![Lyman Stone](data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="20" height="20"><rect fill-opacity="0"/></svg>)
Below-Replacement Fertility Is Not New
We’ve been here before. Definitely in the mid-1300s.
Last week I criticized a recent paper on falling fertility. I focused on the actual model in the paper.
But the paper by Jesús Fernández-Villaverde and Patrick Norrick has served as a springboard for some unfounded claims. Most notably, claims like this:
Ronnie Stoeferle@RonStoeferle
2/ The authors' point: this has never happened in our history as a species.
Not during wars. Not during pandemics.
Every macro model has a variable nobody trades because it moves too slowly.
This one just moved. 👇
7:00 AM · Aug 24, 2026 · 224 Views
1 Reply
The claim here is “we’ve never been below-replacement before as a species.”
To which I say “WTF!?!?!?!”
Yes, we definitely have, many times.
Key points in this post.
2.1 is not replacement rate.
Replacement rate is determined by prevailing mortality conditions.
Human population has declined several times in the past.
Therefore, by definition, we were below replacement rate.
We can learn from prior episodes of below-replacement fertility.
What Is The Replacement Rate?
People often quote “2.1” as the replacement rate, i.e. the number of children each woman would need to have to keep population stable in the long run under the assumptions of a) stable mortality rates near modern levels b) relatively normal sex ratios c) no migration.
In practice, a country’s population growth rate can vary widely from the rate directly implied by TFR/Replacement because a) mortality is not stable, b) sex ratios may vary from nature due to abortion, c) there may be migration. As a result, you cannot easily infer whether a country’s TFR is above or below its replacement rate just from population growth.
However, the story for the human species is quite different. If you observe that the entire human population is lower in year t than it was in year t-100, you can guarantee there is below-replacement fertility. Think about why:
a) A century-long decline implies not just one or two years of bad mortality but a persistent condition of high mortality relative to fertility, which is exactly the dynamic “replacement” captures
b) species-wide sex ratios on century-long time horizons almost never vary widely from natural sex ratios
c) the scale of migration to and from Planet Earth is extremely small and entirely recent via Lunar and orbital missions
So, what is replacement rate? Well, it’s a basic math problem! One that JFV’s co-author, Patrick Norrick, is definitely aware of, because he wrote about it here. Here’s he and co-author Nicholas Eberstadt’s estimates of true replacement rates across countries:
That’s in 2023.
But, what did it look like in 1950, I wonder, the earliest year of the dataset being used here?
As you can see, it’s not just that TFR has fallen over time— replacement rate TFR has fallen over time. In 1950, the global replacement rate was 3 children per woman. Today it’s around 2.1. In the 1970s as war-related mortality was high, Timor-Leste saw its fertility drop and replacement rate rise, such that it nearly fell below-replacement despite having fertility of 5 kids per woman!
There are other fun examples. Consider Japan and Bulgaria. Both fell “below replacement” when their fertility was >2.2!
China’s replacement rate didn’t drop below 2.2 until 2015! I could go on but the point is true replacement rate varies a lot. Elsewhere, I’ve written about how replacement rate was probably below fertility in many specific countries in the 1700s.
But of course, in 1950, global TFR still vastly exceeded replacement rate. But we already could have guessed this because population growth was very high. So let’s turn to population growth!
