For most of human history a desire for sex was quite sufficient to ensure reproduction even without as much desire for children. With the advent of birth control the environment changed and there are new selective pressures. Also the modern world has a lot of new, competing, sources of joy.

R strategies certainly don't always beat K strategies! But humans are currently in a very rare situation where our population seems to be very far below K and most people in wealthy countries aren't living anywhere near subsistence level. So, until Malthus rears his head again, evolution is going to be paying a lot more attention to r.

For those following along at home, for N as the number of individuals:

  dN/dt = rN(1 - N/K)

There are genetic, dietary and lifestyle factors. Genetically and historically, people were used to famines and lower availability of food. So the body evolved to store up whenever food consumption increases. This applies to large parts of the population in Africa too.

With affluence and affordability improved drastically in the last few decades, people tend to eat more and richer food, which forces the body to store as fat. This triggers Type 2 diabetes, hypertension and heart disease. Diet wise, the consumption of dairy is quite high (not as high as some developed countries though) - significantly higher than consumption in the past. This combined with genetics makes things worse, and a lot of lacto-vegetarians also tend to suffer from the lifestyle diseases mentioned above.

I really wish we had more genomic data from across the world.

2. the heat indices of that country can be quite astonishing

http://www.infoplease.com/ipa/A0884238.html shows avg household size has been declining since before contraception, and it doesn't look like contraception changed the slope.

And introduces pension problems for pension systems that are based on an inter-generational contract.

> A more robust, sustainable solution is to improve productivity via other means (including automation).

Sure, that would be nice. It's also completely impossible. Productivity growth has never, ever, ever grown fast enough to bail us out of the hole we're in now, and we have zero (repeat, zero) ideas on how we could possibly change that.

Productivity growth will not save us.

That's how retirement works in most countries. The tax burden mostly falls on the "productive age" (18-65), so until automation pays as much taxes as the humans it replaces (corporate tax dodging considered), retirees in countries with shrinking populations are screwed.

Countries with falling populations will just need more robots. Mass immigration isn't going to help, but this is one of those problems easily solved with tech.

Except: Productivity growth has historically never been high enough to bail us out of the hole we're in, and it has been trending down in recent years for reasons we don't fully understand.

> our material abundance isn't particularly under threat from contraction of the work force.

I wish!

- don't use toilet paper - don't drink milk - don't travel more than a few miles from their birthplace

etc etc, and that will be shifting quite quickly, I believe. It took centuries for industrialisation to get e.g. Europe past such reference points of wealth, after two hundred thousand years of most of humanity living roughly without significant improvements in wealth. But it only took a few decades for e.g. China to lift hundreds of millions out of poverty and towards some form of middle-class.

You can debate some of the details for sure, my overal point is that we're not just going to add a few billion people to this planet, more importantly, we're going to add a few billion middle-class people to this planet. It's not the billions who live off of subsistence farming, who's impact on the environment is not all that much more substantial than any other animal grazing in a field, that is the big environmental issue. It's the fact that those people become more like us, drive cars, run refrigerators, eat energy-inefficient meats, have a shower in the morning and evening, live in large homes that must be cooled and warmed, eat more than necessary etc.

If you look at some figures that state we consume 10x more energy (making up a number here) than a subsistence farmer... it's more the economic shift from poor to rich we may need to worry about from an environmental, than raw additions to our global population.

(disclaimer: not implying I have anything against poor people getting richer, purely looking at it from an environmental perspective. Also, there's opportunities in a richer planet to build environmentally sustainable infrastructure, too, but on aggregate it'll bring some big issues to the table, far bigger than adding 1.6 billion poor people to our population, I think.)

The (human) carrying capacity of Earth was estimated to be around 2 billion, which was surpased sometime in the late 1920's. Contrary to other comments here, that does not mean that after you hit population 2,000,000,0001 we all die (we clearly did not). Instead, what it means is that given the technological level we had at that time, we'd consume renewable resourses faster than they can renew themselves, and we'd also produce waste faster than the environment can degrade it. Otherwise, more than 2B people would cause environmental degradation, which would itself reduce the carrying capacity in the long term.

Please note this definition is tied with humans technological level. It is not set in stone, since we have some degree of control over our impact in the environment, and we have the ability to use the same resources in a more efficient way. The big tragedy of 20th century is that this fact was not recognized but for a handful of theorists, and therefore it was not a political and economic goal to explicitly manage the carrying capacity of Earth. As of 2016, the situation is still the same.

By example, we gained a bunch of technologies that allows us to do the same stuff more efficiently. Given explicit economic incentives, we might have... maybe doubled our carrying capacity (CC=4B). Unfortunatelly, because this was not a goal itself, we engaged in a buch of economic practices that negated much of this benefits, so if we are generous these might have been reduced by half (e.g. CC=3B). Also given that population growth was not arrested back in the 1970's, but only slowed down, the carrying capacity has not improved at all (CC=2B).

Currently, environmental degradation is going in overdrive. We have lost a lot of time, and the resources we need to make an orderly transition are already commited to keep the system going. Population will go down, one way or the other. I don't believe in a single sharp die off many apocalyptic thinkers profetize, but adding and extra 1.6B mouths to feed will make the downward tendency of the curve more steeper than it needs to be.

I am reminded of an old joke:

"A man jumped from the 10th story and is falling to the ground. A woman at the 4th floor sees him from her window and asks, 'Hey, how's it going?'. The man replies 'So far, so good.'"

Yes, technology has saved us in the past. Given that new discoveries are increasingly harder and more expensive to achieve, will technology continue to do so in the future...?

Just please note, aeronautics does not defy gravity. It makes use of knowledge in mechanics of fluids to produce vouyancy, which ultimately is a manifestation of gravity.

To be more clear, infinite knowledge does not allow you achieve any arbitrary goal you care to imagine. It informs you on what means can be used to achieve the subset of goals that are feasible to begin with.

Throwing arbitrarily large amounts of computing power to the same problem may as well result in a flood of responses: "No. Nice idea, but it has been tried before N times, last time at T. It never worked because of X, Y, and Z"; "No. Nice idea, but it has been tried before N+1..."

and

https://en.wikipedia.org/wiki/Total_fertility_rate