For very high HDI countries, between the countries with the highest and lowest cost of living, median fertility rates were not significantly different despite up to a 2.75 times difference in cost of living, as shown below.

Median values for top and bottom 5/10/15 cost of living countries categorised "very high" HDI in 2024 (top = highest cost of living)

Note that the average fertility rates of developed and high-income countries were 1.45-1.48 in 2023. 

Data set: https://docs.google.com/spreadsheets/d/1iGGaQw6hnIETIoYnbjSr-L9pzMy00kIsm4GDOtSXegM/edit?usp=sharing

Sources

Cost of living data (2024 Mid-Year): https://www.numbeo.com/cost-of-living/rankings_by_country.jsp?title=2024-mid

Fertility rate data (2024 list by the World Bank): https://en.wikipedia.org/wiki/List_of_countries_by_total_fertility_rate

HDI data (2023): https://worldpopulationreview.com/country-rankings/hdi-by-country

Polls on voting intention show One Nation has surged to 2nd. A small party gaining so much popularity in a span of a year is unheard of. Immigration peaked in 2023 and has largely declined since then, so I'm surprised they're gaining popularity now and not before 2025 when immigration was a major topic.

Sources:

https://en.wikipedia.org/wiki/Opinion_polling_for_the_next_Australian_federal_election#

https://www.pollbludger.net/fed2028/bludgertrack/

2 charts were plotted with mental health indicators and fertility rates for the U.S. Starting from around 2016, U.S. fertility rates saw a rapid decline. Around the same time, indicators of poor mental health skyrocketed, indicating a potential correlation. The connection between mental health and fertility rates could be due to poor mental health leading to:

• more fatigue, poor sleep, and stress leading to less time, energy, and desire for children
• lower desire and ability to find, attract, and retain a partner
• worse health in general

Note that the 26-34 age group was the most important since that was the range where most people started having children.

Interestingly, this trend and pattern wasn't unique to the U.S. The average developed country experienced a rapid decline in fertility rates starting from 2016, and mental health data from other countries also revealed a large decline in mental health starting from a similar year.

I'm not saying mental health was 100% the cause, but it was an interesting discovery that seemed to be worth sharing.

Sources

U.S. fertility rates: https://ourworldindata.org/explorers/population-and-demography?hideControls=true&indicator=Fertility+rate&Sex=Both+sexes&Age=Total&Projection+scenario=None&country=~USA

Mental health indicators: https://www.afterbabel.com/p/the-mental-illness-crisis-millenials

There have been studies recently linking the smartphone release in 2007 to the sharp fertility rate decline after. The main study claimed that smartphones were responsible for "33–52% of the decline in the general fertility rate among women aged 15–44", which was a high percentage. I think the timing was purely coincidental, and the GFC was the primary driver. There were several strong counterarguments against the smartphone.

Countries without smartphones

Countries that had no to limited access to smartphones also experienced major fertility rate declines starting from 2008, e.g. Africa. If fertility rates still declined in countries without smartphones, smartphones weren't the main factor. Africa was potentially a poor example since it was an undeveloped country, but there was a small pause in the fertility rate downtrend during 2003-2008, and the downtrend continued after the GFC, which suggested that the GFC played a role. Keep in mind that the study used data from 2007–2011.

Delayed social media app releases

Smartphones were released in 2007, but it wasn't used much differently from a flip phone back then. Most of the social media apps were released on iOS after mid-2010, so people weren't using smartphones to doom scroll social media. The internet was also laggy and unreliable since it was dial up. Social media app release dates:

• Facebook: July 10, 2008
• Twitter: May 19, 2010
• Instagram: October 6, 2010
• Snapchat: July 13, 2011
• Reddit: April 7, 2016

The study used data from the United States over 2007–2011. Fertility rates began declining in the US starting from 2007/2008. With most social media apps being released after mid-2010, the extra delay for them to gain popularity/use, and delay in widespread iPhone acquisition (discussed later), the fertility rate decline began too early for the iPhone to explain it.

Unresponsive US fertility rates during 2011-2016

The US fertility rate data showed that most of the fertility rate decline occurred over 2007-2010. Fertility rates then mostly flatlined over 2011-2016. This behaviour was contradictory. If smartphones played a large role in the decline (33–52% as they claim), fertility rates should have continued to decline over 2011-2016 as smartphones and social media apps gained more widespread usage. Smartphone sales exploded over 2011-2015, so fertility rates should have seen a major decline over this period. To me, it just seemed like the GFC caused a major hit to fertility rates during 2007-2010 then fertility rates stabilised after the economy stabilised.

It's also weird that the study chose to use 2007-2011 data where most of the fertility rate decline occurred and ignored data after 2011 when fertility rates flatlined. Perhaps there was no correlation after 2011 and they cherrypicked the years?

Limitations in study methodology

Another major factor was that more developed areas and richer people, which both have lower fertility rates in general, had far greater access to smartphones. More developed areas were more likely to use, need and buy phones. Richer people were more likely to buy and be able to afford the iPhone since it was a luxury item back then. Most people didn't have easy access to it until several versions later, which was evident from the worldwide sales. These were major limitations of the study, which mentioned that "the average control county has 28.2% of its population in urban areas, compared to 66.4% in the average treated county." Also, they didn't directly compare smartphone usage. They tried to indirectly compare it by comparing counties with <10% and >90% AT&T coverage, so the study could just be indirectly comparing low vs high development or poor vs rich areas. From the study:

>Much of our analysis rests on classifying counties as “control,” defined as <10% of the population residing in block groups with AT&T coverage and “treated,” defined as >90% of the population residing in block groups with AT&T coverage. Intermediate counties with 10–90% coverage are excluded from analyses that rely on binary classifications. Following this assignment, 1,399 counties are classified as control and 914 as treated, while 794 counties are excluded. Treated counties are systematically more urban than controls: the average control county has 28.2% of its population in urban areas, compared to 66.4% in the average treated county.

Fertility rates have been falling for centuries. It isn't something that can be quickly fixed and it requires massive changes to reverse. No political party will ever take the large risk, especially since there has been no proof of a solution yet i.e. a developed country returning to 2.1 fertility rate. Another major reason is immigration. There's little reason for governments to take the large risk when immigration already addresses the problem. Developed countries aren't going to act until the immigration tap runs dry, which is unlikely until 150+ years later.

Politics aside, I doubt people will ever reach consensus on a solution. Everyone has their own theory. The lack of consensus will just cause slow progress and insignificant changes. The only problems that are commonly cited are cost of living and housing. Realistically, governments will never solve these issues and there are large conflicts of interest as some people want house prices to rise. Regardless, solving these issues is unlikely to return fertility rates to 2.1 as fertility rates were already <2.1 before 2000 when cost of living and housing were cheap.

Fertility rates reversing will never occur due to unanimous agreement. It will occur when population decline starts having major impacts on the country, e.g. economy crashing, massively diminished populations, and high old-age dependency ratio, which will force people to change. This isn't going to occur until 100-150 years later (depending on levels of immigration). Once change occurs, it will still take 50+ years for fertility rates to rise significantly since it will take a long time for people to adapt to the changes. People aren't going to start having many babies immediately in most scenarios. Also 100-150 years later, I expect the fertility rate to be <1 by that point and people to have become accustomed to low fertility rates, so returning back to 2.1 will take much longer than now. Keep in mind that the UN projected fertility rates to be 1.59 in 2100 in the medium fertility scenario, and based on new data, that's obviously an overestimation. The low fertility scenario projected fertility rates to be 1.1 in 2100 and not change much this century.