Sarah Hoyt has written extensively about that; she totally dismisses the population figures cited by the UN as having been made up by third-world countries to claim higher numbers, because that brings in more aid.

C. S. Lewis

Then there is the phenomenon of Kodokushi—people apparently deciding to or having to, die a “lonely death” (in what one would have thought was a very family-oriented and tight-knit society), and reports that each year an estimated 30,000 Japanese (the actual number may be a lot higher) die alone, and are often only discovered to have died alone by the smell, with an industry having grownup to clean out their homes and apartments, and to dispose of their possessions.

The appearance of phenomena like Kodokushi perhaps another indicator of disintegration in Japanese society.

See https://min.news/en/world/eb18e95cc336b62f4d30c3a66060ffbc.html and

https://www.ndtv.com/world-news/so-many-japanese-people-die-alone-theres-an-industry-devoted-to-cleaning-up-after-them-1804387

If you believe analysts like Peter Zeihan, in many instances past actions taken–as in the case of China and its former “one child policy”–mean that these demographic catastrophies are already baked into a country’s demographic structure, and there is not any easy way–or perhaps any way at all–to reverse them.

If people just don’t feel like having children–if the spirit and the optimism about the future isn’t there–it looks like it’s pretty hard to cajole them to do so.

On a fundamental level, the change from a mostly agricultural society to one based on manufacturing and technology, and the subsequent move of a large percentage of the population from the villages and the countryside to large urban areas—urban settings which quite often feature very small, crowded, and very expensive accommodations–plus the sheer cost of raising children, has discouraged having large families.

From my perspective there are also some really strange cultural dynamics taking place in Japan these days, such as the phenomenon of some young people living sexless lives, or living holed up in their apartments or video game parlors, and never leaving.

And what is this fascination with childish cartoon characters?

Then, there are the reports of “liberated” young women who want and have careers who cannot afford to marry or to interrupt those careers to have children.

How about Japan’s aging “left over women?”

Sex hotels everywhere, what is that all about?

And, from reports, government efforts to promote having more children have just not worked.

The driving spirit that used to animate traditional Japan has, it appears, just dwindled away, and I don’t see any way to recapture and to reignite it.

There are many ways that a society can commit suicide.

Admirable though they may be in many ways, like some other nations, the Japanese are in the midst of a predicted demographic death spiral, which will see the collapse of their population (and, I’d imagine, any real influence that nation might have on the world) in the not so distant future.

This spiral could be averted if they allowed large scale immigration by non-Japanese settlers into a homogeneous, 97.9% ethnic Japanese Japan, but letting in foreigners is not something they are willing to do.