TESTOSTERONE SUPPLEMENTATION DRASTICALLY LOWERS SPERM COUNTS
Men who want more children should not do it.

Numerous studies find that exogenous testosterone sharply reduces testosterone production within the testes, effectively shutting down sperm production. 🧵.

https://x.com/MoreBirths/status/2077425458430230838

A World Health Organization study found that among 271 healthy and fertile men, a large majority became azoospermic (a condition where a man's semen has no measurable sperm) with testosterone injection.

There was just one pregnancy in 1486 cumulative months of study. 2/6

A large part of the low birthrate crisis is that people aren't finding partners. But with dating apps, people have more options than ever. What gives?

A 2020 study found both men and women become pickier and more prone to reject possible partners the more options they see. 🧵
After seeing just 40 profiles, users were much more likely to reject the matches they were presented with.

Meanwhile the average Tinder user goes swipes through 4000 profiles a month.

The leftwing, blank-slatist economist Daron Acemoglu sees how rich countries have low birthrates and argues that low birthrates cause growth and innovation. 🙃.

The opposite is true. The evidence shows larger populations have more innovation and higher economic growth. 🧵.

https://twitter.com/nberpubs/status/2074509000226332942

The most seminal work was Michael Kremer's 1993 paper "Population Growth and Technological Change: One Million B.C. to 1990". 2/7

Kremer showed that societies with larger initial populations progressed faster and saw faster technological change and growth.

In his remarkable 1755 publication Observations Concerning the Increase of Mankind, Ben Franklin put on his demographer hat and foretold the rise of America to superpower status some 21 years before the signing of the Declaration of Independence. 🇺🇸 🇺🇸

🧵, please share!
Franklin worked out demography from first principles.

He began with the observation that marriage drives population growth, and marriage in turn is driven by the ability of young men to support a family. He saw that when people marry early, they are able to have more kids. 2/9

HOW VANCOUVER ENDED UP WITH THE LOWEST FERTILITY IN THE WESTERN WORLD
Vancouver, Canada had a TFR of just around 0.7 in 2025. That's the lowest fertility of any major city in the West, and one of the lowest anywhere on Earth.

What forces lead to such low birthrates? 🧵. The story starts with culture. Canada quite a bit less religious than the United States these days, and that is a big reason why Canada's TFR at 1.25 is much lower than the US (~1.6).

But within Canada, BC in general and Vancouver in particular are especially irreligious. 2/7

What is the impact of grandparents on fertility?

Three different recent studies found that having childcare help from grandparents leads people to have more children. 🧵.

A 2023 study of China looked at how grandparents impacted the odds of going for a 2nd child.

Those who had childcare help from grandparents for their first child had a dramatically higher chance of going for a second. 2/4

Today we celebrate all fathers!

But most of all, we should recognize younger fathers, who are trying harder and doing more to be there for their families than any generation that came before them.

🧵, please share!
All over the world, dads are spending far more time on childcare than ever before. Dads in the US today spend 4.5 times as much of their day on childcare as their own fathers did a generation ago. 2/6

(Chart by the brilliant @AzizSunderji.)

A just-published paper introduces a new metric: the ratio of births observed (Bo) to births needed (Bn) to make up for deaths.

When Bo/Bn is below 1, a population faces natural decline.

With this lens, the dire situation of Europe and East Asia is thrown into stark relief. 🧵.
Most often we rely on TFR, births per woman of childbearing age. But that ignores the age structure of a population.

Japan is an old country with few women of childbearing age, and deaths far outnumber births.

India is a young country, and births still greatly exceed deaths. 2/

Turkey's astonishing fertility collapse
Turkey's president Erdoğan made it his mission to revive birthrates. Instead, Turkish fertility has fallen faster than almost anywhere on Earth to just 1.42 in 2025 (just 1.2 outside of Kurdistan).
What happened, and what can we learn? 🧵.
By every measure, Recep Tayyip Erdoğan has been a failure in the thing that mattered most to him, getting Turks to have more children.

Why? It doesn't help that Erdoğan is deeply unpopular with young people. 2/8
(Chart by @JesusFerna7026)

India's new birth report just revealed a TFR of 1.88, a little below replacement.

But unlike most countries, 🇮🇳 does not have a crisis of low births. With its young population, India had 23 million births, 3x more than any other country.

A look at how India is different. 🧵!

https://twitter.com/BirthGauge/status/2057351376192131166

South Asia is the one region where marriage remains almost universal even as it has collapsed nearly everywhere else. Why? Arranged marriage.

In China, Korea and Japan, arranged marriage was common but became rare.

In India by contrast, arranged marriage remains the norm. 2/7

A shocking new study finds that the desire for children has collapsed among young people in China.

In the most recent data some 32% of 18–24-year-olds, and nearly half of young women, said they don't want any children at all. 🧵.
Fertility desires have long been taken for granted. Even as fertility has dropped, the desire for children seemed strong.

If birthrates were too low, many argued it was only because of structural barriers to having children, like housing and childcare. 2/7

Early birth data for 2026 is out. Europe and Asia are moving in opposite directions.

Many European countries are showing stabilization and even a modest recovery in births.

In East Asia, meanwhile, births are plunging by double digit percentages to extremely low levels. 1/4

https://twitter.com/BirthGauge/status/2051747059317809469

In Europe, Finland, Sweden, the Netherlands, Germany, Switzerland, Spain, Portugal and Greece are all showing an absolute increase in births in 2026.

That is impressive considering that the average age in the EU is 45 and there are fewer women of childbearing age every year. 2/4

A paper published today argues that women's liberation led to collapsing fertility around the world.

Using US data, the authors show 58% of women's sexual relationships are with the most promiscuous 10% of men.

But most of these relationships are short term and childless. 🧵.

Women's freedom, widely viewed as positive around the world, is strongly associated with lower fertility, the authors find (r=.81).

Many forces have led women to pursue short-term rather than long-term mating strategies. But most of these partnerships don't lead to kids. 2/5

HOW ONE MAN REVIVED A NATION
The population pyramid of Kazakhstan is unlike any other in the world. Birthrate decline was stopped and reversed, even as the country grew rich.

Nursultan Nazarbayev led 🇰🇿 for 27 years from its founding in 1991 and got it to grow again! 🧵!

Kazakhstan's population pyramid inverted in the mid 1980s as the number of births declined sharply and fertility fell below replacement in the 1990s.

But then births sharply recovered and Kazakhstan experienced a baby boom lasting almost 30 years even as it's GDP grew 15x! 2/7

A study in Sweden found that firstborn children tend to have more children of their own than those that came later in the birth order.

Why? It is likely because older siblings had exposure to babies growing up, while younger siblings did not. 🧵.

https://twitter.com/uaustinorg/status/2039351993316102448

The large study of siblings (N = 1.5 million) found firstborns tend to have significantly more children than those that came later.

The effect is especially strong for women. When girls have a chance to help with little ones, they have much stronger family desires later on.
2/4

A new map shows the last time each country in Europe reached replacement fertility.

Most western European countries, including the UK, France, Germany and Italy haven't had replacement fertility in more than 50 years.

This is the main reason for Europe's stagnation. 🧵. This map shows the sobering fact that once a country falls below replacement, it almost never bounces back.

Eastern Europe did not fall below replacement until the 1980s but has had exceptionally low fertility over the past 25 years. 2/4

A big fail in the New York Times today with the claim that "31 is the new 21."

The Times leads readers to believe today's young women will make up lost fertility in their 30s and 40s.

But all evidence indicates they will not. 🧵.
First, the Times is ignorant of what recent research shows about age and infertility.

A big 2023 study of three million women by Geruso, Spears and LoPalo found that the ability to get pregnant is much lower in the 30s than the 20s.

So unfortunately, 31 is not the new 21. 2/4

A recent study found that giving men a pay raise led them to have more children, while giving women a pay raise led them to have fewer children. 🧵.
For women, the effect of a pay raise was significantly reduced future fertility. A pay increase at 25 was associated with a large decrease in fertility at age 30, regardless of the skill level.

For men, a pay increase was associated with persistently higher fertility. 2/4

A newly published paper found pronatal policies only worked when supported by culture.

"Maternity benefits increased fertility only among women who grew up in religious families" in the Baltics.

This could explain why many pronatal policies have not boosted fertility more. 🧵.
In 1982, there was a big expansion in child benefits in Estonia, Latvia and Lithuania including maternity leave wage benefits, a cash payment for birth and 18 months of job protection.

Five East European countries with comparable economic systems did not get the benefits. 2/5

A new study finds that work-from-home raises fertility more than any conventional family policy.

"Estimated lifetime fertility is greater by 0.32 children per woman when both partners WFH one or more days per week as compared to the case where neither does." 🧵.
In this chart, a large effect is clearly seen, with fertility higher when either partner has some work-from-home and highest when both do.

The authors say this is not due to selection because fertility rose among those that unexpectedly got WFH, compared to those that didn't.

On this Christmas, we can reflect how Christianity was able to grow out of the ashes of collapsing Rome.

Scott Alexander reviews Rodney Stark's The Rise of Christianity and describes how the new faith won out by valuing women and children.
Important 🧵!
Much like our world today, Pagan Rome faced terrible birthrates.

Sex-selective infanticide was the norm. Women were not valued and many men just wanted to stay single.

It got so bad that Roman General Macedonicus proposed forcing people to marry! 2/7