Not Japanese either, but I have lived and worked in Japan long enough.

> From what I understand, the cost of living is very high

The cost of living in Japan is not as high as it used to be and lower than many other countries. There is relatively high inflation now, but that's due to a weak Yen and relatively low interest rates. If anything, Japan's cost of living has hit rock bottom in absolute terms.

>Women are typically expected to stop working after marriage or childbirth

That's also quite outdated. This is called "kotobuki taisha" (can be roughly translated as "Congratulatory quitting") and it used to be the norm, but nowadays most of the women return to work, usually after taking a maternity leave and childcare leave, which are guaranteed by law[1].

There is still an issue of old (or old-fashioned) bosses and colleagues who either push women to quit after childbirth or passive-aggressively hint them that they should be taking better care of their children by being full-time moms. That behavior is called "matahara" (maternity harassment) and its frowned upon, but unfortunately still quite common[2].

> men have to bring in a fair amount of money in order to support a family

The cost of raising a child is high and probably a factor in choosing to have fewer children, but I don't think Japan is different from most other developed countries here. Public education is quite comprehensive in Japan, and private spending on education is lower than OECD average[3].

> Given the price of larger dwellings in larger cities

Large dwellings in Tokyo and other central metropolitan areas are expensive, but you have to keep in mind modern Japanese houses are quite small by international standards, even in rural areas where land is cheap. Old Japanese houses were larger, but this is just the standard for new Japanese houses. The norm is probably somewhere around 70-120 square of meters of floor space (i.e. all floors combined). I've yet to see any Japanese person complain about the size of Japanese houses, this doesn't seem to be a concern for anyone (except for us foreigners, who got used to a different standard of living).

If you're fine with a not-too-large house, you could get a house in the suburban area around Tokyo with a 2 hour commute, with mortgage for a while. But land prices rose quite a bit recently (not sure if this is true for all areas), and with the recent interest rate hikes, Japanese mortgages are not quite the nearly-free money they used to be.

In short, I think most of the factors above are outdated. High cost of living for single-income families and workplace hostility towards double-income families were probably strong factor in the previous decades, but not now.

I don't know if there is any accepted conclusion on the most relevant factors that affect Japan, but for me the following factors ring true:

1. Lack of daycare facilities (hoikuen): Japan has two types of pre-primary education: preschools (youchien) and daycare centers (hoikuen). Youchien end too early for working parents, and they traditionally structured around the assumption of having a full-time housewife. The mother is expected to do a lot of busywork like making bento (lunchboxes), sewing uniforms, attending various events, handling regular feedback the teacher, etc.

Hoikuen is a better fit for working moms, but there seems to be a shortage, and there are long waiting lists for daycare centers in some areas. The situation seemed to have improved significantly over the last couple of years, but this was a big issue in previous year. If you can't find a daycare center, it's kinda hard for a mom to return to employment after childbirth.

2. Nomikai Culture: Japanese companies tend to constantly have Noimaki (drinking parties) after work. Depending on the company and role, these can range from once a month to 3-4 times a week, and from optional fun to implicitly mandatory. In many companies, they could be optional in theory, but schmoozing during these parties is important for promotion and relationship with your customer. This culture took a strong hit during COVID-19, and seems to have never fully recovered since. So this is another factor that may be improving, but still affected birthrates over the last 10 years.

3. Matahara: As I've mentioned above, women who choose both career and child-rearing might still suffer some harassment at work. Women who don't need to rely solely on their husband's income. Naturally, stay-at-home-dads are barely a thing in Japan.

4. Limited unskilled labor immigration: Most western countries have accepted a large number of unskilled immigrants who by definition tend to be less educated. Women education is widely accepted to be the number one factor in determining birth rate, so having high unskilled labor immigration artificially pumps up your country's birth rate as long as that immigration trend continues (the original immigrants would typically get closer the country's birthrate within a generation or two).

Until recently, Japan severely restricted immigration of unskilled employees, but that has been changing too over the last few years. There are still many limitations that keep education levels higher than "completely unskilled", such as requiring Japanese language proficiency and professional exams and training, and visa recipients cannot bring their family to Japan until 5 years later (and even then with some restrictions). So I don't expect seeing big shifts in that direction for the next 10 years.

[1] https://archive.is/yPcOF#selection-1581.143-1581.149
https://mataharanet.org/en/what-is-matahara/

[2] https://mataharanet.org/en/what-is-matahara/

[3] https://www.oecd.org/en/data/indicators/private-spending-on-...