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Comment by jacquesm | Hacker News Reader

> There is no sensitive chaotic model behind this, it's completely straightforward dynamics.

Sorry for the poor choice of words, no chaotic model was implied or intended.

> Fertility rate per woman, and death rate.

No, there is also technological development, wars and so on. So any prediction that looks 80 years out that only looks at fertility rate and death rate is going to be found wanting. It's a bit long for a long bet, but I'd be more than happy to take one for a 30% or more deviation from the prediction.

The degree to which people are willing to believe simplistic models without further qualification is astounding to me, given the past 80 years in development of medical technology, electronics and our ability to wage war any predictions about the next 80 years should come with huge error bars. And that's before we get into the consequences of Climate change.

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> No, there is also technological development, wars and so on.

Which have an effect on fertility and death rate. There is literally nothing that matters to population dynamics other than fertility and death rate.

We have extensive experience on that now: Technological and social development drives fertility to at or below replacement levels rapidly. This is the main factor. Importantly though, _this is not a prediction_. This reduction has already happened. So the bulk of the prediction is just playing out the consequences of developments we've already seen.

Of course major events like climate change have the potential to invalidate this prediction. But that doesn't mean this prediction is useless. And this is would not be due to "sensitivity to initial inputs".

> Which have an effect on fertility and death rate.

Yes, but given that they are unknowns you can't just extrapolate from the presently available data and call it a day.

> But that doesn't mean this prediction is useless.

It's an open door that the world population will stabilize at some point, it is also an open door that the speed with which the world population is increasing itself is reducing. It has to, simply because more affluent people will have fewer children because they are not looking at their children as their source of income when they are themselves unable to generate any. Children as a pension scheme is a very common thing still, but that is fortunately changing.

> And this is would not be due to "sensitivity to initial inputs".

To a degree it is: any error in the predictions for the next couple of years will result in a much larger error in the future, simply because of the fact that population increase is a non-linear affair to begin with, and that any med-tech development that causes people to live longer will also cause a large increase in the number of people alive at any one moment.

Compounded those two could have an enormous effect.

For reference, the world, 80 years ago, so just before WWII had only 2.2 billion people.

> Children as a pension scheme is a very common thing still, but that is fortunately changing.

It is still the case, just on a larger scale.

Good point.

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I would argue that the primary mechanism is cultural changes effecting the fertility rate. Japan suffers a declining population not because of its technology, but because of its culture. Population growth rate peaked in 1968. Up until 1968, lots of evidence social scientists had to work with indicated that technological and social advancement increased the fertility rate (notwithstanding life expectancy changes and rich vs poor countries). Just because cultural changes over the past 51 years have been toward lower birth rates, does not mean that this will continue.

You cannot predict the direction that culture will change. Any predictions I make about this are extremely speculative, but it is easily imaginable that 20 years from now the zeitgeist reacts against the corpus of current culture to be more family-oriented. Think about how much you hear people complain against "the machine." Whether it's on r/latestagecapitalism, or r/the_donald, everyone hates the piss out of broad society, and may find solace in tight-knit families.

There's nothing "cultural" about birth rates. Raising children is hard, sex feels good, given the option to have sex and not have children, most people choose to have sex and not raise children.

Atheist women choose not to have children, christian women choose not to, muslim women choose not to.

Give people the option not to have children, remove the stigma and maybe they have 1-2 children, but no more.

Given how many single people there are, how tight the fertility window is for women who have careers, how tight schedules are in big cities, how expensive it is to have children today, there's nothing we can reasonably do to increase birth rates.

You state that there is nothing cultural about birth rates and then list several cultural factors influencing birth rates. And then you state that there is nothing we can reasonably do to increase birth rates, even though most of the factors you mentioned are not set in stone.

I'm not even sure what you are trying to say.

I don't think the person being replied to necessarily meant that, but cultural is often used in this context as a racist term, as in: "Africans will always have many kids, it's their culture", where before the more overtly racist "nature" might have been used.

The implication being that the cultures of (some) people are unchanging and fixed. I think you'd agree with a statement like: Cultures and family values do change all the time, due to a variety of factors like better medical care, and economic empowerment, in reasonably predictable ways.

So it's not unchanging cultural differences between Africa and Europe that drive the different fertility rates, but rather different socio-economic factors that drive a different family culture. Changing the socio-economic factors will change the culture surrounding families, just as much in Africa as it did in Europe, the Middle East, Asia, etc... at various times in the last century and a half.

My argument was that all modern, technologically advanced civilizations will face these exact issues, so there is no point in calling those factors "cultural", because cultural facts are meant to distinguish between cultures.

I'm sure an alien species or a computer simulation having a few of the same basic drives as humans do (sex is very pleasurable, contraception exists, raising educated children is extremely expensive and hard, state provides pensions in old age, people are free to have or not to have children, child mortality is low, work provides money, money satisfies needs, quality work is abundant only in high density cities and a coupe of others) will arrive at same fertility problem we have arriven at.

My second point was that there is nothing we can reasonably do without a major overhaul of human traits or shutting down the technological advanced world we live in.

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