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Dean Spears and Michael Geruso are economists, demographers, and associate professors at the University of Texas at Austin. Spears is a founding executive director of r.i.c.e, a nonprofit that works to promote children’s health, growth and survival in rural India. Geruso served as a senior economist at the White House Council of Economic Advisers under President Biden, where he advised on healthcare and population change. Their book After the Spike: Population, Progress, and the Case for People is out on July 8, 2025.
In this week’s conversation, Yascha Mounk, Dean Spears, and Michael Geruso explore whether a higher population really impacts climate change, why more people on the planet means more geniuses, and how we can avoid depopulation.
This transcript has been condensed and lightly edited for clarity.
Yascha Mounk: In 1968, Paul Ehrlich published probably the best-selling book about demography ever written, called The Population Bomb. On the cover, it said: While you are reading these words, four people will have died from starvation, most of them children. Ehrlich’s fear was that unless we took radical steps to reduce the global population—steps that many countries, like China, did take in the subsequent years, in part inspired by Ehrlich—we were going to have terrible human suffering on our hands.
In your new book, you say that actually the concern is the other way around. The concern today is not overpopulation but underpopulation. What did Ehrlich and other people inspired by Thomas Malthus get wrong? Why should we worry about depopulation rather than overpopulation?
Dean Spears: Ehrlich and the 20th-century overpopulation doomers were wrong on their values and wrong on the facts. Let’s start with the facts. Global depopulation is now the most likely future. What global depopulation means is that every decade, every generation, the world’s population will shrink. That’s the path 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? Ehrlich missed that depopulation was coming, and he probably wouldn’t have answered the question of whether we should welcome it right either—because depopulation won’t solve climate change, or reduce global poverty, or make the world fairer. In fact, we stand to lose something important, because people are a core ingredient in scientific and social progress. Other people are win-win. Their lives are good for them and good for you.
Mounk: I think you’re already teasing two of the main questions I want to get at in this conversation—perhaps we’ll add a third. The first question: is depopulation actually happening? I think the instinct that a lot of listeners to this podcast are going to have is: isn’t the population still growing a lot? Aren’t there many countries in Africa that still have five, six children per woman? How can we be so confident that depopulation is actually going to happen?
The second question is: perhaps depopulation is a good thing? What about those concerns about climate change, the use of global resources, etc.? Humans are a cancer on the planet, and there should be fewer humans—right?
The third question I want to get at is, if we buy those first two arguments, what do we do about it? How do we actually respond to all of that?
Let’s start with this empirical premise. I’m a skeptic of how easy it is to project things in the social sciences. Economists famously have predicted 12 out of the last five recessions. Political scientists didn’t see the rise of authoritarian populism and the crisis of democracy coming at all—2016 was a complete surprise to them. Often, we get things wrong in demography as well.
How can we predict with such confidence that even though the global population still seems to be growing—even though it grew a lot over the course of the 20th century—that’s going to reverse very soon and that we’re going to start depopulating?
Michael Geruso: I'll take the first part of that question. Maybe it's useful to just take a step back and give a bit of context on where we've been over the last couple of centuries. For most of human history, going back thousands of years, the global population was pretty small. Ten thousand years ago, there were maybe 5 million people on the planet—the size of Atlanta today. It wasn’t until about 1800 that we had a billion people on the planet for the first time. Then in the century that followed, through 1925, the population doubled to 2 billion. And since then, we've quadrupled—we’ve doubled and doubled again—to the present 8.2 billion.
I understand the intuition many people have that we live in a big, growing world and there’s no reason to think that’s going to reverse, because that’s the only experience any of us have had: living in a population that was big and growing fast. But interestingly, this whole time that the population has been growing—and that we’ve had good statistics recording the demography, recording birth rates all around the world—birth rates have been falling. So even as the population has been growing, birth rates have been falling. And that’s the first clue to understanding how we can know that depopulation is around the corner.
The reason the population has been skyrocketing over the last couple of centuries isn’t because people were having more babies than they used to. It’s because we’ve done a better job of learning how to keep people alive—more children now grow up to become adults who have children of their own. Whereas a couple hundred years ago, maybe 30, 40, 50 percent of children would have died before the age of five, today a very small fraction—even in the poorest places in the world—die in childhood.
These two things have been happening simultaneously: population has been growing, birth rates have been falling. And today, in two-thirds of the countries on Earth—or two-thirds of the populations on Earth—birth rates are too low to sustain populations over time.
What’s the magic number there? It’s a total fertility rate of two—two kids per two adults. Below that level, each successive generation is smaller than the last. And so over the long run, the global population will shrink.
Mounk: I think part of the striking thing here is that, yes, there are some countries that still have rapid population growth—Kenya, Chad, other places in sub-Saharan Africa among them—but lots of countries that I think a lot of the audience here would assume are growing fast have actually now fallen below that magical threshold.
Mexico is below that threshold. The Islamic Republic of Iran, despite its ideology, is well below that threshold. India—which is where Paul Ehrlich starts his book in 1968, describing in pretty unpleasant terms the teeming mass of humanity crushing down on him as he walks through the old streets of Delhi, a scene that helped inspire his fear—is now just about below replacement rate.
Geruso: No, that’s right. And I think some people—understandably, since this hasn’t been the issue they’ve been paying attention to in the last decade—might think this is a phenomenon of rich countries; that it’s something happening in North America, or in Europe, or in a couple of select Southeast Asian countries like South Korea. But India has below-replacement birth rates. Latin America as a whole has a birth rate of 1.8 kids per two adults, so it’s also on course to shrink. Really, the only region on Earth where birth rates are still above two is Sub-Saharan Africa. But even there, birth rates are falling. Whereas in the mid-20th century, there might have been five or six children per woman, today there are four. And four is still above two, but we should expect that sub-Saharan Africa will follow the same course, in time, that the rest of the world has as it’s developed: fewer children, smaller birth rates, and eventually a path to a shrinking population.
Mounk: One of the striking things about sub-Saharan Africa, I think, is that it is by far and away the poorest region in the world. There's a strong correlation between affluence and birth rates. But relative to the level of GDP, Africa actually has lower birth rates than other continents did at the same stage of development.
Spears: Well, education is another thing. GDP is one thing, but one of the things we emphasize is the level of secondary school education—especially for girls. Sub-Saharan Africa right now has a low level of female education, similar to what was true in India a few decades ago. It’s at an average birth rate like where India was a few decades ago.
But right now, schooling in India—secondary schooling for girls—is around 70%, and their birth rate is right at two. Kenya’s at 53%, and their birth rate is at three point something. Mozambique’s is at around one third, and their birth rate is close to five. So it would be a real stepping off the path that other countries have followed if, as education and socioeconomic development of other sorts come to Sub-Saharan Africa, we don’t see birth rates falling there too—which is what the demography community basically projects.
Mounk: Isn’t there one assumption behind this that perhaps we can’t fully rely on? I buy that there’s nothing culturally different or specific about sub-Saharan Africa such that—if the Philippines, which had a lot of kids at one point in time, or Pakistan, or Ireland, which also had a lot of kids at one point, started to have fewer and fewer kids as they became more affluent and as more women gained an education—somehow Africa is going to be the big exception.
But what we’re assuming in the background here is that Africa is going to continue to develop rapidly economically, that the share of women who are able to access education is going to increase, etc. Can we be sure there’s going to be that level of economic progress in sub-Saharan Africa? Can we be sure, for that matter, that there won’t be regions in the world that fall back into abject poverty, where fewer people are able to access education in the future, and where, therefore, we also see a commensurate rise in the fertility rate in the decades to come?
Geruso: No, I think there’s no guarantee of anything. Let’s be really clear about this. When we sound the wake-up call about global depopulation, we’re not guaranteeing that depopulation is going to begin in 2060 or 2070 or 2080. Those are the ranges of forecasts and projections that groups like the UN put forth. Certainly, all sorts of futures are possible. In some sense, if we didn’t think that something different than global depopulation was possible by the end of the century, we wouldn’t have written this book.
Now, the possibility you’re raising—that things just get worse, that we regress on all sorts of measures of progress, socially and economically, and that ends up causing birth rates to rise—I don’t think we’re at the end of history. I think there are no guarantees that progress will continue. But that would be awful. We’re talking about one risk here, which is the risk that even as the world gets better, we’re headed to this unfamiliar future because the globe is going to shrink rapidly. You’re talking about another risk, where we actually fail to continue to make progress and regress. That’s also a risk we should take seriously.
But I want to say just one word, because I think it can be hard to really get a feel for how quickly global depopulation could happen. We’ve had this hockey-stick growth in the global human population over the last couple of centuries. But the decline could be just as steep. Here’s a way to think about it: take the birth rate in China right now. China is an outlier—it’s relatively low—but it makes the math a lot easier to do in our heads. So, for China’s birth rate right now, the total fertility rate is one, which means that for two adults, there’s one child. That means that in a generation of adults of 100 people, there would be 50 children. And in the following generation—the grandchildren’s generation—there would be 25 grandchildren. So over the course of a couple of generations—maybe 60 or 70 years—you’d have the size of cohorts declining by 75%.
Now, if we turn to some numbers we can’t easily do in our heads, but which are maybe more representative of the world: the average birth rate in the 140 richest countries today is about 1.5, averaged over all of those countries. An average birth rate of 1.5 is a future in which the global population declines by 10% every decade—by two-thirds every century.
So this really could come fast, in a way that I think is not very apparent or visible when we look around the world and see a birth rate like 1.5—which is probably not very different from where you are in Maryland or Virginia or D.C.—and think of that as sort of normal. Because that normalcy is going to lead to something profoundly unfamiliar in the future.
Mounk: Yeah, and I think there’s a lot about this math that is unintuitive but is really important to understand. It doesn’t feel like whether, on average, families have two kids or 1.5 kids is going to make such a vast difference. But you play out the math, and it does make a very, very big difference—which is something people really underestimate.
I want to get, for a moment, at this idea of population momentum. One of the things that makes it hard to predict the business cycle, for example, is that things can turn very quickly. Something that happened 20 years ago doesn’t necessarily have a big impact on what the economy is like today, at least in terms of whether it’s growing or declining.
In demography, it’s still difficult to predict, and I’m struck—having done a little bit of research on this—that the four major organizations trying to predict exactly how many people will be alive in 2100 have pretty different views. But there’s this idea of population momentum that does allow us to make predictions with greater accuracy than in some other areas of social science. Because if you want somebody to have a kid 25 years from now, they’re probably going to be born today—or born in 10 years or whatever. The size of the cohort of one-year-olds today tells you something about the parameters of how many children people might have in 20, 30, 40 years.
So tell us a little bit about that—and tell us what you think the most realistic scenario is for what the world population will look like in 25 years, in 50 years, in 100 years—perhaps, if you want to go that far, in 200 years. Given the slightly different estimates made by different organizations, what is the best projection? And presumably that informed your book title, After the Spike—are we coming down from the spike, and what does the world look like once we’ve passed it?
Spears: Population momentum is an idea from formal demography about how the age structure of the population impacts how population growth unfolds—separate from birth rates and death rates.
So, the biggest picture: the Earth is a closed system. We enter it by being born, and we leave it by dying. So birth rates and death rates are going to completely pin down the simplest model of how population changes. But that’s in the medium to long run.
In the short to medium run, the fact that there are, for example, a lot of children alive right now means that they’re going to grow up and have children. So that fact—about how old people are—is going to change how the population grows. The surprising fact about population momentum is that even if birth rates were to go up to two today—or whatever the replacement level is—the size of the population would keep growing for decades, as the children we have now grow up into reproductive age and have children. So there’s only so much the size of the world’s population could deviate from its most likely path over the coming decades, even if birth rates are very different. That’s one thing we know about what’s likely to happen.
But that’s very different from knowing exactly what the size of the world population will be—or the speed at which the population is changing—in the 2080s, or 2100, or 2200. Here, I think the most important thing to say is: no, of course we don’t have a crystal ball. We don’t know exactly how far below two the world’s average birth rate is going to fall.
But in some sense, we don’t need to in order to understand the most important thing, which is that depopulation is the most likely future. If, on average, the birth rate is less than two kids per two adults—essentially anywhere below two for the world as a whole—then we’re going to get global depopulation. The same exponential algebra that governed the upslope would govern the downslope, so it could be very fast. The size of the world population quadrupled in the past 100 years—the decrease could be similar. The world population doubled in the 100 years before that—the decrease could be similar. Exactly which of those speeds it will be depends on just how far the average birth rate falls below two. So, do we have reason to expect it’s definitely going to be 1.5, like in Europe, or 1.6 in the United States? No. Like we said, we don’t know exactly how fast education is going to change in Africa. We don’t even really know how much of that is an effect of education, as opposed to the fact that people who would have had different numbers of kids are more or less likely to go to school anyway. There are lots of details that we don’t know. And those details rightly fill the careers of population scientists, journals, and research papers. But for the biggest picture—which is that depopulation is the most likely future, and that once it happens, it could be fast and exponential—all we need to know is that it’s likely the average birth rate for the world as a whole is going to go somewhere below two for a long time.
Mounk: Yeah, it seems to me—having looked at this a little bit—that there is very good reason to think that. There’s even stronger reason to be confident that all of those countries that now have birth rates significantly below two are going to be depopulating. So, it may be a little harder to predict exactly the speed with which fertility rates will continue falling in sub-Saharan Africa, but it’s very clear that the next generation—and the generation after that—is going to be vastly smaller in Italy, in China probably, and, given the trends over the last few years, in the United States as well. That in itself is a topic worth taking very seriously.
I want to shift now to the second set of questions that I’m sure listeners are going to have. And that is: let’s say we are after the spike—that we’re in the last little moments in which the population is continuing to grow just because of the way age cohorts work—but we’re about to start experiencing this rapid decline in population. Well, why is that a bad thing? Isn’t it the case that we’re so worried about climate change? Isn’t it the case that the Club of Rome keeps talking about all of the global resources being depleted? Isn’t it the case that finding an apartment in New York City or Tokyo or Paris is already incredibly expensive? Wouldn’t it, in some ways, be nicer if we had fewer people?
Why do you think the world would not, in fact, be nicer—let’s leave some poorer parts of the world out of the picture for the moment—if affluent societies like the United States, Southern Europe, or East Asia were to experience a very rapid population decline in the years to come?
Geruso: This is maybe the most common question that I hear from all sorts of people—including our students at the University of Texas—when we talk about population change: this idea that human activity pollutes and destroys. Wouldn’t things just be better for all sorts of things we care about—especially environmental outcomes—if there were just fewer of us? Also, wouldn’t there be more to go around? Maybe let’s hold that second one for a moment and focus first on the environment. The problem with that is that the facts and history just don’t bear out that logic in the way we might expect. So let me tell a little story.
In 2013, the smog in China was making international news. Newspapers were calling it the “air apocalypse.” The particulate matter air pollution—which comes from burning coal, vehicle exhaust, and wildfires—was completely choking the sky. There’s a standard air quality index that goes from 0 to 500, and the reading in Beijing on that scale was 755. So this was an international scandal. That was in 2013. In the decade that followed, China added 50 million people to its population—that’s the size of Canada. It’s bigger than Texas, bigger than California—just that addition. So how much worse did air pollution get over that decade? Well, maybe surprisingly, if what you expect is more people means more problems—air pollution fell by half. This is not just China. It’s not a cherry-picked example. Over the last decade, the world as a whole added three-quarters of a billion people. And air pollution—particularly particulate pollution—has fallen. Facts like that challenge this basic assumption that so many of us have: that more people means more environmental problems. At the core of that, I think, is a bit of a misunderstanding about how environmental progress—where it’s been achieved—has actually worked.
I’ve been thinking recently about the problem of lead in the air during the 1960s in America. In the 1960s, people were breathing in lead that was in the air. The reason they were breathing in lead was that we were burning leaded gasoline in our vehicles. Today, we don’t breathe in lead. An additional person in the United States doesn’t cause more lead to be pumped into the air. The reason is that in the 1970s, through the Clean Air Act and other regulations, we regulated leaded gasoline basically out of existence. It would have been silly in 1970 to think that the way we’re going to deal with lead in the air is by waiting for—or hoping for or trying to bring about—a scenario in which there are fewer drivers. That just would not have been a serious solution to the problem of burning lead and breathing it in. That was something that needed to be taken on directly by targeting the pollution.
I worry that when people think about population and big environmental problems like climate change, they’re sometimes making the same sort of error. They’re thinking that the way to bring down greenhouse gas emissions—which are caused by human activity—is to reduce the number of humans, not to change the activity. But that’s never been the way environmental progress has been made. If we want to deal with greenhouse gas emissions, we need to build solar farms, turn on wind turbines, activate nuclear reactors, build transmission lines across the United States to move energy from where the sun is shining and the wind is blowing to where it’s not. A smaller population, in itself, is not going to accomplish any of that. Nor is it going to make our industrial technologies green. A smaller population is not going to cause the invention of low-carbon concrete.
So the lesson we should learn from the history of environmental progress—and there has been environmental progress; we’ve cleaned the water, we’ve reduced the damage we were doing to the stratospheric ozone layer, we’ve taken lead out of the air, we’ve been cleaning up particulate air pollution—is that all of that has happened by directly targeting the pollutants.
So the way I think about this is: dealing with huge environmental challenges like climate change is going to require that billions of us live and consume differently. It’s not going to require that billions of us never live. You mentioned Ehrlich at the top of the show—and I think for too long we’ve been riding the same line that Ehrlich set up for us in 1968, thinking that people are problems, and every additional person just increases our problems.
Mounk: Yeah, I think it feels like this podcast is turning into a math podcast, which is not my usual style. But again, on the math of carbon emissions, I was really struck when I read about this. Britain is debating whether it should stick to its goal of carbon neutrality by 2050. If we're serious about curbing climate change, we need to find some solution like that—where we’re basically no longer emitting any net carbon in the course of the next 30 years or so.
Now let’s imagine, for the sake of argument—which actually turns out to be wrong, because there’s a lot of pre-existing infrastructure, because more dense cities actually emit far less carbon per person than more rural living, etc.—but let’s assume that each additional person means proportionately more carbon emitted, and one person less means proportionately less carbon emitted. Let’s say that the population of the United States would shrink by 25% by 2050, which would be incredibly rapid and set up a huge population problem down the line. Well, that’s just not enough. If we reduce the carbon we’re emitting by 25% by 2050, this is one of the nightmare scenarios where climate change is probably going to go completely out of whack.
If you’re thinking about what the levers are to solve climate change, the dimensions we’re talking about with human population—unless there’s a catastrophic nuclear war that decimates the population by 99.9%—just aren’t going to get us where we need to be. I think that is really instructive. If the U.S. population shrank by 25%, and it meant—which it doesn’t—that carbon emissions also dropped by 25%, we still wouldn’t be anywhere close to the scale of solution needed to deal with climate change. Going beyond the environmental question—what other reasons do we have to worry about this? Why is it that a society which is rapidly depopulating might not have the economic dynamism we take for granted, might struggle to provide for its older people or other people who are needy, might be less inventive, or might just be less appealing in all kinds of different ways?
Spears: Here’s one reason why depopulation matters and why we should want to avoid it: we’re all made better off by sharing the world with more people—people alive alongside us, not just those who came before—because other people make the discoveries and have the ideas that improve our lives. Other people are where science and knowledge come from.
It sounds counterintuitive to think that other people could improve our living standards and be good for us. But let’s start with a big question. Why are our lives today better than lives 200 or 300 years ago? Why do we have shoes on our feet, glasses to correct our vision, plenty to eat, shorter work days, medicines to treat our diseases, a huge library of literature, and so much social insurance for people who are elderly or disabled? Why do we have all of these things? It’s the same dirt beneath our feet, the same sun shining on us, the same wind. There’s so much that’s the same. What’s different is that we know more. We now know what to do with the resources on Earth. We now know how to organize ourselves better. We know how to farm more efficiently, how to make computer chips, how to make soap to keep us from getting infections, and how to make antibiotics to treat infections if we get them anyway. Most importantly, we have a germ theory of disease to better understand what an infection is in the first place.
All of these things are ideas, discoveries, notions or creations that people had over those couple hundred years between then and now. Two hundred years ago, there were an eighth as many of us—about a billion people. So imagine if all along, every year, we got to age 20 or 25 and there were only a billion of us. Does it really seem plausible that we would know and understand the things we do now? Would we know what we now know about how to organize a cancer drug trial, or a kindergarten that teaches phonics, or a parliamentary democracy? We learned all of these things together—as one generation iterated on the advances of the last, as a mentee carried further the ideas of a mentor, as team members discovered together what none could do alone.
Paul Romer is an economist who won a Nobel Prize in economics for working out the formal theory behind this idea. The critical economic insight is that knowledge is a special resource: it gets used, but it doesn’t get used up. So if a child’s infection is treated with an antibiotic—something no one could have done 150 years ago—then that specific pill is gone. No one’s ever going to take that antibiotic pill again. But the formula behind that pill—the formula of the drug, and more importantly, the scientific knowledge and the germ theory of disease that made it possible—that all remains. That’s still there for someone else to use again.
So when we think about the economics of population, we might first think about resources getting used up. But what Romer’s theory of endogenous growth tells us—and what we see in the world and in the evidence—is that the most important resource for improving our living standards is these sorts of ideas. They’ve come from the work and the collaborations of other people. Without other people, in a depopulating future, we wouldn’t continue to make so much progress. Each of us would be worse off than in a stabilized future that didn’t depopulate.
Mounk: Let me push you on this idea for a moment. There’s something plausible about this idea that there are rare geniuses in the world who are able to have ideas that perhaps other people might not. Or, to put it differently, perhaps some ideas just take a lot of collaboration—it takes a lot of people working on them. The more people we have, the more likely we are to be able to do that.
There’s also a kind of different intuition. When you look at the contributions of some of the great figures in history—Michelangelo, Plato, whoever—partially they could contribute to so many different fields because there wasn’t as much knowledge then, there weren’t as many people competing for these ideas, and so on. So perhaps if we didn’t have as many people, then the people who are really brilliant could make more contributions to more different fields. And if there are ideas out there to be had, perhaps an affluent world with two billion people might end up discovering a very similar number of them as an affluent world with ten billion people.
How convincing is it that the rate of progress is really going to slow so significantly just because there are fewer people? Is the main concern about the sheer number of people who are there to make those discoveries? Or is it about how dynamic a society is?
Another element of a shrinking society is that if we want to make sure people are well taken care of when they’re elderly, we just have to allocate an ever-growing percentage of our wealth to the healthcare system, to people who look after those elderly folks. Perhaps if there's a need for social changes, those are very hard to get through in a population that’s really old, because they’re much more likely to resist various changes. When I look at Europe today, there’s a huge need for investment in the technologies of the future, in revitalizing the European economy. One reason that’s hard to do is that a huge percentage of state expenditure goes to very generous pension schemes. And since pensioners are such a big share of the electorate—and since they’re more likely to vote—any politician who tries to change that might run into trouble.
So is the main concern really this idea about the sheer number of inventions we’re going to have in the world to make it better? Or is it more about the dynamism and the age structure you’re going to have in that kind of society? Or perhaps is it both—and if so, why?
Geruso: I think it is both, and that these things operate on different time scales. In the short run, there are going to be fiscal pressures on governments as populations age and as there’s a smaller share of people who are working, and a larger share of people who are retirees who need social support through public pension systems, public healthcare systems, etc.
What we’re most interested in, in our book, are the other longer-term consequences. One thing you mentioned was this idea that people like Michelangelo could contribute to so many fields because there just weren’t that many other people innovating. You might say the same thing about someone like Newton, who contributed to calculus and to physics, right? I’ll try not to get too deep into the economic weeds, but one of the active areas of research in economics is the question of whether ideas are getting harder to find. In some sense, people like Newton might have picked all the low-hanging fruit. So now we have to work harder and harder—to mix metaphors—to squeeze the next bit of juice out of the fruit on the tree. We have to climb higher.
That means two things can be true. It might’ve been the case that progress—measured however you want: in terms of longer lifespans, better lives, higher GDP—was not so much slower long ago, when there were fewer people. But in part, that’s because it’s now taking larger teams of people, more research effort, a higher fraction of the population dedicated to innovation and R&D and pressing the frontier forward, just to keep making the same annual rate of progress that relatively few people could manage in the past.
So I actually think looking back to the past and saying, look, we made a lot of progress back then with these rare luminaries, is not a good model for thinking about what might be true of our future, when we need to assemble a huge, complex web of niche specialization to get our next innovation. The mRNA vaccines, for example, came just in time to rescue us from the worst outcomes of COVID—that was a collaboration, a synthesis of work that spanned states and countries and decades and specialties. That’s not—unlike Newton writing the Principia—something a single person can do. So I actually do think there’s something serious at risk when we talk about shrinking a global population that relies on such niche specialization to make progress.
Mounk: Yeah, I do find that convincing. One slightly naive version of this argument is to say that we need geniuses in the world to discover stuff. And if we only have a billion people, then that’s not going to be enough geniuses. That doesn’t strike me as particularly convincing, in part because in a much smaller world in the past, there were people we now think of—rightly, I think—as geniuses. And in part because there would still be a lot of people. Because that’s not exactly how discoveries work today, right? In fact, the reason why it’s hard to have geniuses today is precisely because they’re so specialized.
But there’s another set of arguments, which isn’t just about the availability of the super smart, specialized researchers who are able to develop those unique skills it now takes to discover ideas that are harder to reach. It’s also about the demand for that, actually. In order to have this team of 200 people working on commercializing the mRNA vaccine, you also have to have a market where you think it’s going to be financially viable. If you have a lot fewer people in the world, a lot fewer of those kinds of activities are going to be financially viable. You're just not going to be able to marshal the economic resources to have all of those different teams deployed to do that.
There’s another argument which is a little bit more philosophical, and I wonder how you feel about it. Should we care about the size of the population in itself? Obviously, we would care if 9 billion people died in some horrible accident. Let’s assume there are 10 billion people in the world—which there aren’t quite yet. If 9 billion people died in a nuclear war or some kind of terrible natural disaster, of course we would care—because those are humans who are already there. I hope the listeners to this podcast would share that intuition. Now, I think intuitions go further apart when we say that people having fewer children isn’t some horrible national disaster. It’s not some awful accident in which people die a horrible death. It’s just lives that were never born. And if by this natural process that doesn’t involve suffering, we end up with a world of one billion people—is that just as good as having a world of 10 billion people, assuming those lives are similarly thriving? Is there a reason to think that if we can have a world in which there are 10 billion people, most of whom lead thriving lives, we should prefer that to a world with only 1 billion people who have thriving lives? How do we think about that kind of slightly abstract—but I think important and interesting—philosophical dimension?
Spears: A big part of the case for people—and why it matters whether the future is on a path toward depopulation or toward stabilization—is that it’s not the same for living standards. It wouldn’t be the case that we’d have the same level of thriving or poverty reduction. We wouldn’t be on the same path toward abundance and of continuing the global fight against infant mortality and other problems—because of what people do for one another. And part of that, as we’ve talked about, is innovation—which really isn’t about geniuses. It’s about the way we all build upon the ideas of one another. Part of it is, as you say, the fact that when other people want and need the things that we want and need, they make it more likely for us to get it. If other people need the same specialized medical treatment that you need, they’re making it more likely that that kind of hospital service is available. If other people want and need the same transportation network that you do, they’re making it more likely to exist. Other people make us better off per person. So we wouldn’t have the same kinds of progress toward better living standards and abundance without them.
But there’s another part of the case for people to ask about, which is that many billions of lives that could be excellent—full of well-being and joy—would never be lived on the path to depopulation. One of the biggest and toughest questions for people to grapple with is, would that silence be a peace to welcome, or a loss and an absence to mourn? What does it matter if another person gets to be born to live a good life? And if it matters—what does it matter if they don’t? What if billions of people don’t? We can stack up and tabulate the consequences—for parents, for the environment. We can do the economics about technological progress and living standards. But this question is: should we also give some consideration—maybe not every consideration, but not zero—to the would-be people who would exist if the population stabilizes, and who would not exist if the population declines? Does that figure into our thinking about what would be better? Or should we only care about the well-being of people here and now?
Our answer in the book—and the one we talk through—is that we think the chance to live a good life matters. We think it does count for something, in comparing stabilization to depopulation, to say that more people would get to live a good life. Now, that doesn’t imply that anybody should try to force anybody else to be a parent or not to be a parent. Something can both matter and be a matter of free choice. We think that whether or not billions of people have a chance to live a good life is one of the things that matters when we’re considering whether stabilization or depopulation would be a better future.
Mounk: I know from experience talking to people about this that my intuitions on this differ quite widely from those of many others. I think probably a lot of people listening to this just don’t have that same instinct. There’s a very complicated, interesting philosophical literature on how to think through this question, inspired by Derek Parfit and then taken up by many other researchers over the last decades. But to me, there’s a relatively simple thought experiment that indicates why we should care. If you ask: is the world better in which there are 10 billion people living thriving lives, or one in which 1 billion people are living thriving lives?—a lot of people don’t have a strong intuition. And I think that’s partially because, as people who think about economic policy often say, anything with too many numbers just looks the same to people. Whether some bill costs 5 billion dollars or 1 trillion dollars is a huge difference—but it kind of sounds the same. I think about this stuff quite a lot for my work, and even to me, it’s not the same, but it feels the same. And I think there’s something similar going on here. A billion people is so unimaginably large that maybe 1 billion people is just as good as 10 billion. Who cares? It’s all big numbers, right? But let’s spread the numbers out a little bit further. Let’s imagine one world in which there are a billion people living worthwhile lives—starting families, producing art, hanging out with each other, having a thriving human life—and another world in which there are ten people living thriving lives.
Now, I take your point, Dean—it’s actually very hard to imagine that ten people really could lead such thriving lives, because there wouldn’t be the economies of scale and all the specialization and so on that you need to make that possible. There’s not going to be many antibiotics in a world of ten people. But let’s set all of that aside. We’re in a world where ten people live these thriving lives. I’m glad those ten people are having good lives, but that’s very few people. There’s so little variety of human experience. Surely, it would be better to have a world of a billion people than a world of ten people. So it seems to me that that indicates we do have a strong intuition that the number of people matters, and that if we can increase the number of people who live worthwhile lives by 10x, that’s something we should care about—even if we’re talking about numbers that are so abstract we don’t immediately feel them.
Spears: I think that makes a lot of sense. One of the starting points in our book is that we say the general welfare matters. Then the question is the one you're asking: if there are 10 billion people or 100 people or 10 people, and they somehow had the same quality of life, would that matter? Would that make a difference? I think that’s one intuition you have, and other people have other intuitions. I don’t think a few minutes of podcast is going to settle the question for very many people. But we can go beyond intuition, and that’s one of the things we talk about in the book.
For example, some people say that what matters is the quality of life, not the quantity. So you’d want to measure that with how well people are doing on average. If 10 billion people and 1 billion people had the same average quality of life, then by that logic, both futures are equally good. If all that matters is average well-being, then that would be a tie. But the average—along with any other metric that only cares about quality of life and not quantity—turns out to be a bad way to assess things, a bad measurement of general welfare. And here’s how we can see that.
Imagine a child born in Uttar Pradesh, a disadvantaged state in India. It happens to be the place where I do my research and work. Imagine this child is born next year and she’s lucky. She’s poor by your standards, but her family loves her. She manages to avoid serious illness. She’s happy and healthy as a child, enjoys her siblings, and rides the tide of improving gender equity to go to a school she likes. She finds a job she’s proud of—maybe as a nurse. As an adult, she still enjoys her siblings and gets to call and talk with them on walks to and from work. She has an arranged marriage, but her parents give her a say in it, and it turns out to be a good match. She’s friends with her sisters-in-law. She likes being a parent. Things go well for her in her adult life. She’s healthier, wealthier, and safer than most women who’ve grown up in that part of the world in decades or centuries past, even if she’s not as healthy or wealthy as we are—or as most people reading this book. In short, she grows up to live a life that’s happy, but also below average for the world in 2025. She wouldn’t trade her life for non-existence. After thoughtful reflection, she’s glad to be alive. But if her well-being is below average—not only for 2025, but for all time—then what? Did her life, her existence, make things worse? Would the general welfare have been better without her? Not according to her. She wants her life. And we say that makes a lot of sense. But if you're fixated on average well-being, and you're ignoring the quantity, then you’d be forced to say that the universe would be better off without her—because her existence would nudge down the average.
That’s one way to see that this average-only approach goes wrong. And more broadly, it shows that any approach that only looks at the quality of lives and not the quantity of lives has to go wrong as an account of the general welfare. So what we say is that both the quality of lives and the quantity of lives matter. That still leaves plenty of open questions—how exactly should we weigh quality against quantity, how should we make trade-offs? We talk about these things. But we don’t need to fully settle them to know what to do between depopulation and stabilization. Because a stabilized future, compared to a depopulating future, is going to be one where both the average quality of life is greater, and the overall quantity of life is better. Things are going to be better on average and in total.
That’s a theme of our book: other people are win-win. Other people are good for them and good for you. We’re used to rejecting zero-sum thinking in so many other areas. We know to reject zero-sum thinking about international trade. We know to reject zero-sum thinking about the environment—that we all have to decarbonize together. But we still have this zero-sum mindset when it comes to population and to other people. So part of what we’re hoping to accomplish is to help people get out of that zero-sum mindset about others—and to see that other people are win-win. Their lives are good for them and for the rest of us too.
Geruso: Let me just bring this back to the practical question about birth rates and depopulation. One thing that came up in this discussion is comparing a 10 billion person population to a 1 billion person population. Although I think we haven’t been super clear about this so far, what Dean and I are advocating for is not unending population growth. It’s a stable population—at some level. We’re not here to say exactly what that level should be, only that it shouldn’t be depopulation forever.
But on this question of a 10 billion person population versus a 10,000 person population or a 1 billion person population, something that I think may not be obvious is that to achieve any of those states, if that’s what you want, then at a practical level you want the same thing as we do. You just want it at a different point in time. For any population to stabilize at any level—even a tiny population—there needs to be about two births per two adults on average, over the long term, across the globe as a whole. So the practical question of what this means for birth rates actually does not hinge on whether you think the right population size is 10 billion people or 8 billion people or 1 billion people.
In the rest of the conversation, Yascha, Dean, and Michael explore why birth rates are so low, and how to encourage people to have children. This part of the conversation is reserved for paying subscribers…
