It's very dangerous to try and compare human behavior to any pattern seen in nature— particularly human behavior in aggregate. While humans are animals like any other, we are also very much not simple beasts beholden to environmental conditions.

To wit: the current human population is beyond the natural carrying capacity of the places we live. The only reason we can sustain 7bn people today is because we've artificially increased local carrying capacity through artificial fertilizer. If we lost that technology today, a majorty of humans alive now would starve to death.

There's really no reason to assume any environmental factors that don't physically preclude human occupation will have any effect on overall population numbers. We can artificially extend our ecosystem to support essentially unlimited people. The only real hard limit is space to physically put bodies and the amount of energy our society can use without boiling the oceans with waste heat.

If population growth levels out, it won't be for any natural reason because we are already well beyond any natural limit.

I think the real problem is the age structure of the population is increasingly skewing older and this problem becomes worse the lower the birth rate. I don't know how we're going to keep supporting more and more people getting past the retirement age and collecting benefits on a shrinking working age population being squeezed harder by taxes. Either retirement spending goes down maybe with higher retirment age or increased healthspan, or we become much more efficient at taking care of the elderly with fewer resources, or the working class gets squeezed harder & harder.

> I think it's more likely, drawing from biology, that we end up at a stable global population level without having to worry about moving backwards along the metrics of education, income or contraceptive access.

There's absolutely no inherent equilibrating force that will stabilize global fertility rates at replacement. Many countries have blown by replacement (the USA included) and continue on a downward trend year over year.

Yea my crackpot theory is it’s genuinely something that’s inherent which is causing these declines. That’s why no attempts to reverse them have been successful. I think like you’re saying we’ll end up at some equilibrium.

>All sorts of animal population follow a sigmoidal growth pattern where there's exponential growth, some degree of overshoot and then a return to a steady level somewhat below that peak.

Animal populations usually decline because they lack food or have predators and other external factors. Not usually because of a lack of will to reproduce due to social or economic reasons.

I don’t think we’re going to find a number and stay there. Too many factors impacting population size are changing. Healthcare, climate, food science, etc. It’s likely to always fluctuate, and it’s likely to continue to be something people worry about.

The economy depends on some level of growth, so if we can't accomplish that with a stable or shrinking population then it's gonna be a bad time for a while.

EDIT: I did not think I'd have to state this explicitly, but:

yes, I am in fact talking about the capitalist economy the western world currently operates under

growth = economic growth

Actually growth patterns of animals vary wildly. There's a whole set of animals that get "unstable" growth - Cats are famous for this, for example. That means that cat numbers in specific areas actually grow to the point that cats die out in the next generation, destabilizing the entire food chain in the process (happened in Australia, for example)

The problem with this instability is that the numbers bounce around wildly. Up and down, by a lot, in as little as 2 or 3 generations. But there's a process that stops the bouncing: hitting zero.

> If you consider starting a family with no hope of ever getting out of renting, as landlords constantly raise monthlies, you might reconsider children.

Generally the less money you make the more kids you have. It's really a question of prioritization. People say they're holding off on kids for X or Y reason but I think this is more of an expressed vs revealed preferences situation. They would rather chase material wealth for themselves than have kids, and to be clear I'm not judging just observing. Through most of human history mud huts weren't a blocker to having kids.

https://www.statista.com/statistics/241530/birth-rate-by-fam...

Right, I think we’re running into the limitations of a scarcity-based system here. Even many well compensated couples would face having to make major tradeoffs with their economic stability, careers, time spent with the kids, retirement, quality of life, etc, and are accordingly choosing the path of least risk.

Even the most generous countries aren’t fully compensating for the costs of raising a family, and the assistance offered by many is less than pocket change. It’s only natural that incentive is going to be low.

This trend has been going on for much longer than the current worries about microplastics and whatnot. Lower fertility doesn’t necessarily mean lower physical fecundity. It can also just mean that generations of kids have been raised to believe having kids early ruins your life, and should only be done much later after you graduate university and your career is well established (by which time you’re in your latter 20s and your fertility is naturally lower.)

I believe lower fertility is most closely associated with education for women. Women with an education sometimes find interests other than being a baby factory.