Depopulation and Longtermism

By MikeGeruso @ 2025-09-09T16:24 (+7)

This post dovetails with the ‘Essays on Longtermism’ essay competition. Below, I outline a chapter on global depopulation that I contributed with Dean Spears to the open-access volume Essays on Longtermism. Readers are invited to enter the competition by writing their own essay in response to our chapter. Here's an overview of our chapter:

What if the challenge for a flourishing future is not too many people on a crowded planet, but too few people to sustain the progress that the world needs? What if human history, counted not in years but in human lives, is four-fifths over?

This chapter on the risks of global depopulation brings facts from population science and population economics into dialogue with longtermism. It references and builds on facts and arguments in our 2025 book After the Spike: Population Progress and the Case for People. The starting point is this: No future is more likely than that people worldwide choose to have too few children to replace their own generation. 

Many longtermists would agree that, to eventually achieve a flourishing far future, it is valuable that, over the coming few centuries, a complex global economy flourishes and the number of people does not become small enough to be vulnerable to extinction (or a big, enduring loss short of extinction) from a threat that a larger population could sustain. In this sense, depopulation poses a risk. Fertility rates that are normal in much of the world today would cause population decline that is faster and to lower levels than is commonly understood, threatening the long-term future. 

The global population would fall fast under circumstances that would feel familiar and normal to many of us alive today. 

If the whole world reaches and sustains a fertility rate like the US has now—or instead matches the average fertility rate in east and southeast Asia, or Europe, or Latin America—then there would be fewer than 30 billion future human lives, ever (Spears et al. 2024). That’s because there have been about 120 billion human births so far, since the beginning of our species. So that would mean that humanity is now four-fifths over, only one-fifth remaining. This outcome would not require low fertility to be sustained for millennia, or even for more than a few centuries. By 2350 CE, there would be only 20 million births per year compared to around 140 million in 2022. The last time so few people were born was sometime in the 9th century. The Mayan civilization was waning then. The Vikings were just getting started.

A natural question about any long-term population projection is: ‘How can you be so sure?’ 

No one can be sure about the shape of the long-fun future. That uncertainty is itself important. Like all threats to long-term flourishing, depopulation is a risk, not a certainty. The uncertain possibility is sufficient to make understanding depopulation an urgent cause. We are sure, in contrast, about the conditional statement, that if birth rates fall below two and hold there, then depopulation is the inevitable consequence. We discuss the reasons–grounded in demographic research, historical fact, and economic theory–why no one should expect any equilibrating force to automatically stabilize the global population by raising birth rates to a global average of two children per two adults. Over the long run, this would cause exponential population decline. 

Why should longtermists be concerned with depopulation? 

There are many reasons to pay attention now. In After the Spike, we treat these in depth, including an argument from population ethics that there is important value in future human lives being lived for themselves, ignoring any positive or negative effects these lives have on the wellbeing of others. In this Chapter, somewhat in contrast, we excerpt abridged arguments from our book focused on material wellbeing for the average person. Evidence from macroeconomics and other social sciences indicate that a depopulating future might not be able to sustain the material progress that has prevailed over the past century when the population was large and growing. We investigate that risk. 

None of our arguments require fertility rates to stay low forever to threaten the long term future. Timing is often neglected in discussions of long-run population, as if any population path might lead us to the same bright future. But timing shouldn’t be ignored, as we discuss in the chapter. Whatever future longtermists hope for, they should not be confident that progress towards these outcomes would not be closed off by a much smaller population in the next several hundred years.

We discuss possible policy responses. There are no shovel-ready solutions to reverse this phenomenon. Governments are not (and possibly never have been) effective at making and sustaining large changes to population level fertility rates. In this sense, responding to depopulation is not yet tractable at the level of policy, and is particularly ripe for progress. 

Humanity’s understanding of depopulation is today where climate science was a half-century ago: It was important then that scientists were measuring carbon concentrations, recognizing the system-level challenges, and sounding a wake-up call, even though they had neither the computing power to produce an integrated climate assessment model nor the technological foundations for a clean energy infrastructure. 

This chapter is intended as a call for more research and better understanding of depopulation. It encourages longtermists to include on their agenda the neglected possibility of slower, sustained depopulation—generations of exponential decay in population size over a few centuries until there are only a few hundred million of us. Whether fertility rates, after that time, are such that the population stabilizes or continues falling, it might not require a huge disaster to close off our flourishing future.

From Toby Tremlett: ‘Essays on Longtermism: Present Action for the Distant Future’ was recently published by Oxford University Press. It’s open access — you can read individual chapters here, or the full collection here. Contest details here.