- Imagine you dug a hole 1 mile deep and 1 mile wide. How many humans do you think you could fit into it? The answer is about 60 billion, many times the current global population.

- If you dropped a human on a random spot on this planet, even if we just assume land, the median number of people within a mile would be exactly 0.

I think the second thought experiment is quite informative because it's easy to lose track of the reality when we mostly travel from packed city to packed city. But these cities, and the pathways that connect them, make up a negligible percent of the Earth. China is smaller than the US, and has pretty rough geography, yet was food self sufficient with a population upwards of a billion. They only became food dependent as their economy expanded and they replaced farms with iPhone factories, metaphorically speaking at least.

Finally, population growth in the longrun will always be determined by the groups with high fertility. It's never longrun stable unless everybody has the same fertility rate. So for instance the Amish today maintain a fertility rate of around 6. This is why there are now hundreds of thousands of them, and within a few decades they'll number in the millions. As highly educated, highly secular, high income individuals remove themselves from the gene pool, they'll simply be replaced by Amish. And as the former die off, and the latter grow - we'll eventually see population growth resume.

One last point. Fertility rates also determine age ratios within a society. Low fertility = most people are elderly, high fertility = most people are young. So it's not only economic collapse due to fewer people, but you create societies where there's a much greater social cost (due to elderly care), and far fewer people to take care of them. South Korea is a great example of this. If they don't fix their fertility rate, they'll collapse as a nation within the lifespan of some people alive today. 'Breaking News 2094 - North Korea successfully invades South Korea with one tank.'

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bkandel a day ago | parent | next [-]

You can't even assume that Amish population growth will remain constant! See e.g. https://medium.com/migration-issues/how-long-until-were-all-.... We also don't know what will happen to Amish retention as the population grows, or what will happen if the population spreads into previously non-Amish areas -- it's just really hard to predict what's going to happen multiple decades in the future.

	▲	somenameforme a day ago \| parent [-]
		In general I don't disagree, but more because of the rest of society than the Amish. Every country that's entered into fertility crisis has basically started freaking out and none are anywhere near the worst of it yet. It's only the countries that will be in fertility crisis, but aren't there yet that still have some people think it just comes down to being kind of like now, but with fewer people. I think there's no way the status quo simply continues on. The country to keep an eye on will be China. They're having a catastrophic fertility crisis, have long-term oriented leadership, and have a strong precedent for some fairly batshit means to 'adjust' fertility. There's also the issue that I don't think I'm a snowflake. Fertility has become a big issue for me and is part of the reason I now have children - not as many as I'd like, but infinitely more than I was thinking about having not that long ago. I've come to view it as a core social responsibility since cultures/societies literally cannot exist without people having enough babies, and I think more people are coming to see it this way as well.

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vladms a day ago | parent | prev [-]

> And as the former die off, and the latter grow - we'll eventually see population growth resume.

Previously (centuries ago) most of the people were like the latter group. And things changed. I would not bet that a group can remain completely unchanged in the next tens to hundreds of years. There is of course a chance, but don't think it is as clear cut as you make it sound.

> Fertility rates also determine age ratios within a society

Plus immigration. Some like and accept that, some hate it and don't like it, we will see how it plays out, but there are more things involved than just fertility rates.

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somenameforme a day ago | parent [-]

Immigration is largely irrelevant at the current rates of decline for many reasons. A simple one, and without any controversy, is plain old numerics. A shared fertility rate of 1 will trend towards you losing approximately 2.5% of your population per year, compounding. This is why Japan's current deterioration will be seen as the 'good ole days' in the future - they're currently "only" losing about 0.5% per year. So after 80 years, a single human lifetime, your population declines by 87%.

There simply are not enough people to replace this sort of decline, even if you just grab anybody regardless of skills, education, or even the ability to speak your language - which is not exactly a great recipe for a healthy country. Like if 60% of the world, by population, is losing 80% of their population over that timeframe, that'd be a global population decline of 48% with only 40% of people in the rest of the world. You'd need all of them and then a good chunk of their babies too, and that's for less than one human lifetime of change.

I'd say immigration is just kicking the can, but it scarcely even counts as that given how little of an effect it can have even at theoretically absurd levels.

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vladms a day ago | parent [-]

The problem is rates never stay the same. What you say is correct, if nothing changes. But things do change (for one, population is decreasing).

Let me put it another way, if a pandemic would reduce the world population by 50%, what do you think birth rates would do?

Now, if your argument is "it will soon be a shitty time to be old and it will last for a while", with that I do fully agree. The old people of today have it much easier than the old people in 30 years. But people in 80 years might have enough (resources, space, etc.) to start making children again at different rates.

I would say we will live and see, but doubt it is the case for that long periods...

	▲	somenameforme 21 hours ago \| parent [-]
		Not that long ago it was believed that fertility rates could never fall as low as they are currently falling because it was thought that people would start having kids again at a sufficiently low fertility. That wasn't just a lay view either, but was part of all population models. It's why the UN had to recently make major revisions to their population models. It turns out that people didn't just start organically having children again because some stat fell below some level. Go figure. My framing is snarky, but is this really a surprise if we think about it objectively? All of the claims about causal effects on fertility from education, income, and so forth are nonsense. They have endless major counter-examples, the least of which not being the past where they completely fall apart. The cause is solely and exclusively cultural. And completely changing the trajectory that a culture has been headed down for many decades, arguably centuries depending on exactly which component(s) of culture one thinks is responsible, is only viable with very extreme actions. Would a "real" deadly pandemic shift culture? I imagine probably, but even that's still almost more of a hope than a guarantee - akin to population models of the past. --- Finally, the population effects of declines in fertility only really become visible once the first low fertility generation starts significantly dying off, so about 60 years from the advent. This is why Japan looks so terrible relative to other countries, even though their fertility rate is significantly higher than many other places. Japan fell below replacement around the 70s while South Korea lasted all the way til the 80s. The point I make with this is that the future isn't speculative. South Korea's fertility rate has been around 1 (and even less) for about 20 years, the approximate length of the fertility window for a generation. So they're going to pay a huge price for that in the future, and that is now inescapable. It doesn't matter much if a bunch of 40+ women decide they should've had children - it's now simply too late. Even if a later generation decides to try to 'fix' the past, the burden is unrealistically high to manage. To get back to the initial population equilibrium, they'd need their next generation to have a fertility rate of 4. A generation at a fertility rate of 1 cuts the generation size in half, so a generation at 4 would double it and return to the initial equilibrium. That's wholly unrealistic.