Essentially, there's a sort-of phase diagram for the optimal human reproduction strategy where the axes are things like "income inequality" and "environmental stress". You can then color in patches of the diagram with what strategy members of a society will settle on.
For example, high-stress, high-inequality societies practice polygamy. The game-theoretic logic is that women need a partner that can provide consistent excess calories. If you can't feed your kids in just one year out of twenty, they die. It doesn't matter what the average income is, just how steady the income is. In societies like that, only the richest 10% of men can provide this, so they all have 10 wives. The other 90% of men remain bachelors and often participate in tribal warfare, exacerbating the problem. Saudi Arabia used to be like this, and parts of Africa still is.
Similarly, in "one corner" of this graph is super rich men in stable societies, where their optimal reproduction strategy is serial monogamy to young wives. Look at Hollywood A-listers or billionaries to see this play out in real life.
Japan got stuck in a part of the phase diagram where the optimal reproduction strategy is "don't."
Women aren’t having kids - because they can avoid having kids. While still having sex and having societal benefits, including retirement. That is a very, very new and unusual situation when looking at the span of human history.
Birth control is a pretty new invention (mid 60’s approximately), and societies globally have been steadily rolling it out/making it available more easily.
Childbirth is dangerous (especially the first time), usually ruins one’s figure (and hence ‘market value’), and is economically very expensive.
At a minimum 4-6 months of ‘non productive’ work time per pregnancy, not counting the several years of near constant caretaking for a newborn. Which kills normal careers, where taking ‘time off’ pretty much guarantees you’ll be surpassed by all your competition - and 2-2.5 years pretty much guarantees it. And that is for an office job, not labor.
Before birth control, the only way to do that would have required celibacy or constant abortions - and good luck suppressing that drive for most people!
It’s like wondering why almost everyone is fat and out of shape when there is no public transit, everyone has to drive everywhere (and gets stuck in traffic), food is terrible and junky, jobs are mostly sedentary, etc.
Of course that’s how folks will be on average when that’s the environment! And birth rates are by definition based on the average behavior.
Coupled with the fact it’s relatively easy to survive/live in society without kids now, and it’s a double whammy. At least until the consequences of having no kids for a long time come home to roost - then everyone is going to be fucked. Figuratively.
I think social safety nets are an under-reported cause of this. 80 years ago, children were your retirement plan. Long term stable value pensions and 401ks, along with government ensured healthcare and social security, weren't a thing. If you got to 70 without kids, you would eventually not be able to work and probably die poor.
Now, almost everyone thinks not having kids could be an option, because of course they'll have some savings (the rest of society's labor), and government healthcare, and the government won't let them become destitute.
Socialized retirement destroys most of the incentive to sacrifice your youth to grow and raise new people. Why sacrifice your youth and money being a parent when you can leech off the labor of other people's kids in retirement?
I think we'll need to eventually experiment with things that try to give some disproportionate benefit to people who have kids. e.g. everyone gets some minimal socialized retirement and healthcare, but your benefits increase above that baseline in proportion to how many children you raised. Maybe your benefits increase at 3+ kids and decrease with 0-1 kids.
The challenge comes if the kids hate you and refuse to acknowledge you - something which has been being encouraged more and more by western society, BTW.
Historically, people would secretly understand (yeah, they are an abusive alcoholic!) but call you a bad kid and tell you to suck it up and deal with it. Since otherwise they’d have to deal with it. Going no contact is increasingly normalized.
Probably why many societies, especially conservative ones, lean so heavily on filial piety type concepts and frown on people cutting off their parents. especially when those parents are a burden or huge pain in the ass to those around them. Like Chinese mother-in-laws.
I can’t imagine that proposal going over well with the LGBTQ contingent either - which near as I can tell gets a lot of the angst from conservatives precisely because they’re ’having fun’ by opting out of the system and doing what they want instead of ‘comply or die’ like the rest.
If the crunch is big enough, anyway.
Same with the tax base used to fund gov’t healthcare and similar programs.
That said, Japan has… problems. But they aren’t apocalyptic. At least not yet.
Yeah. For that reason, old age pensions should be tied to how many kids you've had, not how much money you paid into the system during your working years. If you have no kids, you get no pension. If you want to support in old age, you need to contribute to the next generation's labor pool to provide that support. Otherwise, you need to work to support yourself.
Or, the more subtle approach - inflation. Hmm, wait a minute….
Birth control is available in countries where women are having over 5 kids on average each despite that. Similarly, legalisation or the illegality of abortion has no significant effect on child birth rates. Availability of birth control (or abortion) alone doesn't explain the "big picture", it just alters it a little bit. These are Catholic and conservative talking points, not scientific theories with evidence, charts, graphs, and equations.
Similarly, childbirth has always been dangerous, but is less dangerous now, so that has no explanatory power either.
Etc...
You're also confusing something else: Women's careers being interrupted by childbirth is the much closer to the truth. Women didn't work previously on anything but raising a family! In the economy/stress phase diagram, this is a different place, where a single income is no longer sufficient. THAT is the problem. If women are forced to enter the general workforce, then that alters the game-theoretic optimum for family planning.
Similarly, the mere existence of a social safety net such as a government pension substantially alters the equation. Individual families are no longer incentivised to "invest" in their future by having kids themselves, because they expect "society to take care of it" with unemployment benefits and retirement funds. In the past, people had individual incentives to have kids, because that was their retirement plan.
Previously, women (unless celibate or okay having abortions a lot) had no choice but to spend time at home, which forced the market in that direction.
And I’m also not confused about birth control availability. You’re confusing ‘theoretically available’ vs ‘socially acceptable available’, which those societies haven’t hit yet. Ideologically, they’re very resistant - but changing.
Just like the changes in the US (starting to be possible in the 60’s) didn’t become the norm until the 80’s to 90’s or later. American Catholics have been okay with birth control since then (roughly), where South American Catholics are still against it.
Which, notably, is when the US started to become more heavily two-income household dependent. And when wages stopped keeping up with growth.
Without birth control being possible, none of it matters - which is why it being possible is actually important. And why we’ve been seeing steady change in societies since it has become possible.
The only "allowed" solution is importing people from poor countries into richer ones, but disguised as something humanitarian, taking in refugees of war or whatnot.
Eventually, likely in the direction of gun control - birth control being licensed or the like. Or being religious being a requirement again to be a ‘respected member of society’.
Unlikely to be popular of course, but eventually society will change to protect its interests - or die.
I wish we could have real conversations about how to solve this issue, and not bury our heads in the sand. I'm not proposing any particular solution, just for our collective attention to the problem and liberty to discuss solutions.
People seem to think "oh, I'm paying my social contributions, that should be enough for my retirement", but the whole formula really doesn't factor in a sharp drop in workforce we're about to experience. We're papering over the monetary deficit with credit all over the world, but that's not feasible long-term, and the "who's actually gonna do it" aspect will become much more important. Writing a check doesn't change the senior's diaper, nor anything else.
As for practical solutions, first you have to make it possible for young people to obtain real estate again somehow (crucial), then to subsidize kindergartens / child care of all kinds heavily, and only then go more radical.
Another radical angle could be some sort of education reform where people are somehow incentivized to do all the post-BSc education in their 30s instead of 20s. How to achieve that without horrible side effects is up to debate. Subsidies for parents on every corner and every aspect of life (including education) would probably work, but how much is the optimum and how to implement it is a serious question.