Is it plausible that humanity goes extinct due to not making enough children? If not, what would make it more plausible?

With current technology, no. I have nothing to add to Mark's answer regarding why.

Here go two possible technology change scenarios that could bring that about.

1. Our robot overlords won't let it.

Machines have taken over. Either as a compromise or as plain mercy, they'll allow humans to live - possibly with a lot of luxury and a lot of freedom - but no reproductive freedom. The "problem" will take care of itself in due time, even if lifespan was greatly increased.

2. Entertainment got way better, child care is still very hard

Advances in technology made video games, food, travel, TV, social networks, and Stack Exchange much more appealing than today. Not to mention the new entertainment options that came about later.

Child care is still basically the same. No robot will change a diaper.

Humans nature has limits to the super-stimuli it can resist, and, referring to Mark's answer again, the Mormons could go the way of many Catholics and just succumb to their own preferences, failing to follow their religion in that particular aspect. If the change is strong enough, no human subculture will remain with a fertility rate beyond replacement. I don't think a lack of propaganda would be plausible, but individual preferences can still overcome it.

As a matter of fact, yes.

A recent study published in The Lancet, and reported upon by the BBC, projects that the global birth rate will soon drop below 2.1 - the "replacement rate" at which births outnumber deaths - and fall to 1.7 by 2100. The result is that global population will peak at just under 10 billion and then begin to decline, with 23 countries projected to lose at least half their current population by 2100.

Professor Christopher Murray, the director of the institute behind the research, was quoted as stating:

I find people laugh it off; they can't imagine it could be true, they think women will just decide to have more kids.

If you can't [find a solution] then eventually the species disappears, but that's a few centuries away.

It's worth noting that in reality, most of those countries projected to be worst-affected are not only aware of the issue but actively fighting to reverse it, including with propaganda (Japan's efforts have actually become a meme). The first BBC article notes a recent upswing in Sweden's birth rate, for example.

However, you've stated that in your world, this isn't the case, and humanity remains blissfully unaware of the declining birth rate. If they remain that way until it's too late, then yes: humanity could genuinely die out. It'll just take a while.

Genetic engineering would be one cause. We aren't too far for being in control of our evolution and while the obvious removal of genetic diseases from our gene pool would probably be the first thing we did, it's not the only way we could make our lives easier. And if genetically engineered offspring became the norm, we might eventually just choose to become biologically, but not medically, infertile. Especially since the menstrual cycle is not a biological feature you would want to have if you could live without it.

Now how would this lead into extinction? We would have to lose the technology somehow or access to it would have to be limited. Wars or natural disasters would work as possible limiting factors.

Now the beauty in this idea is that infertile humans wouldn't only knock themselves out of the reproductive cycle, they would also limit their partners. That's if monogamous relationships were still a thing.

I think a remote colony could die out because of lack of kids, even though they don't have issues with resources or dangers.

For example, a colony on Mars or the outer system if that's not remote enough misses out on prime child-bearing years because they are pressured to work on building and maintaining the habitat, doing science, and whatever-they are persuing careers not having families. Meanwhile, economic decline leaves them more isolated than intended, with no fresh colonists.

Very few youngsters eventually have fewer kids: why, if they are doing OK? Maybe they are still waiting too long, not worried or in denial about a problem, as mail-order brides will be coming Next Year For Sure.

Or, they do try and breed and increase the number in the next generation, but the gene pool is too shalow. How would that show? As disease that affects everyone, or genetic flaw that becomes prevelent. But then the primary cause of dying out would be blamed on the disease, only secondly explained as being possible due to lack of diversity.

A sufficiently scattered human population could make this more plausible. Having millions of humans alive isn't helpful if they are scattered around in groups of a few hundred each and are unable to intermingle. So if something were to happen that would prevent humanity from traveling freely between many small settlements (which is no small feat if the settlements are on different planets around different stars), that could have disastrous effects.

And shrinking populations could have issues beyond genetic diversity. As the population ages, the ratio of working people vs retirees (assuming they retire much like we do of course) will fall, so a smaller group of youngsters suddenly have to support a relatively large elderly population. This would suck many resources out of many more luxury pursuits, potentially including maintaining the infrastructure necessary for interstellar travel.

Not in real life - @Mark has right answer. All sci-fi around this concept are just that: a sci-fi novel to get some interesting setting, not to investigate a probable outcome.

That said: Yes, in a sci-fi, because but it would involve some extraordinary intervention. Like virus making women sterile, targeting only women, or making the birth fatal. And in this situation, focus of all humanity, all resources, would be focused on solving the problem: curing the virus, or developing artificial womb.

Virus able to kill women selectively is the fastest solution - you need to find a cure withing a single generation, or the game is over. I recall such sci-fi novel (with world wars to protect enclaves with uninfected women).

Making the birth fatal is the most interesting sci-fi setting, assuming you need say 100 years to develop solution. Any aspiring sci-fi writers: this one is for you! And please let me know! :-)

Question: Is it conceivable (with the aggravating factors proposed below) to see this tendency become global in the future, and the population ending up decreasing up to extinction level?

No. Not because of the factors you mention, anyway, specifically:

• Humans go live on a range of planet around close stars, thus the population becomes more scattered.

I don't see how this would have any meaningful effect at all on human population on Earth, except for the reduction on Earth of people who leave. So unless Earth becomes a horrible place to live, so that most people want to leave Earth, I don't see this being able to endanger the survival of Earth's human population. That's quite possible, but that would be a factor you did not mention. The populations on other colonies might be in trouble from conditions there, but that too would be a problem you did not mention.

• There is little to no awareness of the matter, nor any propaganda about making more children.

This isn't plausible because already humans have awareness and propaganda about the lower birth rate for more highly educated women, generally for economic/workforce/healthcare worries, but not for extinction worries.

With the increase in living standards, the number of children by woman is decreasing in highly developed countries.

I tend to think this has less to do with "living standards" and more to do with higher levels of education, and more choices for women in terms of professions, ability to support themselves, access to reproductive rights and services (birth control, abortion, marriage rights).

So, the issue you mention, which is that women tend to have fewer children when they have more education and choices, is something humans have been studying, but is in no danger of resulting in human population evaporating simply do to that trend, and certainly not without humans noticing it.

Most of the many actual threats to human survival have to do with the opposite problem, of increasing human population, leading to disasters from us destroying our planet's ability to support life through non-human habitat loss, non-human extinctions, climate change, pollution, WMD proliferation, pollution, clean water use, GMO pollution, nanotechnology, overfishing, ocean acidification, etc. We'd be better off with a declining population, and would have a much easier time surviving as a species and a planet if we lowered our population even to 1% of what it now is.

Industrial food consumption and other modern life conditions may lower fertility, and some future accident might make that situation far worse. But again, that's also not what you were asking about.

However, I suppose if we consider a future where humans overcome our current problems and survive long, long into the future and become very civilized in some form that has a situation where the fertile women are choosing not to have children very often, and perhaps that far-future civilization has become disinterested in maintaining its population levels, I could see it being a kind of issue, but I think attitudes towards reproduction and extinction would need to change, and/or some other factor such as greatly reduced fertility, before this cause would threaten humans with extinction.

The biggest problem might actually come from humans extending their life expectancy to hundreds of years. If we lived for a very long time, we would generally reduce the number of children we had (or at least try to spread them out over a long period). Otherwise we could have 30 generations living at the same time.

What might happen as people postpone having children until later, they might die before procreating. Another problem, might show up in the human reproductive system might not survive hundreds of years, so too much postponing and you might have a near immortal group of humans who can't procreate.

No. The current trend of having less children (than our grandparents did) is tending to less, but not tending to zero. People like having children - well, most of them do. People are fine having as many children as they can care for and still survive. Our grandparents had no reliable form of contraception and children could be an asset as unpaid labourers.

In (what I shall lazily define as) modern Western culture, people want children, but they also want everything else. This results in the 1 - 3 child household so common these days. In countries moving up from 3rd world status the number of children per family declines but is not zero.

Anecdotally I would say, if there were more programmes supporting young parents, people like me and my neighbours would have had one more, maybe, but not 6.

Its interesting that you as that because it is already happening on earth as far as the Western world is concerned.

It turns out that you need a birth rate of about 2.1 to break even - you have to account for sterility, accidents, homosexuality etc. The West and those other groups close to the west such as Singapore Japan and Korea are running between 1.5 and 1.8 depending on the cycle as a consequence of feminism, contraception and abortion.

To make the math easy consider it is 1.5. That would be 8 grandparents, 6 Children and 4.5 grandchildren. So in the time between grandparents and grandchildren - lets say 50 years you have a halving.

Now I am lowballing the figures here but certainly you would see a halving in less than a century.

In the west at the moment we are about to see all the baby boomers die - born around 1947 they are about 65 years old now. These guys will drop off the twig fast once they hit seventy. I saw it happen with the WW1 guys and WW2 guys. One minute they were there then poof! gone.

When that happens the demographics of the whole western world will shift dramatically.

So yes a population could certainly become extinct - but would also be displaced by more fertile groups just as we are seeing at the moment.

It puts a whole different spin on classical "Moral" arguments - contraception, abortion, homosexuality etc - but that would be begging to go off topic....

Many thanks to all of you! I think you all have good points, and the real answer to my question would be a combination of these.

No: Economic or climatic (lack of ressources) pressures couldn't do the trick
As @WorldSpinner and @Vandroiy point out above, these types of pressure can only force a population drop for a limited amount of time. Indeed when the humans are less numerous, then the pressure fades and restrained fertility rise again.
As the population is spread on a lot of planets, the risk for a great catastrophe to kill enough humans at once to get passed the minimal amount for specie perpetuation is also really unlikely.

No: It's all about culture!
As @Mark remarks in his answer, even if the mainstream culture discourages having lots of children, in time it would be replaced by the subcultures with higher fertility rates.
Even if global human fertility is decreasing, (for w/e reason) the individuals from the fertile subcultures willing to have children will try everything in order to do so, and probably succeed thanks to advanced science. (in vitro fecondation and so on) As @Vincent points out, individuals mostly do see their interest before such things as their specie's destiny.

Yes: It would require intervention of a greater institution or entity
As @WorldSpinner and @Vandroiy emphasise, the only way to go past these barriers is to postulate that an extremely well informed and patient institution or entity works in the shadows to lower the overall fertility (using viruses or chemicals) and break the most fertile subcultures. (using manipulation)
I think we proved here that this is necessary as else evolution or fertile subcultures (that we can see as a social mutation towards specie surviving) would always take over.

Alternative possibility: coincidences
The only other way is if an extremely unlucky chain of coincidences happens. For example first the economic pressure makes couples less eager to procreate, then a few catastrophic events dynamite the most fertile subcultures, while a very hardcore sect takes over...
With the dramatic combination of all these economic, climatic and cultural factors, humanity could end up being a very infertile and fanatic small group of individuals.

Short answer: No, it's not plausible for humans as we know them.

You cannot look at the population as a while when determining the long-term growth curve. Ideally, you would look at individual fertility, but that's neither practical (at the present) or possible (for the future). Instead, you need to look at factors that influence individual fertility, and (on average) the strongest of these is the local cultural expectations. So, you project fertility on a culture-by-culture basis, along with factors (such as wealth) that change cultural expectations, to get the future population curve.

For example, consider a population that, as a whole, has a fertility rate of 1 (half the replacement rate), while a small subculture has a fertility rate of 4 (twice the replacement rate). Fast-forward a few centuries, and the "small subculture" is now the dominant culture, and the population fertility rate is approaching 4.

There are a number of cultures on Earth with high fertility rates. Probably the most prominent are the Mormons, with a strong religious imperative towards large families, and thus a strong resistance to the normal influences towards smaller family size.

In your future scenario, if you go away for a thousand years or so, you probably won't come back to a dying humanity. More likely, you'll come back to a rapidly expanding one, driven by the need for new homes for its ever-growing population.

It's All About Fertility!

The possibility of humanity's population reaching near extinction levels from economic pressures alone seems infeasible. As economic pressures reduce birthing rates, our population will stabilize and then begin to reduce. As it reduces the economic pressures will subside and birthing rates will rise and the population will increase. This cycle should oscillate and an equilibrium population would be reached, according to basic economic principles.

There are several scenarios in which reduced fertility could cause humanity's population could decline to extinction levels and be masked by economic explanations. The economic explanations for humanity's declining populations might just be an Occam's razor rationalization of an unrecognized, systemic and growing fertility problem. I will lay out a few plausible examples in order of descending probability:

The Natural Selection by Economic Pressures:

Perhaps the economic pressures of raising a child of high reproductive value create a fitness criterion for lowered fertility or libido. If such a fitness criterion were persistent though several generations, humanity may evolve a biologically lower fertility rate. Once the economic pressures which induced the fitness criterion of lowered fertility have subsided, it may be difficult for humanity to revert back to a higher fertility rate. One such reason is that with a lowered fertility rate, evolution occurs at a slower pace because fewer births mean that there is less chance for variation in each generation. It may be conceivable that if not reverted quickly enough, humanity passes an evolutionary tipping point, past which we cannot evolve a higher fertility rate before reaching extinction levels.

The Folly of Man:

What if mankind, by accident or malice of forethought, introduced a fertility reducing agent to our species. There are many possibilities such as weaponized bacteria or viruses, synthetic chemicals inducing an auto-immune response, genetic modification of our food supply all causing reduced fertility.

It may go unnoticed for a few decades before economists & actuaries denote that the population is declining greater then expected. Identifying the agent, eliminating it's exposure to our species, and remedying the damaged caused may be millennia long, Herculean task that brings humanity's population to near extinction levels.

The Extraterrestrial Soft Kill:

Suppose an extraterrestrial species desires earth for reasons unknown. They could fling some asteroids from our kaper belt at earth, decimating human life at the expense of the rest of the biosphere. That's easy for an interstellar species, but what if they don't want to destroy other life on earth? They could attack in a more conventional warfare to selectively target humanity and preserve the biosphere, but they would likely suffer some casualties and humanity may adapt to the foreign threat.

A better way for an extraterrestrial species to kill humanity would be to introduce a fertility reducing agent to our species. Then they need to only wait a few centuries as our population declines. They could attack us via conventionally warfare when our species is militarily weak from generations declining population or just wait for us to die out naturally.

Yes. At least in a science-fiction setting, you can construct a plausible scenario where this might happen.

If there is one child per couple on average, the population will halve with every generation. This would result in a long, slow decline. For example, after 14 generations (say, about 420 years), the Earth's current population of 7 billion would have fallen by a factor of 2^14 = 16384, to less than 500,000. If the birth rate is closer to replacement level, the decline will be even slower.

It is possible that no individual generation would see the fall in population as an urgent problem, especially if robot servants were able to make up for shortages in labour.

This scenario forms the background to the novel Saturn's Children, by Charles Stross, in which humanity is (possibly) extinct and a robot civilization occupies the solar system. Some of Asimov's robot novels explore similar ideas, notably The Naked Sun and The Robots of Dawn.

The animated series Futurama has humorously observed that if you want human beings to have children and perpetuate the species, an important rule is: Don't Date Robots. A clip illustrating the dangers is here: http://vimeo.com/12915013

There is little to no awareness of the matter, nor any propaganda about making more children.

I don't think that part is plausible unless demographers and economists are not doing their job properly. Most developed countries today are under the replacement rate of 2,1 children per women and they are aware of this. The increase of life expectancy or what we can also call the aging of the population contributes in maintaining and even increasing the population on the short term. But the median age is increasing. It's probably over 40 years old in most developed countries.

Measures to counter the problem:

• Some countries, if not all will try to attract immigrants to lessen the demographic problem. In some case, it's not enough or it's just a temporary measure. In Germany, on the long run, the immigration rate will have to increase in order to keep the same population level. But then the local population will be flooded with strangers and it's not going to work well.
• Countries can also adopt birth policies to increase the natural number of birth. Most already have but it's not always enough. China decided to abandon the one child policy but the birth rate remained stagnant in the urban areas. It's because people do not feel comfortable enough financially to have a second child. I have to say that the inflation rate is fairly high in China and predicting the economic future is no easy task. And China doesn't have a lot of measure to help young families like family leave or daycare centres.

Is it conceivable (with the aggravating factors proposed below) to see this tendency become global in the future, and the population ending up decreasing up to extinction level?

It's already becoming global. In almost all the countries, the birth rate is decreasing. Most countries in Latin America are close to the replacement rate and so does other counties like Thailand or South Korea. Other countries are already facing a decline in population. Most are located in Eastern Europe. We know that this is coming and yet, it's is something that is very difficult to stop. Just like the climate changes.

Does it mean it will lead to the end of mankind?

That is very hard to tell. We would need to predict the future. Some said that reducing the population could be a good thing. On the long term, the living conditions could improve even in industrial countries. More resources per capita, more space, less pollution... Good standard of living could increase birth rate enough to keep the population stable but at a lower level.

On the other hand, a society affected by a problem like the aging of it's population is under a huge economic strain. For example, if the number of children per women is only 1, it mean that each new generation will be half as numerous that the older one. The gap might be hard to fill and the young will have to take care of the old. This is likely to become impossible if things get really bad and will lower the life expectancy. Eventually, it will reach something close the the age of retirement, around 50 or 60, like it was before WW2. Before 1945, people worked almost all their live and many did not have the luxury of retirement. If this happens, we could get rid of the economic problem.

But would people want to have more children, I'm not sure. They do not think at the survival of the species when they consider having or not having a child, only at their own survival.

No. Not under any conditions I can imagine occurring at random. (Excluding the case where a lack of resources, or competition for them, renders children unaffordable.)

This follows from basic evolution: if there exists any trait that randomly occurs, increases the tendency to have children, and can be passed to children via genes or cultural memes, it will expand exponentially.

It is highly unlikely that no such trait would exist.

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Plausible explanations for the outcome described in the question are extremely hard to find, because all evolutionary mechanisms will work against you. If there is the tiniest loop-hole to the conditions -- anything culture or genes, amplified by the use of intelligence, can exploit -- it will be increasingly be used against the conditions, with disrespect of the things we normally value in life. Remember, everyone who goes with the flow dies; after every generation, only the children of the previous generation will be left, the others are gone from the gene pool. This is a very very strong force to drive evolution.

Finding conditions for this to work in a large, heterogeneous population is probably too hard. The most plausible way to get there I can think of is a highly intelligent entity that is systematically detecting and removing groups of people that go against the desired outcome. Otherwise, there would have to be a reason that only few people are alive at the onset of this, and that they are fairly homogeneous, so that they all fall into the same trap.