I think peak baby is less about good or bad and more about a turning point for growth demand.

Conventional wisdom on population growth is that it’s a solved problem and that the population will inevitably stabilize or decline. I say let’s not count our chickens just yet. There are still countries and groups where the growth rate hasn’t stabilized at all, and even if these are small today, exponential growth means they soon won’t be. Population growth will continue until everyone agrees it shouldn’t.

Due to the dynamics of demography, a population is either exponentially growing or exponentially shrinking. A stable population is an unrealistic goal.

You’ll have to provide a lot more context for this to be believable. Maybe it is an unrealistic goal for animals who aren’t interested in having a stable global population.

It follows naturally from differential equations that describe population:

dPop / dt = fertility rate * pop (birth rate) - deaths

Note the "current population" factor in that. As long as the number of births depends on the current population (i.e. anyone has a chance to give birth), this equation will result in an exponential curve. If the fertility rate is greater than replacement rate (that "deaths" term that I've handwaved away), the population will exponentially grow. If it's less than the replacement rate, the population will exponentially decay. But it is necessarily exponential, because it's a constant rate multiplied by the current population, and if you remember your calculus, an exponential is defined by a constant growth rate.

There's a whole subfield of population dynamics, and I remember lots and lots of predator/prey models, resource constraint models, etc. from my applied calculus class. Some of those do have non-exponential growth curves, eg. a basic 2-species predator/prey model gives rise to sinusoidal population curves as the increase in prey leads to an increase in predators, which results in more of the prey being killed and eaten, which results in a decrease of food, which results in the predators dying off. But what they all have in common is death rates that are proportional to some function of the current population. In other words, you have to kill off proportionally more of the current population based on how many people they are, something which would be ethically unacceptable to most humans. If you just have natural deaths (i.e. a death rate proportional to Pop(t - life expectancy)), you always get either exponential growth or exponential decay, because that's the way the math works.

I suppose there was one other population curve that gives a sigmoid function (i.e. a logistic curve that asymptotically approaches a limit but never reaches it). This is what you get when there is a natural resource limit to giving birth, i.e. when instead of births being fertility rate * total population, it's fertility rate * (exponentially decreasing fraction of the population). Many people find this scenario ethically challenging as well: in plain English, it means that only the elite can afford to have kids, or alternatively - cap the number of houses so that only the top X families can have a house and raise a child, and then create social stigma around having kids when you cannot afford those increasingly scarce markers of stability. It is disturbingly close to reality, though.

Right, but what most people mean by stable population is a population that doesn’t fluctuate outside of a range. A population that grows and shrinks but has some kind of negative feedback that causes it to oscillate around a value, is stable enough, even if the local behavior of the curve is exponential.

That sort of sinusoidally oscillating population is certainly possible, but be careful what you wish for. The negative feedback, as far as population is concerned, is "death". Before the exponentially-growing post-Industrial population, we'd have periodic wars/famines/plagues that result from the population of an area outgrowing its local carrying capacity. That's not generally considered a good thing.

Death is not the only thing to inevitably reduce population - there's also taxes. As in, give subsidies to childbearers in eras where the population is too low, and take them away when the population gets too high.

The problem with taxes & subsidies is that their discriminatory power is not as great as policy makers would like to believe. They tend to either not work at all, or they work too well and overshoot the target. (This is the issue with many other economic interventions as well.)

Economists like to pretend that everything is a continuous function of price, but this very rarely reflects real markets. Most utility functions are step functions; there is some point at which changing your behavior becomes economical, and above it everybody switches, while below it nobody switches. People can either afford to raise a child in the lifestyle they expect, or they can't - and many of the sacrifices in lifestyle don't move the needle in fundamental living costs. For having a child, that point appears to be in the hundreds of thousands of dollars. If you make childcare and college free and give 2 years of maternity leave and ensure that you can buy a 3BR+ house on an average income, then sure middle-class people will start having kids again. All the middle class people will start having kids again, because they all face the same economic constraints. And then you'll have fertility > 2 again, and the population will start exponentially growing.

You could have a large per-child subsidy and cap at 2, but this also is a lot harder to generate the desired effect than expected. Accidental pregnancies are a thing. Twins are a thing. Infertility is a thing. Careers are a thing. People who want to remain childfree regardless of cost are a thing. If you cap at 2, you'll get a fertility rate under 2 because of all these other considerations.

And this is what's playing out empirically across many countries right now. A lot of European countries have generous subsidies for childbearing; they still find that they can't raise the fertility rate above 1.5 or so. China lifted its one-child only policy, moving it to 2 children in 2016 and 3 children in 2021, and finds that fertility still collapsed (even after instituting subsidies) and they can't convince people to have babies. There are significant non-monetary costs to parenthood; it's a whole other lifestyle, and the money involved needs to be pretty large to alter that.

Luckily there are many fertility rates that are indistinguishable from "stable population" over a span of a couple generations, say 100 years or so. That means there is plenty of time for policy shifts to incentivize or disincentivize having children.

At current fertility rates, Europe's population is going to halve in 3 generations, and who remains is going to be quite old on average. I don't know your age, but if you're young enough this will happen within your lifetime. It's not a distant and abstract problem.

I am not accounting for immigration of course, but from the wider point of view, immigration is musical chairs. Sri Lanka is well below replacement and still falling fast. Who is going to immigrate there? Ditto for most other countries.

Fantastic explanation, better than I could have put into words. Thanks for filling in.

It’s nothing about agreement. Everywhere has declining birth rates once they hit a modern standard of living for at least a couple of generations. Not because people think fewer children are a good idea, but because having many children in modern world is unpleasant and very few want more than two.

I think it mostly has to do with increasing desires for a high standard of living when it becomes possible and the pressure of high density cities making lots of kids an unattractive prospect.

While we do observe a trend towards lower fertility rates as wealth/health/modernity increases, this isn’t universal. For example, there is a big gap between the fertility rates of religious and non-religious people in the arguably modern USA. It only takes one such group to carry on the tradition of exponential growth.

A shrinking population is fine, this planet has its limits.

A rapidly shrinking population is not ideal.

There is a lot of panic over shrinking population where people project it out 1000 years and find there are only like 5 people left in the world.

Steady state population should be the long term goal, at least until we start moving into orbital colonies or something. But unfortunately that's an anathema to growth oriented economies which makes it the bad guy in economics and politics.

At fertility rates of 1.6, it takes just 25 generations for the global population to go from 8 billion to 900k. Just 10 generations for the global population to drop below 1 billion. Pretty scary stuff.

assuming fertility rates stay constant, which they rarely have. 250 years is a lot of time for things to change. (and of course there's no need for fertility rates to grow to as high as they have been historically, due to the massive drop in child mortality).

I'm not convinced it's a self-correcting problem.

That's an extreme drop rate for the whole globe to have, and a very long time, so I'm not scared by it.

The population only went over 2 billion in 1927. Dropping from current numbers is fine, and less competition over space would be nice.

The issue isn't just the total number, it's the demography. The population is getting very old at a breakneck speed. I'm not sure how healthcare and pensions are supposed to be provided for old people when the tax base gets smaller and smaller every year. The end result is pretty terrible elder poverty.

Not to mention the changing political dynamics of having a lot of old people outnumber younger people by 2 or 3 times in the voting booth. The gerontocracy is bad enough now. Imagine it 3x worse.

> The population is getting very old at a breakneck speed.

https://www.indexmundi.com/graphs/population-pyramids/united...

Looks fine to me. What's your definition of "old" here? Are you specifically worried about Korea like you mentioned in another post?

I'd worry about the implications of a global 1.6 birthrate if they were actually going to happen. I don't think that's very likely, though. I don't know if we're even going to get down to 2.

Since the mid 1980s, the median age in Italy has gone from 34 to 46 and the trend is accelerating. This is despite massive immigration. Expect this to happen everywhere in the next 40 years. India and China are already below replacement. All of LatAm, Europe, and South East Asia as well.

Unless someone thinks of a democratic, scalable solution it's going to be a lot of pain for a lot of people for a long time.

This is exactly my point. People assume that human behavior won't change if the population starts shrinking noticeably, especially if that shrinkage results in reduced housing pressures.

Moving to orbital colonies won't save you in the long term if growth remains exponential. You can expand at most in a sphere growing at the speed of light so at O(t^3). Eventually the O(c^t) population growth will outpace the rate of expansion.

Projecting out exponential growth into the far future is great because it has happened exactly never in history. All growth is on an S curve. Always.

I mean Japan hasn't been doing great but it's been doing fine, so I don't think the end of growth has to mean anything but a period of stagnation. The economy will be fine.

In your lifetime, Korea will have more than two 60+ year olds for every working person. They are screwed.

Korea also has a cultural practice of extreme ageism. So much so that the elderly are frequently impoverished to the point of food insecurity and committing suicide by jumping from bridges.