we be worried about overpopulation or depopulation?

Episode Date: July 8, 2025

Daniel and Kelly talk to the authors of "After The Spike", who argue that humanity is on a dangerous path to depopulation.See omnystudio.com/listener for privacy information.

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To some, this is a disaster. Elon Musk, for example, says that collapsing birth rate is the biggest danger. civilization faces by far. With 14 children at last count, he seems to be attempting to avert this crisis single-handedly. Folks concerned with the decline in birth rates advocate for policies that incentivize having more children. But on the other hand, a decline in birth rate is music to the ears of others. As someone who adores both human beings and nature, I would love to see living standards rise as population size falls. It seems reasonable to assume that more humans translates into a greater environmental impact. So if humans are choosing

to have fewer babies, that sounds like a win-win for humans and for nature. Plus, I've heard humans are going to level out at 8 billion people anyway, so any decline in population size would be short-lived. And anyway, this seems like a problem for future generations, right? Why should we start worrying about population size now? Well, today we're going to talk to Dean Spears and Michael Jerusalem about their recent book After the Spike, Population, Progress, and the case for people. They are concerned that the future

will see a precipitous decline in human population size, and they want to see us start taking this problem seriously now, so that we have plenty of time to find compassionate solutions to the problem before society start to feel the pinch. Welcome to Daniel and Kelly's

Extraordinary Universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I have two children and a dog. Hello, I'm Kelly Weiner-Smith. I study parasites and space, and I have two children, two dogs, seven goats, two cats, and as of next week, I will have five ducks and two geese. Is that it really? How many rats are living on the property?

Well, you know, there used to be a lot of them, but we took care of them. And I know, I know, I know, I know, you and I really like rats, but not when they're eating the chicken's food. And it was just complicated and it was us or them. So my question for you, Callie, is why did you decide to have two children and not three or 17? And do you consider goats part of the Weiner Smith clan? I love the goats, but not as much as I love my kids. And so there's a hierarchy there for sure. Wow. Speciesist much?

Yes. It's true. I like humans. I do like humans a lot. We decided to have two because after we had the second one and he was old enough to be like sleeping a little bit better, I thought to myself, I do not have the energy to go through those long nights again. And my second pregnancy was high risk. I ended up in the ICU with hypertension after my son was born. And so I was like, I think my body is telling me two is good. And yeah, what about you? Why did you pick two? I feel like two is a special number because it's the same number as parents in the family. And so I think people really underestimate, like, how much more work a third child is.

And, you know, when we talk to the economists today on the program, they're talking about these numbers, one, two, three. But I really feel like there's a barrier there past two where it just really becomes much, much harder. And also, if you have, like, two academics and young careers, and it's just, you know, just didn't seem plausible for us. Plus, every pregnancy for us is high risk because Katrina's type 1 diabetic. Yeah. So it just felt like more than we could handle. So you and I both read the book, and they do address how difficult and time-consuming it is to have children also, like, how beautiful and amazing it is. And one of their suggestions is that, you know, society needs to modify in ways that make it easier, you know, easier to get child care, or it's sort of less work

for the moms and for the dads and just trying to make the whole thing easier. But I agree. Two was a lot more work than one, and I thought, I thought, well, I'm already doing this parenting thing. It won't be that much more. It's a lot more. And I'm still glad I did it, for sure. But yes, three is, to me, inconceivable. So thank you to all the parents out there having more than two children and helping us maintain the human population. That's right. Yes, it's an important job. It needs to be done well. And so speaking of the human population, the book that we're talking about today sort of projects forward. what the human population might look like as we go through multiple generations. And so we wanted to know what our audience thinks about how many people will be on the planet in 300 years, and here are their answers.

That's a good question, but realistically unanswerable, in my humble opinion, because there are too many unknown variables that can occur in 300 years that could wipe out a portion of all of humanity, from our own deeds to an asteroid impact like the size that took out the dinosaurs, or a combination of things like successive volcanic eruptions, wildly violent tsunamis, tornadoes, and plenty of earthquakes. So I don't think the question is realistically answerable. So the optimist in me would say that overall population would be in a slow decline by then, since as education levels increase around the world and the standard of living increases around the world,

people tend to have fewer children on average. In fact, they tend to have a negative birth rate, so I would say, one billion individuals. I kind of feel like we'd go back down to something like that. The pessimist in me says that we blow ourselves up with a nuclear disaster. Birth rates in all countries have dropped. I expect in 300 years we will have less population than today. It's easy to naively imagine living like the Jetsons 300 years from now, but considering the unsustainably outrageous rate of our populist over-consuming all natural resources except sunlight. I anticipate a bleak existence for however many humans can subsist on what remains. Choosing a number, I doubt even first-order

billions. Well, I think that in 300 years, there will be social pressure to just replace yourself. I think with longevity, people will be living for a very long time, and we will avoid overpopulation. I personally think we have a horrible future as a species. We are terribly overpopulated right now without enough resources. My guess is in 300 years there'll probably be less than 10% of the population left. Sadly, I don't think there'll be any people left on planet in 300 years. We are our own worst enemy, and considering the trajectory we're going in right now, I would say less of 5% of the current population.

Precisely zero. There's no way we'll make it that far. So, Kelly, why did you pick 300 years? It's a really specific timeline. Oh, I went to one of the figures in their book, and I looked at a point where the projected population size was quite a bit lower. And so I thought, you know, at this point, people can say up or down, and according to their projections, the answer should be definitely down.

But I got to be honest, I did not expect answers to be so dark from our audience. Usually there's like a bunch of jokes thrown in there like, I don't know, but here's a poop joke. And I'm like, oh, I love you guys. But this was like, we're probably going to blow ourselves up. Yeah. And I was like, oh, man, a couple answers like that. And so, wow. Well, I don't know.

Maybe digging this deep into science, into physics and biologies made people existential. You know, they realize how fragile our meatbags are on this tiny speck of dust. we're floating on in this vast universe. So it's sort of hard in that context to imagine we're going to be around in 300 years, 3,000 years, 3 million years. It seems like incredible we're here at all. Yeah, no, I absolutely agree. And I share concerns about what our species is going to do because we have nuclear weapons and are we going to like just totally accidentally blow ourselves up? I hope not. But from a like resources and population standpoint, for anyone who's feeling negative but would like to have their spirits slightly lifted. I really liked

Not the End of the World by Hannah Ritchie. She runs our world in data. And so she really digs into the numbers for a bunch of these like resource problems. And it's like one of my favorite books because I love the way she addresses these problems with data. And in the end, she decides, look, we can handle this. And it doesn't need to be too catastrophic in terms of all of us changing our lives. But we do need to make some changes. And here are some things. suggestions. And anyway, I really enjoyed her book. Awesome. I'll check that out. Well, today we're talking about the book After the Spike, Population Progress and the Case for People by Dean Spears and Michael Garuso out today. Go check it out. Kelly and I both read this book. I really enjoyed it

because I love books that take apart sort of popular misconceptions, like here's the standard lore everybody believes. Why did you believe that? Why is that not true? And what's more likely to happen? And this book does a great job of that. It's a little bit nerdy. and economical and data-heavy, but only as much as you need to convince yourself that these guys know what they're talking about and the issues they raise are real issues. It's also fun and their jokes. And as you'll hear in our conversation today, these guys are good at explaining stuff. Yeah. And I also felt like they were sort of kindred spirits with us because for a lot of the answers, it's, look, we just, we don't know. We don't have the data. And they're very honest about

what we do know, what we don't know. And I really appreciate that they're taking on this problem because it has sort of become politicized, as I mentioned in the intro. And I feel like they might be sort of making themselves targets, which is maybe why they are so, so tied to their data, the data and their arguments. And anyway, they make some pretty interesting arguments. And let me go ahead and give their bio real quick. Spears and Jerusalem are professors of economics and demography at U.T. Austin. Jerusalem served as a senior economist in the Biden White House. Spears runs the nonprofit research institute for compassionate economics, which helped establish a give-well-backed newborn health program in India. Welcome to the show, guys. Thanks so much for having us. Glad to be here.

My husband, Zach, has been absolutely raving about your book. He loved it. So I was excited to have a chance to read it. So let's jump right in. One of the things that I like about this book is that it gets me to think about population size way differently than I thought about for like literally all of my life. So I was born in 1982. When I was born, population size was about 4.6 billion, and it's currently over 8 billion. And I'm an ecologist. So I usually think about population size as being the thing that, like, kills animals and destroys ecosystems and stuff like that. So when I started hearing that, you know, oh, eventually population size is going to go down, that sounded like good news to me. And so your book was sort of like mental whiplash for me. So explain to us why

population is going to go down? And why, instead of feeling like, oh, this is maybe good news, why this is something that we should be concerned about? So we were also born in the 80s, and we also grew up hearing about overpopulation. But right now, global depopulation is the most likely future. And what global depopulation means is that every decade, every generation, the world's population is going to shrink. That's the path that we're on. Within a few decades, the world's population will begin to decline, and there's no reason to think that once that happens, it'll automatically reverse. So a big question before us is, should we welcome that, or should we want something else to happen? But even before that is why it's happening. And the fundamental explanation is that

birth rates are low and falling. So that's a way that humans are different from other animals, if you're thinking about ecology, that humans have cultures and society and decision-making and decide how many children to have. And over time, we've been deciding to have fewer children. And you know, when I say low birth rates, you might think something like South Korea, where there is 0.7 of a kid on average per two adults right now. But actually, it would be enough for global depopulation to have low birth rates like in the U.S. where it's 1.6 kids per two adults or like Texas or Latin America, where it's 1.8. Because depopulation will be the unavoidable consequence of there being fewer than two children in the next generation to replace two adults

in the last generation. And two-thirds of people now live in a country where the birth rate is below that two births per two adults average that would stabilize the population. And what exactly is new about that piece of knowledge? Like, is this something that demographers and economists have known for a long, long time, but the general population isn't aware of? Or have we recently learned something about birth rates? Why this conflict between our knowledge that birth rates are falling and the general sense among the whole population that the population is growing out of control? Yeah, I think that's a really good question. The fact of the matter is that falling birth rates are not some entirely new phenomenon that just popped up on the scene in the last

five or ten years. During this period of rapid global population growth, you know, over the last 50, 60 years, that was a period during which birth rates were already falling. It's just really hard to see and appreciate that birth rates are falling in a time when the global population is spiking up so quickly, right? So you mentioned you were born in 1980, Kelly, or sorry, 192, I was born in 1980, but the birth rate has been falling for a very long time. And it's been falling for as long back as we have good statistics to document it. So birth rates were falling in France in the 1700s. Birth rates were falling in other places in Europe in the 1800s, birth rates have been falling for the globe overall for as long back as we have any good

statistics. Now, like, there have been temporary blips, like the post-World War II baby boom, but on average, over the whole world, over time frames that span decades rather than individual years where things might bounce up or down, birth rates have been falling. And so I just think it's really hard to come to grips with the idea that even as the population is rising, birth rates have been falling. Can you break that down a little bit for those of us who don't always think in terms of differential equations? How was it possible for the population to be rising and the birth rate to be falling? Okay, good question. Population change at a basic level is the difference between birth rates and death rates. So there's two ways that population can increase. Births can go up

or deaths can go down. And the revolution that's happened around the world in the past two centuries has not been birth rates climbing. It's been death rates falling. And in particular, the death rates of children falling. That sounds like good news, yay. Yes, that's very good news. Yay, science. I mean, it's really good news. It's sort of hard to appreciate from a modern perspective just how awful the mortality

rates of children and babies were, not all that long ago. So a couple hundred years ago, the sort of the global average for mortality for a kid would be something like one in three kids dying before. age five, right? And so as we've learned to keep ourselves and our children alive through better sanitation, through an understanding of the germ theory of disease, through better water, through better nutrition, eventually through vaccines later at our history, what that's meant is that more people have grown up to become adults and to have children of their own. So the part of the equation that's been changing this time is that death rates have been falling. And birth rates have

falling two, it's just that they've been chasing death rates downward. And the net effect of all of that over the past 100 years, 200 years, has been just a skyrocketing of the global population. So just to put that in some perspective, in 1800, there were about a billion of us worldwide. About a century later, in 1925, we passed 2 billion. And then since then, in the last 100 years, we've quadrupled to the present 8.2 billion. And all of that action has been about declining death rates that have been declining faster than birth rates have been declining. And so is the idea then at some point we're going to max out on what we can do in terms of reducing death rates and now the birth rates are going to start being more important?

Yes, yes. We're going to min out on what we can do for minimizing death rates. Thankfully, mortality rates for children are so low that they're pressing up against a zero lower bound. So there's just not much more room for them to fall. Meanwhile, fertility rates continue to fall. Can't we all just transform into immortal cyborgs and solve this problem? That would be, that would be, that would be something. I love seeing you search for an adjective there. Well, I mean, okay, so there is something interesting there. I want to jump in on that because that is a question that we get, which is could lower mortality rates solve this problem?

We don't mean lower infant mortality rates, what sort of brought us up to spike? People say, what if people just stop dying in older age and live longer than? Like, what if instead of dying on average at 75 or something, we live twice as long and died at 150, right? Would that be enough to knock us off the path and make it so it's no longer the case that depopulation is the most likely future? And the answer to that question is surprisingly no. Now, you asked out differential equations, and I won't get into it, but just think of it as like this. If everybody still has two kids or 1.6 kids on average per two adults, then we're going to have, the same phenomenon where the next generation is smaller than the generation before.

A given generation will live longer, so the number of people alive at any one time will be greater, but depopulation will still be what's happening as generation after generation gets smaller and smaller. You're just stretching out the time scale of the effect. Exactly, stretching out the time scale. Now, it could be if we live to be 150 instead of living to be 75, and also the biology of reproduction change, that inside of having, you know, kids in our 20s and 30s and 30s and maybe 40s, you had a couple kids in your 30s, a couple kids in your 50s, a couple

kids in your 70s, right? Then we would get off the path from depopulation, but it wouldn't be because we were living longer. It would still be because we were having more kids. I'll pass. I don't know. I mean, what are you going to do with all that time when you're 140? And sleep, read, read. We might need to be cyborgs that to keep up with our toddlers when we're 130. That does not sound like a lot of fun. That sounds hard. I've been telling a bunch of my friends that I read this book and have wanted to, like, start conversations with them about it. And a lot of them will say to me, well, that doesn't match up with what I've heard. What I've heard is that the Earth's population is going to settle at $8 billion and we're going to stay there.

And a couple of people have said this to me. Do you know where that figure came from? And could you explain why it's wrong? Yeah. So I think it's just that people have known that population growth was slowing. And there's just like a trope or something that's come out of, you know, population growth is slowing and it's going to hit zero. And there just isn't a lot of thought about what happens next. So you might read a newspaper article that says, you know, demographers that come out with new information that shows that population growth is slowing and going to hit zero.

Yes, but then what happens next? You throw a ball up into the air. And at some point, it's, you know, vertical velocity is going to hit zero. But that's not the end of the story, right? the same force that slowed its ascent is eventually going to bring it back down. And I think people just aren't seeing that second side of the story, that just as falling birth rates have slowed the growth rate in the population, falling birth rates are also going to bring the population size down. So are you suggesting that we haven't had any sort of realistic model in the sort of popular

discussion of population? That it's all just been assumptions about how things are going to stabilize? I think there has been a lot of thinking that because the population size is growing, it's going to continue growing, even though there have been signs all along that that probably wasn't going to be the case. So all the way back in 1980, a long time ago, which we've already discussed on this, one in five people already lived in a country where the birth rate was below two. You know, in the 1970s, Europe passed below two, Japan passed below two, Cuba, Australia, Canada passed below two in the 1970s. And so not only have birth rates been falling for not just decades, but centuries, it's long been the case that we could have seen this coming. But I think

because rapid exponential population growth due to low mortality was so new on the historical scene, that got everybody's attention. And so it just wasn't that many that. thought through the if then, if the world's average birth rate falls below two, then what's going to happen next? And the evidence pointing towards that being the likely future. And you mentioned earlier that birth rates vary a lot from country to country, South Korea versus other places. And, you know, I've heard anecdotally about subpopulations that have very high birth rates. I remember my wife in grad school learning about some population where they have like seven, eight children on average. What do we know about the sort of variation

among sub-populations? Are we going to see the global population decline, but some populations grow very rapidly to compensate? What's sort of interesting, and I think flying under the radar in the popular discourse about this, is that really almost every region on Earth has either low fertility or falling fertility.

So it's really easy to think that this is a phenomenon that's special to Southeast, the rich economies of Southeast Asia, or maybe to Europe or the U.S. But, you know, we're here in Texas, and the birth rate in Mexico, just south of the border, is lower than the birth rate in Texas. The fertility rate in Latin America as a whole is something like 1.8, which is below the replacement rate. Let's see. So the three largest countries in the world are India, China, and the United States, and all three countries have birth rates below replacement.

And those are places that are very different. They're different socially. They're different economically. They range, you know, India is a development. country, China's a middle-income country, the United States is an advanced economy. But basically, everywhere in the world, we have low birth rates. And the exceptions to that, places like sub-Saharan Africa, birth rates are still high, so it's still something like four kids per two adults. But that number has also been falling over the last 50 years. And demographers

tend to project that will continue to fall into the future as sub-Saharan Africa continues its development in terms of rising incomes, rising opportunities, rising education. So let's take a break, and when we get back, we'll talk about what we know about why birth rates fall and how good we are at predicting this stuff. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers. The pilot is having an emergency, and we can. think you could do it? It turns out that nearly 50% of men think that they could land the plane with the help of air traffic control. And they're saying like, okay, pull this. Do this, pull that,

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have gone from four to, are they dropping from four? Or did they go from higher than four to four? They've gone from something like six in the middle of the 20th century to something like four today. So how good are we at predicting this kind of? stuff in like a 10-year frame, for example. So in a 10-year frame, professional demographers are good at predicting population trajectories. The reason that demographers are pretty good at predicting population sizes over a course

like 10 years is that the population just has a lot of inertia in it. So no matter what happens to, maybe we're sticking too much on calculus here, but a fertility rate is a flow and population is a stock. And so you can change the flow as much as you want instantaneously. And it's going to have no immediate effect on the stock. So in terms of predicting the population tomorrow, I can predict that very well, even though I don't know how many babies will be born today, because most of that prediction is built up in the stock of people who are already alive.

We know how people age and die in their later years. We can predict that with pretty good statistical accuracy. And so the uncertainty is really about fertility, and changes in fertility actually take a long time to play out into changes in population size. Now, how good is anyone predicting whether fertility rates will stay low, not just in the next decade, but in the next half century, the next century? I think we should be pretty clear that nobody knows for sure. But what we're seeing around the world is a convergent pattern of low fertility, fertility below two. And history is not a guarantee of future performance. Is that what they say in stock market predicting? But

if we look around, we shouldn't ignore history, and if we look around the world at all of the countries where over a full child-bearing lifetime, fertility rates have ever fallen below 1.9. There's 26 countries in that category. In none of those have lifetime fertility rates, meaning over the course of women's entire life, ever risen above two again. So that zero and 26 record isn't a guarantee of what will happen, but it would be something that we would do well to take seriously as a risk for the future. want to understand what we know about why this is happening and then also talk in a minute about things people have tried and why that might or might not work. But what do we know about

the overall trends that are causing this? So surprisingly less than you might think. You know, everybody has a theory of why birth rates are low and falling. But everybody's theories are different. And none of them really captured the bigness of falling birth rates. And by bigness, I mean, we've talked about how it's something that's been going on not just for years or decades, but for centuries. And also that's happening in societies that are very different from one another around the world. And so... So we can't blame it all on Joe Biden, for example. You can't blame it all on Joe Biden. You can't blame it all on any one thing. There's going to be no silver bulletics. So you hear people, you know, depending on, as you say, there are sort of preconceptions thinking about, well, maybe it's about capitalism or the free market or neoliberalism that's causing the problem. maybe somebody might say it's about feminism or the retreat from marriage or the decline of religion that's causing the problem. But whatever your theory is, you can find an exception. So for me, I learn a lot by thinking about India, which is the country that I study in my research

and where I work. And so India is now not only the world's most populous country, but the world's most populous below replacement country. It's below two also. Even though India was the place in the middle of the 20th century, that was the center of these overpopulation fears. And what makes India so interesting is that it's a society where religion continues to be important in almost everybody's lives, not just on a piece of paper, but actually something that's part of their practices. Almost everybody gets married and at young ages. Almost everybody gets married at young ages and starts having kids at young ages. And here's what's particularly striking is that female labor force participation in India,

is still pretty low in international comparison. So in a place like the U.S. or Europe, there's an important story about conflicts between career and family being part of why people aren't having so many children. And in fact, that's the title of Claudia Golden's book. She's a Nobel Prize winning economist, Career and Family. And this is very important in many people's lives. But even that's not the explanation that works everywhere because it couldn't be what's going on in India because so many of the women aren't working in the paid labor force.

but still having below replacement birth rates. And so no theory is right or applicable everywhere. And so part of what's so striking is that, yes, there's variability across countries, but that variability is shrinking as birth rates fall everywhere. Countries are becoming more similar in this way. And so what we're seeing is a convergent outcome. And that's part of what makes us think that depopulation is so likely. So Mike kind of already addressed this. But while I was reading the book, this was a question that kind of kept going through. my mind, so I want to sort of bottom line this. So we don't understand why birth rates are falling. So we don't know for sure that things won't change when we hit a population size of $6 billion,

for example, but we have really good data on trends, which suggests we should be concerned about this. Would that be a fair way of sort of summarizing the argument? Yeah, and I think we could say more, too. I think we can sort of do a little bit of introspection here. Sometimes we can too quickly jump to the idea that, well, this is like an unsustainable trend. You know, birth rates can't keep falling. I mean, the first thing to say there is that birth rates wouldn't need to keep falling where they're already low. They would just have to stay low. And then the question is, couldn't it be the case that people could continue to choose to have the sort of families that they do now? I mean, in a world that was shrinking, that fell to $6 billion or $4 billion,

I think it's quite plausible that if nothing else in society changed, people could look around and think, you know, lots of people thinking that two kids is the right number from them, some people thinking one, some people thinking zero, some people thinking three or more. I think people could continue to make those sort of choices, even as the population was falling. Sometimes people think that there's going to be some natural rebound to two, that two is some sort of special number when it comes to fertility. And two might be the number that a lot of people choose, but two is not a special number in terms of a population average.

We've never had a population average in any population, any time that just held exactly at two. When birth rates fall, they fall right across two and there's no like magnetic force pulling them back up to two. So I think there's lots of reasons to think that low fertility could continue even in the face of a falling global population. I think you're bumping up on something really interesting there, which is implicitly doing differential equations. You know, you're telling us that people don't respond to the density of people around them. It's not like families get together and they're like, hmm, there's lots of kids in the kindergarten, let's have fewer kids. Or while kindergarten seem empty, let's have more kids, that's not sensitive to the population itself, right? And it's more than that. It's that it might not even seem that different.

And so, you know, in Texas right now, the average is 1.8. You know, what if the average instead were 2.1, you know, that would make the difference for the world as a whole between depopulating and growing. But I don't think life would seem very different, living in a society with an average of 1.8 from living in a society with an average of 2.1. When you're, you know, jogging around the park or going to the store, it would feel pretty similar. And I think, you know, back to the question of how did this all slip under everybody's radar and how did we get on the path of depopulation without everybody noticing? I think part of it is because, at least in the United States, where we were right around this two level for a long time. One point eight's just not going to feel that different for. 2.2, but it's going to have very different consequences for the long-term future. And the story we're sort of getting to is that birth rates and rates of production of children somehow depends on a lot of complicated factors that it's not always obvious to dig out. And it reminds me of this story I remember from my wife being in graduate school. She had a friend who was working to study this population of Hutterites, a small religious

community in the Midwest. And they have like an enormous fertility rate, like huge numbers of children. And I remember these researchers studying like, why is it? Why are they so fertile? And eventually the answer was they just have sex every day. And that's why they have so many kids. And so it's not like, you know, we want to populate the earth or kindergartens are, you know, too empty. It's just like a product of lots of choices people are making about their days. But their numbers are going down too. Yeah, we talk a bit in the book, not about the Hutterites, but about like the Pennsylvania Dutch, the Amish. And we talk about it because I think a sort of a certain kind of math-inclined person would hear these arguments and think, huh, well, it's one thing to say

that many populations have birth rates that would lead to decline, but there are these subcultures like the Amish that just have tons of kids, right? So even if we're not having a lot of kids in the rest of America, the Amish are having a lot of kids. And you could do a sort of extrapolation to think, well, if the Amish keep having more and more kids, they're going to build up to be a larger a larger fraction of the population. And, you know, so, you know, we look this up and the recent population growth rates of the Amish or something like 4% a year. And if you just extrapolate that naively, then, you know, in 200 years, there's going to be a billion Amish in the world. But we don't think that argument makes an awful lot of sense for a couple of reasons.

One is that sort of high fertility enclaves, whether it's the Amish or any other, their fertility is also going down like Kelly alluded to. So, you know, maybe last century the Amish were having eight kids per two adults. Today, the estimates put it at something like five or five and a half kids per two adults. It's hard to know there's not a census of the Amish the way that there's like a census of Texas. So reason one is that even among high fertility subcultures, what counts as high fertility is changing over time. And, you know, Sunday, high fertility might mean two kids instead of one rather than five kids instead of three. The other reason that we shouldn't count on the Amish outnumbering grains of sand on the beach is that people don't always adopt the cultural practices of their parents. We see cultural change all over the world.

South America is changing from being more Catholic to becoming more Protestant over time, just as one example. But, you know, in our own lives, I think we've all had the experience of being teenagers and projecting one cultural practice or another of our parents. So these links that connect generations aren't perfect. And you really have to push this idea of cultural heritability pretty far to think that even as the Pennsylvania Dutch ran out of Pennsylvania farmland because, you know, they're spilling across continents because there's just so many of them that somehow these cultural practices and preferences are going to be inherited one for one from parent to child. I think some of that cultural stuff is at play with the hutterites because we, hutter rates are communal. So my husband and I spent a lot of time reading about them when we were trying to decide if communes would be a good model for early space settlements. And so we became sort of experts in hutterites, and they were also lamenting their declining birth rates. I keep waiting for somebody to make a weird al joke here.

Nobody's going to talk about how we're not going to be living in the future Amish paradise. No, we missed that cultural touchstone. Let's talk a little bit about why depopulation could be bad. So, you know, as an ecologist, the idea that we might have four billion people, like, I don't want there to be zero people. I like people. And I don't want any people to die. But if people are choosing to not have as many babies and we end up with something like $4 billion, it doesn't sound so bad to me initially to have fewer people that we invest more in. But you all make a pretty good argument about more people being better for a lot of reasons. So let's hear about why we don't want billions of people fewer on this planet. Well, first I want to hold on that $4 billion thing for a second, because I think it's important. If somebody's listening to this and they're thinking, yeah, let's have some depopulation and what I want is a future where the world stabilizes at some number like $4 billion or $3 billion or $2 billion, then that sort of future would fundamentally require the same thing we're talking about here, which is an increase someday in birth rates. The population is not just going to suddenly

hang and hold still at $8 billion or $10 billion, and it's not just going to suddenly hang and hold still at $4 billion either, unless birth rates go back up. So in our book, after the spike, what we're considering is, would it be better to have a future of depopulation generation after generation, or would it be better instead to have a future of stabilization? And not to hide the ball, we make the case for a future of stabilization. Now, we don't say where. That's beyond what anyone could know given the science we have, but let's take $4 billion. For the world's population to ever stabilize someday at $4 billion, that means after a period of depopulation, birth rates around the world are going to have to go back up to two and stay there. And I just want to

bottom line that to make sure people caught that because you're arguing that if birth rates stay below replacement, we won't just go from $8 billion to $4 billion, we'll go to $2 billion and $1 billion and half a billion and you just keep going down. Right. It's, you know, when your rocket ship gets to the planet that you want to be on, how do you slow down and stop? When the global population gets to the size that you want to stabilize at $4 billion or $3 billion or whatever, if you want to stay there, then just like at any other size, birth rate you have to be an average of two. So this is my conciliatory message that we're on the same team here if you want the size of the population to stabilize somewhere. And we face the same question of, you know, how would we

ever get back to a birth rate in an average around two. Okay. Now, another question, and it's the one that you asked, which is, why would we want that? Why would we want the size of the population to stabilize or to avoid depopulation, generation after generation? And here's one reason why we think that depopulation matters and that we should want to avoid it, because we're all made better off by sharing the world with more other people. Other people alive alongside us, other people. Other people who are alive before us. Other people make the discoveries and have the ideas

that improve our lives and other people are where science and knowledge comes from. I like people, too. People are awesome, yeah. We can drill down on that, too. I mean, why are lives better now than a couple centuries ago?

You know, why do we all get to have shoes on our feet and glasses to correct our vision, plenty to eat, climate-controlled work environment, shorter work days, a library of literature, AI in our pocket,

you know, social safety net systems to protect the elderly and disabled. Why are living standards so much higher today than not all that long ago? It's the same sun overhead. It's the same wind blowing. It's the same dirt underneath our feet. But knowledge is the difference. We know better what to do with these things. We know how to extract the minerals from the rocks and make glass and solar panels and computer chips. We know how to farm more efficiently to grow more food, more calories per acre of that dirt. And we know how to do that with less human labor. We know how to make soap to keep us from getting infections. We know how to make antibiotic pills to treat infections.

And we have a germ theory of disease so that we know what an infection is. And we know all of these things because of people that came before us. We know better how to organize a kindergarten or a cancer drug trial or a parliamentary democracy. These things, ideas, discoveries, process improvements, innovations. They are why we all, on this call and listening to this podcast, get to expect more out of life and better things out of life than people did not all that longer, though. This notion is something that economist Paul Romer won a Nobel Prize for in 2018. And his realization that he sort of formalized in the language of economic math was that

an idea isn't scarce in the same way a parcel of land or a physical object like a hammer is scarce. Instead, an idea can be reused and shared endlessly. You know, we've been talking about differential equations. I took differential equations as an undergraduate engineering major. Maybe Kelly or Daniel, you had the same textbook as I did. But when I read the words out of that textbook, it didn't cause the words to disappear from your copy of the textbook, right? We get to reuse these ideas over and over again. And so once Newton or Leibniz invents invents calculus, or maybe if they didn't live,

somebody else would have, then we all get to benefit from that over and over and over again. So ideas and scientific knowledge are a sort of special resource that never gets depleted no matter how much it's used. And that's sort of one of the core reasons why sharing the world with a lot of other people or getting to live at a time after a large population has already existed. It's just a huge benefit to us. And it's more than just expanding the population and creating new innovations, right? It's about maintaining our quality of life. If we need a certain number of people to keep making t-shirts and pencils and teaching our children, like the world, our infrastructure can't survive on a tiny population. But it makes me wonder if we're going to do a ridiculous extrapolation.

Tell us about sort of the numbers that people we expect to have on Earth. If birth rates don't change, how many people will we have on Earth in 10 years, 50 years, a thousand years, extrapolate all the way out to, you know, 25,000 years. Are we going to end up with like 20 people on the planet? Okay. We don't know, right? We don't have a crystal ball, but we can make if-then assumptions, right? You know, if the birth rates end up like this, then we can say something about how the future would evolve. And so one easy and plausible case is to imagine the worldwide average birth rate of 1.5, which is what it is right now in Europe, you know, a little bit less than in the U.S. and a little bit more than in Canada. So 1.5, that's the next generation being 25% smaller. The next generation after that being 25% smaller again. So if the world were like that, then every decade, we would see the size of the world population fall by 10%.

And every century, the size of the world population would fall by two thirds. And that cumulative exponential decay would continue and we could get, you know, quite small in just a few hundred years. If instead of it being 1.5, it were an average of one, like in China today, then you can see the next generation is going to be half the size. The generation after that, the grandchildren's generation is going to be a quarter of the size. And so the fall could be very fast. Now, how does all of that play out? How does that, you know, cash out in something big breaking or something breaking down or not having the discoveries we need or even the librarians to staff the libraries that we can maintain the knowledge we have? That's hard to know. But we can learn something about it by thinking about sort of comparing more populated places like cities and less populated rural places. You need enough other people around who want to need the same sorts of things that you want to need in order for those things to exist, to be feasible, to be provided by a government or a business or a nonprofit.

Other people wanting to need what you want is how you get it. And that's true if what you want is well-functioning public transportation because a network of trains and buses can't operate without enough riders. It's true if what you want is a green energy infrastructure built on the work of scientists and engineers and implementers. And it's true for all of us as a whole if what you need is a vaccine for a novel virus or a cure for a rare disease. It's the sorts of things that only the niche medical specialization of a big world could produce. And so as there were fewer and fewer of us, we're not going to be able to provide those sorts of discoveries, products, innovation, creation that are only possible with other people. So other people may sometimes be your, you know, another person may sometimes be your competition. But other people, there being other people over the long run are why things like this can exist and the resource of abundance.

Let's take a break. And when we get back, we'll talk about some, let's say, less than compassionate methods. that have been used in the past to try to improve birth rates and what are some more ethical options we could consider in the future for improving birth rates? Imagine that you're on an airplane

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Listen, I am not here to judge. It is so expensive in these streets. I 100% can see how in just a few months, you can have this much credit card debt when it weighs on you. It's really easy to just stick your head in the sand. It's nice and dark in the sand. Even if it's scary, it's not going to go away just because you're avoiding it. And in fact, it may get even worse. For more judgment-free money advice, listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. I had this like overwhelming sensation that I had to call it right then. And I just hit call. I said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation. And I just want to call on and let her know there's a lot of people battling some of the very same things

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I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcast. And we're back. And we are talking about how the average birth rate globally has fallen below two and the implications of that. So when I think about ways for trying to improve birth rates, the first thing that come to mind are some authoritarian governments who have tried to control birth rates in ways that I'm not super comfortable with, to put it lightly. So let's review some methods that have not worked when countries have tried to improve their birth rates in the past. What do we know does not work? Well, all of them.

All of them. There isn't an example of, you know, government coercion, whether trying to coerce higher birth rates or trying to coerce lower birth rates, you know, resulting in the sort of lasting, big picture, sustainable change to birth rates that could knock the population off of a path of depopulation and onto a path of stabilization. I mean, I want to take a step backwards here. We're making the case that a stabilized future population would be better than global depopulation. We also think that a stabilized population is compatible with commitments to environmental stewardship, to reproductive freedom, and to progressive priorities more broadly. And we want other people to think so to be a part of this conversation. So we don't

want anyone to misunderstand that if somebody chooses to have no children or few children, that's not for anyone else to say whether they're making a mistake. And that's a question of an ethical starting point. But it might still be that all of us together are making a mistake when we make it hard for people to choose larger families and choose to have children. So you can talk about this and talk about whether stabilization is the future we want without it necessarily invoking the idea that anyone should be coercing anybody else. Now, that said, it's an important and reasonable thing to worry about because there have been examples where governments have tried to compel people to have kids they don't want to have or to compel people not to have kids. So, you know,

the classic example that people think about is China's one-child policy, where the government of China made it, you know, illegal and with harsh and coercive implementation to have what they consider to be too many children. And sure enough, if you look at the birth rate in China over this time period, it fell. However, that doesn't turn out to be evidence that the one-child policy is why birth rates in China fell. And the surprising fact is that if you take the birth rate in China over time, so make a plot where, you know, you're looking at the birth rate in 1980, the birth rate in 1985, the birth rate in 1990, what you see is the birth rate in China fell over time while the one-child policy was in effect. But if you also look at what happened to

birth rates over time in any other middle-income developing country that was also undergoing the same sort of increase in living standards, increase in education, specifically increase in female education, you see birth rates falling in basically the same parallel way, that that was happening all around the world in countries about that sort of level of socioeconomic development. And so what that tells us is that it probably wasn't the one-child policy that was responsible for the big picture decline in birth rates in China. That's not to say that China would have ended up in the exact same place without the one-child policy, but somewhere pretty similar.

And it's certainly not to say that the one-child policy didn't do a lot of harm to people's lives in China, but the birth rate is probably where it would have been otherwise. And wasn't there, I can't believe that I'm forgetting the name of the country and the leader, was it Chow Chescu? Somebody tried to increase birth rates by making birth control unavailable or something like that. Correct my historical and act. Accuracies, please, and tell us that story.

No, that's your, you are correct. Yes. So this was Romania. This was 1966 or 67, Chichescu. He implemented Decree 770. And what Decree 770 said was that birth control was no longer going to be available or legal, that abortion was going to be illegal. And just to put this in context, in Romania prior to that time, abortion was more freely available than it's ever been. for like the whole of the United States. So this was a big change in Romania. And that big change

overnight did cause birth rates to rise overnight. So, you know, you pull the rug out from people. You change what's available to them. And birth rates did climb between 1966 and 1967. I think they, something like almost doubled. Wow. But in the years that followed, birth rates started tacking back down. And, you know, within something like a decade, they've fallen two-thirds the way back from where they started prior to the policy as people made other plans and built their family planning

around the new coercive regime. So that policy did a lot of harm to people and, you know, just a lot of harm. I mean, women died. Pregnancy became very unsafe. So it was awful, but it was awful without achieving the goal of boosting birth rates super high.

And, you know, Chichescu was overthrown he was lined up against the wall and shot. And so if your formula, and I know this is not anyone on this call's formula, but if it's anybody's formula to try to bring birth rates up by, you know, a dictator coercing women to have children they don't want, that's not a sustainable option for anybody, including that dictator. So you want policymakers in the audience to hear that as a cautionary tale. It's a good message.

So in either direction, trying to. push numbers up or push numbers down. We don't have good evidence that it works, and it's incredibly unethical the way it's been done. Yeah, I think we even have, I say even something a little bit stronger, we have good evidence that it doesn't work. What about other things like trying to provide more child care or making college cheaper or more family planning? Aren't there ethical ways we can make it easier for people to have more kids? There are lots of ethical things that we can try and that countries have tried, unfortunately, the track record there isn't as great

as one would hope it would be. So I think we're still a long way to go to figure out what might work. You know, on this question of making childcare more affordable, more available, you know, if you look to Scandinavian countries, for example, places where college tuition is free, where there's universal health care,

where there's childcare that's not only heavily subsidized or free for, you know, pre-K child care, but it's also provided, like it's available. I don't know what your experiences are, but my experience as a parent is like it's just hard sometimes to organize child care for when work is on, but school is out or, you know, summertime, it's a patchwork of things.

But even in places where child care is better organized, free, where tuition is free, where health care is more available or free provided socially by the government, these aren't places with high birth rates. And so, yes, I think there are things, that there's plenty left still to try in the toolkit of what, like, free liberal societies can do to make lives better and easier for parents. But I don't think anybody, not governments,

not social scientists, have figured out what that special sauce is yet. I want to sort of say that maybe that's not so surprising. I mean, think about, you know, the sort of policies that we hear of, you know, a few thousand dollar baby bonus or something like that, right? You know, let's do a thought experiment. Would you have agreed to marry somebody other than the person who you married for about $5,000? I don't think very many people are going to say yes to that, right? Our spouses listen to the podcast, so we can't actually answer that question. Okay, so it's not a fair test.

Anonymize, I didn't do my research procedures, right? But I think, you know, these are people's biggest decisions forming their family. What's their life story about? What autobiography am I writing, right? You know, a few thousand dollars will help a lot. And, you know, if you ask me or you ask, Mike, we'll say that society should do more to help parents, must do more to help parents, right? But when we say do more, we're meeting something a lot bigger. And it just shouldn't be a surprise. If we don't think that a few thousand dollars is going to change who you're going to marry, it's probably not going to change your biggest picture decisions about how to form your family and form your own life story and other ways, too. And so, yes, Yes, by all means. Let's support parents, support families. Let's do that. But let's not be surprised when that doesn't, you know, shift us from the path to depopulation, the path of stabilization. I was listening to left, right and center the other day. And one of the hosts, Sarah Isgir was saying that, you know, one of the reasons population is going down globally is that when women have more reproductive choice and more options in life, they choose to have fewer kids. And one of the things that I love that you all address directly in your book was that increases in population size would, you know, in the absence of artificial wounds, have to come from women having more babies, which has a health impact, a career impact, et cetera.

So one, my question is, is it true that when women have more choice, they have fewer children, and that's the mechanism? It sounds like maybe we don't understand that mechanism as well based on our conversation we've had. And what do you propose as the method for having more babies and in a way to do it that sort of distributes this responsibility more evenly? I think that we all have a shared interest in there being future generations and so that we all need to be pitching in to making there be future generations and sharing and lifting those burdens off of one another. And I think it's no surprise that in a history where we have long, put a lot of that burden on a subset of the population. I mean, mothers in particular, right, that we have people saying, no, thank you. But because that's been the past, that doesn't necessarily mean that that has to be the future. I mean, sometimes we hear people talking about

these conversations about low birth rates, and they act like there's a choice we have to make. Either we can have regressive gender politics that asks more of women, or we can have global depopulation. I think people both on the left, and the right start these conversations with a presumption that if we're going to stabilize the population, the extra burden has to fall on to women. And I don't think that's right. I don't think that we should start by asking more from mothers. I think the place to start or a place to start is by asking more from fathers. Males can't get pregnant. That's right. But it takes more than nine months to make a new person. It takes decades to raise a child into a new adult. And that

means there's plenty of time and plenty to do for men to even things out. Dads can get up at 3 a.m. to sue the crying baby. They can cook even though they're tired. They can process the laundry and pack the snacks and figure out about the child care when school is out and drive to swim lessons and, you know, do so much. They can accept the work and the responsibility and being the person who's on call on sick days and summer days. And they can do that for years and years and years and years. And, you know, I think this is, it sounds nice. I mean, it's more than just hopeful rhetoric.

I think it's a challenge, and we shouldn't consider there to be any sort of guarantee that this is what's going to happen. But, you know, the future of birth rates is going to be chosen. Whether or not we depopulate and whether and where we stabilize is going to come down to the choices that people, and especially women, make for their lives. That seems pretty clear. And so if we're going to expect people to choose parenting, then there's work that we all need to do, to make parenting more attractive. And that starts with lifting and sharing the burdens that for so long have only fallen on some of us.

I loved reading that part of your book. And is this why you've written a popular book instead of like making this argument to policymakers and within academia? Are you writing this to the general public to make people aware of this coming issue? Well, there's that and the differential equations. But no, I mean...

That's really your goal in the end, right? This is really just a matter. listen. This is a topic that more people should be talking about, not just, you know, more numbers, but a broader set of people. I mean, we're at the very beginning of facing up to this challenge. Most people haven't yet come to the terms of the magnitude of what we're facing. But, you know, we wouldn't have written this book calling to avoid depopulation if we didn't think it were possible for humanity to change course. Depopulation is the path that we're on, but it's not the inevitable future. Sure. People ask us, so what's your policy recommendation? What legislation would you write? And I think jumping to a policy solution at this point is probably the wrong move. This isn't something that's going to turn around in one presidential term or because of one set of legislative considerations for parents or, you know, a new bill about parental leave. I think we're facing something big. And the first step is many more people sharing a belief that we should want something to change. That's a necessary precursor. If you're already convinced and you're asking what to do next, I don't think it's advocate for a particular piece of legislation. It's invite more people into the conversation and convince your friends and family and classmates and colleagues that people are fundamentally good for another and that depopulation is not going to get us off the hook for challenges like climate change and our environmental problems that we face. start a conversation about what could be good about a future in which more people feel like they're able to and supported to choose parenting. That sort of reorientation to thinking that stabilization might be a good future has got to be the first step.

And in the meanwhile, there's a lot of social science to do that we should be building out to better understand what might work so that once we're able to build the social and the political support for a stabilized future, we'll know what to do. But there are a lot of minds to change first. Can I end us on an aliens question? Of course, it wouldn't be a DKEU episode without it. So the fact that we see this trend across so many different communities in so many different circumstances makes me wonder naturally if this is something that exists in, I don't know, all intelligent technological civilizations in the galaxy. And do you think maybe what you've done here is stumble on the solution to the Fermi paradox?

I'm so glad you asked that. Wait, wait, wait, wait, wait, we need to stop just for a moment. But you've really made Dean's Day. He wanted to put the Fermi Paradox in the book. I don't think we ended up putting the Fermi paradox in the book. Big mistake. We cut it. There was a cutting room floor.

But I was interviewed on a podcast from someone at SETI, and I brought this up. And he was just, it was almost like, like, how could I say something so silly and besmirch the podcast? But yeah, I mean, I think it makes a lot of sense that, you know, individuals have to sacrifice to make the next generation. There are shared public benefits of there being a next generation. You're going to learn how to control your birth rates. And so I do think that this sort of dilemma is probably a common one.

Now, I am just a humble demographer of Uttar Pradesh, India. I don't, you know, I can't speak to the larger questions. Dean is glowing right now, everyone. He's very excited to be talking about this. We love it. All right, Dean and Mike, thank you so much for being on the show. This was fascinating, and anybody who wants to learn more should go grab the book after the spike. Thanks very much, guys.

Thank you so much. Thank you. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on Reheart. recent shows, suggestions for future shows.

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