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Comment by martindbp | Hacker News Reader

My concern is once fertility rates are approaching 1-1.5 which is true in most of the developed world, what would be the mechanism for eventually reaching an equilibrium of 2.1? People today just don't want to have 3 or more kids, but there are plenty of people that don't want to have kids at all. I suppose eventually you'd expect natural selection to kick in, but how long would that take? Where's the bottom? I also worry as the age distribution gets more and more skewed, young people will have even less time to have more kids?

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Imagine the population is split in 2 subpopulations. 99% of the people want to have 1 kid per family, and 1% of the people want to have 3 kids per family. Initially the population will decrease quickly, but then it will start increasing quickly. After about 4 generations, the two subpopulations will be roughly equal (55% of the low fertility population, and 45% of the high fertility). After 9 generations, the relative ratio flips. At that point the total population is only about 40% of the starting one, but it's growing fast, because it's basically only high fertility individuals remaining. In 4 more generations it's above the starting level, and then in a few more it's 10 times the orginal population.

At the lowest though, the population is only 10% of the original population, and it appears to be imploding.

Seems like an unwarranted assumption that "wanting a big family" is a heritable trait

If there is any way for genetics to influence family size, it'll quickly spread. Cultural factors and resource constraints dominated for millennia but we'd now expect things to go the other way.

Religion isn’t a veritable trait, but the correlation is strong.

Partly heritable

The part where you inherit big enough house and other assets to make it feasible

If humanity survives then this trait will emerge and become dominant.

Humans are part of a lineage that's survived for hundreds of millions or billions of years. We've already had that time. The pressure is simply more complicated than "have big families whatever the cost" and humans have agency.

You might enjoy "The Mote in God's Eye."

The forces driving lower fertility are based on the balance of resources and jobs available per person now. If we have several generations of population decline that equation changes. There’s no telling how people make the calculation for themselves and I don’t see much purpose in worrying about it.

My theory is low fertility rate is the "cost" of equality and women being in the work force. It becomes quite difficult to have a career if you have more than 1-2 kids. And balancing two full-time careers with even 1-2 kids is very stressful.

Either way we should definitely worry about it, we should try to find ways to reach an equilibrium. Whether that's more subsidies for families or not I don't know. In Sweden, free (pre)-school, college, 18 months maternity/paternity leave, free healthcare and dental care up to 18, plus monthly allowance per child has not had much effect on the fertility rate as compared to the US, so it seems that doesn't quite work. But maybe there's something else/more we can do.

Mine is - waiting until you can afford housing, which current run up might mean 40s

Then there’s public transport that doesn’t run past midnight effectively making it a curfew. And you can’t drink drive like you used to.

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