What does the literature say about population density and fertility?
Across numerous countries over time, there is a consistently negative and statistically significant relationship. High density ➡️ low fertility.
Thus, policy should favor low density! Important 🧵, please share.
First up, Lutz et al. (2006).
Examining 145 countries and controlling for several socioeconomic variables, Lutz et al. conclude, "Population density is now the most important factor explaining the fertility level, with only female literacy coming close in significance." 2/11
Lutz et al. reports that fertility rate declines with increasing density across numerous countries.
Not only this, in 94 regions of Europe, ideal family size was significantly negatively correlated with population density. 3/11
Next, Rotella et al. (2021).
In a study of 174 countries over 69 years, the authors find a robustly negative relationship between density and fertility both within and between countries, even when controlling for a range of variables. 4/11
Rotella et al. write that "between-country differences and within-country changes in densities over time predicted fertility rates, accounting for 31% of the variance in fertility." (p < 0.001).
Across all developed, most developing countries this negative correlation holds. 5/11
Next, de la Croix et al. (2017) report that across 44 developing countries an increase in density from 10 to 1000 inhabitants per square km results in a decrease in fertility of about 0.7 children. 6/11
Using 20 density clusters, the authors find a significantly negative relationship between density and fertility that is significant at the p<0.01 level. 7/11
Next, Testa (2004) finds in a paper entitled Population Density and Fertility, that for Indonesia, fertility is strongly negatively correlated with density across the provinces. 8/11
Numerous other papers show similar negative relationships between density and fertility.
Demographer @lymanstoneky noted this week, "There are basically zero studies that have been able to argue that density was pro-natal, on any measure of density." 9/11
Pundits including @mattyglesias and @bryan_caplan argue for densification on the basis of reduced housing costs.
Tokyo has done this, building tall towers and bringing down prices. Yet its fertility rate remains lowest in Japan (1.04 in 2022)!
Why? Density hurts birthrates! 10/11
With fertility rates collapsing in 🇺🇸 and most other countries, it is not enough to simply build housing.
Housing must be of a pro-natal rather than an anti-natal form. Above all that means lower density.
Since housing endures for generations, getting this right is crucial. 11/11
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On this Easter, thoughts of Rome!
How collapsing fertility caused the fall of the Roman Empire, how early Christians had higher numbers of surviving children, and how the sect of Christianity grew to inherit the Roman world.
And parallels to the world of today! 🧵, please share!
Of all the explanations for the fall of the Roman Empire, low fertility seems to be the most compelling reason. We know Rome's population plummeted. But this wasn't a sudden thing due to sacking but was continuous over a number of centuries as this chart by
@daveg shows. (2/12)
And our best evidence is that low birthrates were the cause. Fertility among elite women was less than two births per woman, while replacement fertility (due to high child mortality) may have been above six. (3/12)
The global low fertility crisis strikes at the heart of scientific and technological progress.
Why?
It is precisely the places with the highest innovation that are hit hardest.
South Korea has both the highest innovation and lowest fertility rates in the world.
🧵, please share!
Innovation is the engine of economic growth. The Industrial Revolution that kicked off the modern world tracks a rise in patent grants in Great Britain at the time.
Going forward, if the high-innovation countries are all shrinking, that bodes very poorly for global growth. 2/7
In scientific output (as measured by scientific publications per million people) the picture is similar.
With lone exception of Israel, every country with high scientific output has below-replacement fertility rates today. This portends a future of scientific stagnation. 3/7
All of Europe struggles with very low birthrates, and this new map by @Landgeist helps explain why.
The average marriage age is higher in Europe than almost anywhere else, and this leaves a lot less time for people to have children within the fertility window. 🧵, please share!
Of course, people don't have to have children within marriage, and it is quite normalized for Europeans to have kids outside of marriage.
Still, the odds of having kids is much higher within marriage, even for today's young people. 2/5
In most of the world but in Europe especially, people have a lot fewer children than their ideal, and getting started late is a big part of why.
Fecundability (the ability to get pregnant) is much lower by the mid 30s, for both men and women. 3/5
Some of the lowest fertility rates in the world outside of East Asia are found in Mediterranean Europe. Spain's fertility is just 1.14 births per woman while Italy's is around 1.21.
Why are birthrates so low in 🇪🇸 and 🇮🇹, and what lessons can we learn?
Important 🧵, please share!
A first cause of low birthrates in 🇪🇸 and 🇮🇹 is high unemployment and unstable work, especially for the young. Youth unemployment exceeds 20%.
Spain leads all of Europe with nearly 18% of work contracts being temporary, and it is disproportionately young people in temp jobs. 2/10
A second cause of low birthrates is 'failure to launch.' An astonishing 47% of young people in Italy ages 25-34, the prime childbearing years, live with their parents!
Here is a 🧵 on how living at home too long stifles fertility, in both 🇪🇺 and 🇺🇸. 3/10
What is the effect of divorce on fertility rates? Following the liberalization of divorce starting in 1970, birth rates fell below replacement in Europe, never to go above replacement again. A🧵!
The paper Divorce Laws and Fertility by Bellido et al. found that "The introduction of more liberal divorce laws permanently reduces the value of marriage relative to divorce. Results suggest that divorce liberalization has a negative and permanent effect on fertility." 2/6
Following divorce liberalization, marital births fell, and non-marital births rose, but the modest rise in non-marital births could not make up for the dramatic fall in marital births.
After 1970, marriage rates in 🇪🇺 fell by nearly half and stayed low. 3/6
We keep seeing a lot of press around polyamory. Polyamory is apparently very negative for birth rates on average, and @Aella_Girl has the data! (N=23K, 1500 poly.)
This lines up with different data from the General Social Survey which I discussed two weeks ago. 🧵, please share!
Thanks to Scott Alexander (@slatestarcodex), through whose blog I found Aella's data. It turns out that polyamorous people have only half as many children as those who are monogamous, on average.
Scott Alexander did his own survey, which found the same thing (below). 2/6
It turns out that Scott Alexander, whose writing over the years brought attention to polyamory, wound up with a family by returning to tradition.
He recently married, and two months ago became the proud father of twins!
(Photos from Astral Codex Ten, his latest blog.) 3/6