What is the ideal size of the human population?

One common answer is “much smaller.” Paul Ehrlich, co-author of The Population Bomb (1968), has as recently as 2018 promoted the idea that “the world’s optimum population is less than two billion people,” a reduction of the current population by about 75%. And Ehrlich is a piker compared to Jane Goodall, who said that many of our problems would go away “if there was the size of population that there was 500 years ago”—that is, around 500 million people, a reduction of over 90%. This is a static ideal of a “sustainable” population.

Regular readers of The Roots of Progress can cite many objections to this view. Resources are not static. Historically, as we run out of a resource (whale oil, elephant tusks, seabird guano), we transition to a new technology based on a more abundant resource—and there are basically no major examples of catastrophic resource shortages in the industrial age. The carrying capacity of the planet is not fixed, but a function of technology; and side effects such as pollution or climate change are just more problems to be solved. As long as we can keep coming up with new ideas, growth can continue.

But those are only reasons why a larger population is not a problem. Is there a positive reason to want a larger population?

I’m going to argue yes—that the ideal human population is not “much smaller,” but “ever larger.”

Selfish reasons to want more humans

Let me get one thing out of the way up front.

One argument for a larger population is based on utilitarianism, specifically the version of it that says that what is good is the sum total of happiness across all humans. If each additional life adds to the cosmic scoreboard of goodness, then it’s obviously better to have more people (unless they are so miserable that their lives are literally not worth living).

I’m not going to argue from this premise, in part because I don’t need to and more importantly because I don’t buy it myself. (Among other things, it leads to paradoxes such as the idea that a population of thriving, extremely happy people is not as good as a sufficiently-larger population of people who are just barely happy.)

Instead, I’m going to argue that a larger population is better for every individual—that there are selfish reasons to want more humans.

First I’ll give some examples of how this is true, and then I’ll draw out some of the deeper reasons for it.

More geniuses

First, more people means more outliers—more super-intelligent, super-creative, or super-talented people, to produce great art, architecture, music, philosophy, science, and inventions.

If genius is defined as one-in-a-million level intelligence, then every billion people means another thousand geniuses—to work on all of the problems and opportunities of humanity, to the benefit of all.

More progress

A larger population means faster scientific, technical, and economic progress, for several reasons:

Total investment. More people means more total R&D: more researchers, and more surplus wealth to invest in it.
Specialization. In the economy generally, the division of labor increases productivity, as each worker can specialize and become expert at their craft (“Smithian growth”). In R&D, each researcher can specialize in their field.
Larger markets support more R&D investment, which lets companies pick off higher-hanging fruit. I’ve given the example of the threshing machine: it was difficult enough to manufacture that it didn’t pay for a local artisan to make them only for their town, but it was profitable to serve a regional market. Alex Tabarrok gives the example of the market for cancer drugs expanding as large countries such as India and China become wealthier. Very high production-value entertainment, such as movies, TV, and games, are possible only because they have mass audiences.
More ambitious projects need a certain critical mass of resources behind them. Ancient Egyptian civilization built a large irrigation system to make the best use of the Nile floodwaters for agriculture, a feat that would not have been possible to a small tribe or chiefdom. The Apollo Program, at its peak in the 1960s, took over 4% of the US federal budget, but 4% would not have been enough if the population and the economy were half the size. If someday humanity takes on a grand project such as a space elevator or a Dyson sphere, it will require an enormous team and an enormous wealth surplus to fund them.

In fact, these factors may represent not only opportunities but requirements for progress. There is evidence that simply to maintain a constant rate of exponential economic growth requires exponentially growing investment in R&D. This investment is partly financial capital, but also partly human capital—that is, we need an exponentially growing base of researchers.

One way to understand this is that if each researcher can push forward a constant “surface area” of the frontier, then as the frontier expands, a larger number of researchers is needed to keep pushing all of it forward. Two hundred years ago, a small number of scientists were enough to investigate electrical and magnetic phenomena; today, millions of scientists and engineers are productively employed working out all of the details and implications of those phenomena, both in the lab and in the electrical, electronics, and computer hardware and software industries.

But it’s not even clear that each researcher can push forward a constant surface area of the frontier. As that frontier moves further out, the “burden of knowledge” grows: each researcher now has to study and learn more in order to even get to the frontier. Doing so might force them to specialize even further. Newton could make major contributions to fields as diverse as gravitation and optics, because the very basics of those fields were still being figured out; today, a researcher might devote their whole career to a sub-sub-discipline such as nuclear astrophysics.

But in the long run, an exponentially growing base of researchers is impossible without an exponentially growing population. In fact, in some models of economic growth, the long-run growth rate in per-capita GDP is directly proportional to the growth rate of the population.

More options

Even setting aside growth and progress—looking at a static snapshot of a society—a world with more people is a world with more choices, among greater variety:

Better matching for aesthetics, style, and taste. A bigger society has more cuisines, more architectural styles, more types of fashion, more sub-genres of entertainment. This also improves as the world gets more connected: for instance, the wide variety of ethnic restaurants in every major city is a recent phenomenon; it was only decades ago that pizza, to Americans, was an unfamiliar foreign cuisine.
Better matching to careers. A bigger economy has more options for what to do with your life. In a hunter-gatherer society, you are lucky if you get to decide whether to be a hunter or a gatherer. In an agricultural economy, you’re probably going to be a farmer, or maybe some sort of artisan. Today there’s a much wider set of choices, from pilot to spreadsheet jockey to lab technician.
Better matching to other people. A bigger world gives you a greater chance to find the perfect partner for you: the best co-founder for your business, the best lyricist for your songs, the best partner in marriage.
More niche communities. Whatever your quirky interest, worldview, or aesthetic—the more people you can be in touch with, the more likely you are to find others like you. Even if you’re one in a million, in a city of ten million people, there are enough of you for a small club. In a world of eight billion, there are enough of you for a thriving subreddit.
More niche markets. Similarly, in a larger, more connected economy, there are more people to economically support your quirky interests. Your favorite Etsy or Patreon creator can find the “one thousand true fans” they need to make a living.

Deeper patterns

When I look at the above, here are some of the underlying reasons:

The existence of non-rival goods. Rival goods need to be divided up; more people just create more competition for them. But non-rival goods can be shared by all. A larger population and economy, all else being equal, will produce more non-rival goods, which benefits everyone.
Economies of scale. In particular, often total costs are a combination of fixed and variable costs. The more output, the more the fixed costs can be amortized, lowering average cost.
Network effects and Metcalfe’s law. Value in a network is generated not by nodes but by connections, and the more nodes there are total, the more connections are possible per node. Metcalfe’s law quantifies this: the number of possible connections in a network is proportional to the square of the number of nodes.

All of these create agglomeration effects: bigger societies are better for everyone.

A dynamic world

I assume that when Ehrlich and Goodall advocate for much smaller populations, they aren’t literally calling for genocide or hoping for a global catastrophe (although Ehrlich is happy with coercive fertility control programs, and other anti-humanists have expressed hope for “the right virus to come along”).

Even so, the world they advocate is a greatly impoverished and stagnant one: a world with fewer discoveries, fewer inventions, fewer works of creative genius, fewer cures for diseases, fewer choices, fewer soulmates.

A world with a large and growing population is a dynamic world that can create and sustain progress.

For a different angle on the same thesis, see “Forget About Overpopulation, Soon There Will Be Too Few Humans,” by Roots of Progress fellow Maarten Boudry.