There's no absolute number, it's relative.

1. Rapid change of any kind is bad in any ecosystem. Growing the population too fast creates one set of problems, shrinking the population too fast creates a different set of problems.

2. modern humans have the ability to expand consumption to use all available resources. 10 million jet-setting billionaires have a far worse impact on the environment than 1 billion poor people. Adjusting the environmental impact per human is going to have a far greater effect on the environment, and more quickly, than population changes.

Absolutely (note I referred to gradual change)

And you are right - but poor people _want_ to have material luxuries, by and large, so we should consider what we want our equilibrium state to look like.

Personally, I'm hoping for "8-10 billion people eating low meat diets, living in well insulated homes powered by renewable electricity, in places where they can walk, bike, or take public transport for almost all of their daily needs, and working a 20 hour (or less) week, with the remainder of their time filled with their friends, families, creative passions, and any other joys not yet comprehended"

but it doesn't seem like that's where we're headed.

because people who want "eating low meat diets" and "walk, bike, or take public transport" go extinct first.

Why? (I speak as someone who fits that description and has kids)

> Why? (I speak as someone who fits that description and has kids)

I’m not the original commenter, but I assume that the reference is that this/your group fairly reliably across most cultures does not maintain the 2.1 children per woman birth rate / replacement rate needed to sustain a population.

At a minimum, this seems to be true for this particular socioeconomic class in most parts of Europe, East Asia, and US/Canada. Not sure about South Asia.

Doesn’t make sense. All groups are trending towards sub-replacement fertility. Even the mormons.

Israel has a TFR of 2.9, which has held steady for 40 years.

That's not even the right question.

The question is - how can be free up a higher percentage of people who are able to propel humanity forward from the drudgery of wage slavery in a sustainable manner?

And you need to effectively allocate cognitively proficient people to this as well, not just anyone.

We do have an answer. It’s 2.1 children per woman. That’s the replacement rate.

We are not on track for equilibrium or a slow decrease over centuries. We are on track for a demographic cliff.

I've seen that "2.1" figure so many times. How is it calculated? (Or is it just a concise and informal way of saying "a bit more than two"?)

It can be calculated, based on statistics of how many people die before reaching fertility.

But given that the statistics have wide error bars, and there are so many other intervening factors, "a bit more than two" is just as good a way to calculate it.

Well, generally it is "how many people do you need to replace the 2 people that created those people". So the baseline is 2 since you need 2 parents and to replace them you'd need 2 children in an ideal world but since not every child survives long enought to themselves be part of this cycle the .1 comes in.

I've thought about it a bit in the meantime .... In the UK only about 30 children per 100,000 die before age 18 so that would be much less significant than the fact that only about 49% of children are female. Taken together, those facts would imply that you need about 2.0414 children per woman, so 2.05 would be more than enough. So how do we get to 2.1? Perhaps the 2.1 is for other countries with higher infant mortality?

You get 2.1 because they only want to show 2 significant digits, but it's important to show that the number is greater than 2.0.

From individual families' perspectives, you can only have a whole number of kids anyway, so 2.1 vs. 2.04 isn't a meaningful difference.

Being dead isn't the only reason someone can't have kids.

enough for what?

well that's exactly the point - apparently our population needs to keep growing forever "just because". But if any other animal had no bound on the upper limit of their population the entire globe would be taken over by it.

I like people! I have two kids myself and love them. But it's not like life was horrible when Earth had 5 billion people.

(Note - I am NOT saying that people in poor countries, who tend to use a minute fraction of the resources of rich people, having lots of kids, is necessarily an issue. But if we want to make a higher standard of living possible for everyone in the world it will be easier to do, ecologically speaking, with 9 billion people than with 15 billion)

I'm not aware of a lot of people that want unconstrained growth. But rapid decline has the potential to be a massive problem, and at the moment a rapid decline is what we're headed for.

How about «enough to have a supply of extremely talented people, able to devise novel solutions to crippling long-term problems we face»?

You won't have many brilliant people in a small natural population.

Without them it's «not with a bang, but with a whimper»

OK, how many is that?

To spend most of their lives working for others.