The title of today’s post comes from an excellent recent talk in London given by my AEI colleague (and University of Pennsylvania professor), Jesús Fernández-Villaverde. Today I look at the simple math of population projections to clarify the nature of the “crisis” — which is indeed worse than I thought.
Fertility is falling everywhere: rich and poor countries alike, booming and stagnating economies, secular and religious societies. The decline is happening far faster than anyone anticipated, even me, ten years ago!
For example, Colombia’s fertility rate is 1.06, Iran’s is 1.44, and Turkey’s is 1.48, all of which are below the U.S.
The decline accelerated around 2014, well before the COVID pandemic.
As a result, humanity’s fertility is likely already below the replacement rate.
Many assume the replacement rate is 2.1 children per woman. That’s true for rich, advanced economies. But not for emerging economies, where selective abortion and higher young female mortality push the replacement rate higher. Thus, for humanity, the replacement rate is closer to 2.2.
The 2024 UN World Population Prospects are riddled with data and forecasts that, frankly, make little sense to my coauthor Patrick Norrick (at AEI) and me.
Most of the differences in economic growth among advanced economies over the past 35 years can be attributed to demographic factors. Once adjusted for this, Japan’s economic performance is roughly on par with the U.S.
The replacement rate refers to the average number of children born to a woman required to sustain current population. The slide below from JFV’s talk explains required replacement rates.
JFV concludes:
[W]e estimate that humanity is already below the replacement rate . . . a more likely scenario, given current policy, is that the peak of world population would be around 2055. By the way, this is the low-fertility scenario from the WPP 2024 [UN World Population Prospects 2024].
The figure below, from a recent report by Goldman Sachs, illustrates the recent downward trend in United Nations medium population projections for the rest of this century, along with the UN low-fertility scenario that JFV estimates to be more likely.
One reason why the UN has higher projections than JFV is a projected “rebound” in birth rates in countries whose populations have already peaked. The UN WPP 2024 explains:
For the group of countries and areas with populations that have already peaked as a whole, the medium scenario of World Population Prospects 2024 assumes that between 2024 and 2100 the level of fertility will gradually increase to 1.4 births per woman (with 95 per cent uncertainty ranging from 1.2 to 1.8 births per woman in 2100) (figure 2.5). This assumption is informed by trends from 39 countries that have experienced declines in total fertility below 2 children per woman, followed by a subsequent rebound over at least two consecutive periods of five years (see United Nations, 2024b). The “rebound” in future fertility for low-fertility countries is consistent with an expectation of continued progress towards gender equality and women’s empowerment and improving social and economic opportunities for young people and families.
JFV argues that the assumed rebound “does not make a lot of sense” and provides multiple examples of what these rebounds look like for particular countries — Below is the South Korean rebound.
If birth rates stay below the replacement rate, over this century there will be disruptive consequences for the structure of the economy. However, the longer-term consequences for humanity are profound. Let’s look at some simple population math.
The figure from Goldman Sachs shows projected births per woman out to 2080, along with two estimates of replacement rates. The lowest projection is from the same UN low fertility scenrio shown above.
Spears et al. 2024 explored stylized scenarios of what would happen to global population if fertility rates remained below replacement rates far into the future. Their answer is summarized in the figures and excerpt below. The figure is a little complicated so let me explain.
For a given long-term fertility rate — on the X-axes — the left panel shows the year in which global population drops to 1 billion, 100 million, and 10 million. You read that right. With a long-term fertility rate of 1.2, global population would drop to 1 billion in 2240 and to only 10 million people by 2500.1 The right panel shows the same but for births-per-year.
For example, what would happen if the world converges to a TFR of 1.2, the average fertility rate that East Asia, taken as a whole, exhibits today? A vertical line traced up from TFR = 1.2 crosses the (short dashed, green) iso-population curve for 1 billion people in 2340. Thus, within 250 years of the lifetime of most children born today, there would be fewer than 1 billion people alive. Panel (b) tells us that by 2280, there would be only about 10 million babies born each year, compared to about 135 million births in 2022. Only four decades after the global population reached 1 billion, by 2380, it would further halve to 500 million; this halving time of 40 years would be comparable to the world population’s doubling time of 37 years in the mid-20th century. If, instead, global fertility converges to 1.5 babies per woman (like in today’s Europe), these milestones would be delayed by only 25 and 95 years, for the 1 billion and 500 million thresholds, respectively.
These simple calculations are what lie behind the pronatalism movement, which has received more attention lately due to Elon Musk, who has argued, “The birth rate is very low in almost every country, and so unless that changes, civilization will disappear.” He is not wrong. Musk famously has (at least) 14 children with (at least) four different women. 2
Should the global fertility rate be a focus of policy? Does the world need a “Paris Agreement”-style international commitment to increasing fertility rates? Is population decline a true existential threat?
At a long-term fertility rate of 1.2, Spears et al. calculate that there would only be an additional 26 billion people to be born in the future. With ~120 billion people having been born in all of human history to date, they conclude that under this idealized scenario, “four-fifths of all the people who will ever live have already been born.”
Is a total fertility rate of 1.2 a plausible scenario for the long-term future? Let’s return to the recent talk of Jesús Fernández-Villaverde, who included the slide below showing 2024 fertility rates for various countries.
I expect we will be hearing much more about pronatal policies and advocacy. It might just be the world’s next big existential threat. Watch this space — and go make some babies!
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1 Spears et al. 2024 explain: “Although population momentum bounds the population sizes that would be feasible in the coming few decades, global depopulation in the 22nd century and beyond could be rapid—just as rapid as the population explosion of the last 200 years—because exponential growth and decay are governed by the same compounding dynamics.”
2 This explains the image and caption at the top of this post.
