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Let's say a new science-based cult takes over the world and convinces the vast majority of humans that 10 Billion is an unsustainable number of people for our planet. They want to reduce the number to 2 Billion. There are no dead-lines, as some meager sustenance is enabled by science. However, most people see the point and are committed to making this change.

Immediately, I am thinking about the Chinese "one child" policy, which caused some problems:
a. Small number of young people need to take case of a big number of ageing people
b. Having only one child pushes people to want a boy, reducing the females born and causing social problems

If you were a population engineer, how would you attenuate human reproduction? What do you reckon would be the major problems that might arise and how do you suggest to prevent/treat them?

One thing on my mind is robots: they are already replacing people and enabling a less human-dependent economy.

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I like that you say majority, because there are definitely groups of people who wouldn't support this kind of action, and these groups are normally ignored on world building SE. – XandarTheZenon Feb 8 at 14:33
    
Oh, I must ask a clarifying question. Do you also want to maintain governmental viability? As in, the US doesn't becom a communist nation to enforce depopulation? (I'm sorry, but I know that would necessarily invalidate a few answers.) Because I think that is very relevant to your question, since social, cultural, and economical aspects of nations are closely related to the type of government you have. – XandarTheZenon Feb 8 at 14:51
    
@XandarTheZenon, I think a change like that necessitates that the people accept it. Hence, depopulation does not need to be enforced through fascism. Let's say the motivation to reproduce excessively is as common as the motivation to rape or perform major fraud. Nothing is hermetic. By government and cultural viability - I mean enough of it remains functional to provide the things we are used to: police, waste management, arts, celebrations. I think if things go too fast too soon, humanity might enter a survival mode, where governments will be dethroned and culture will be very low priority – DraxDomax Feb 8 at 15:18
    
So, that'll do. You want governments to remain almost entirely the same, possibly with a little bit of encouragement towards depopulation. Watching out for religions that believe in large families, such as the LDS church. – XandarTheZenon Feb 8 at 15:53

10 Answers 10

If there are no deadlines, a two children policy will work and prevent both issues you mentioned. With about 2.3 children per mother needed for a stable population, you would have a steady decline.

As shown by all of our developed countries, a low natural birthrate can also be achieved without strict limits, but a number of conditions need to be in place:

  1. A social welfare/safety system has to be in place, so that people are not dependent on having (enough) children to care for them in old age.
  2. Living conditions (food and water especially), health care and security need to be good enough that almost all children survive to adulthood, so no spares are needed.
  3. Raising successful children has to be a costly investment. Outlawing of child labor and the increasing need for highly educated people have achieved this in developed countries, to the point where some couples feel they cannot even afford to raise one child (Hi Japan!).

In short, if having few or no children is an economically viable and "safe" choice, enough people will choose that over breeding like rabbits to lower the birthrate below the replacement rate.

While robots and automation will inevitably promote the third condition by making unskilled labor obsolete, the major problems will be political in nature: How to fairly distribute resources and ensure and end to poverty and war, because those are the main drivers for people to have many children.

Consequences of a declining population

A population that will be "graying" for many generations also needs to get serious about pensions and housing/city-building. Each generation would need to set aside enough to care for itself in old age, to not put an undue burden on the next, smaller generations. This requires long-term planning and commitments, neither being a strong suit of our current democracies, but hopefully your technocrats are better at it.

As the population decreases, many cities will shrink or be abandoned. However, it won't be a move back to villages and rural areas. It will be too expensive to service a widely distributed population with all the infrastructure and technology we have come to expect in our daily lives. The high level of education and specialization needed are also hard to achieve in thinly populated areas.

So the bulk of the remaining population will gather in the most attractive cities, turning them into vibrant developing metropolises. All supporting agriculture and industry will be relocated nearby and the cities will be connected by air or high-speed mass transit without intermediate stops.

Most or all infrastructure criss-crossing the land as well as ghost cities would be abandoned to nature, as the cost to maintain them would be way too high.

A small percentage of the population would choose to live in small independent communities that use solar/wind power and 3D printing to maintain some capacity for technological production, but would have to fly/drive for hours/days to reach a city if they need advanced tech or medical care.

Several other answers refer to the decline in arts, science and general development because there are less "geniuses" born. This may happen, but nothing approaching an 80% drop. A large percentage of "geniuses" born among the 10 billion will be born into poverty and never realize their potential, because there simple aren't enough resources to go around. A population of 2 billion with universal education and social security would produce a number of realized "geniuses" not too much lower.

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Note: from 10B to 2B is not much of a drastic change actually, if you look at how the World population is spread today (US and EU ~350M each vs China and India > 1B each), the change could be nearly unnoticed in developed countries (already accustomed to a low birth rate) and completely life-changing in the newly developed countries. – Matthieu M. Feb 8 at 16:05
    
@MatthieuM. You got US population about right, but the EU is much bigger - at 500M EU proper, and 750M in Europe (including non-EU countries) – Ordous Feb 8 at 16:43
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@MatthieuM. This is only true in a naive sense. India and China have over 1B, so if they 'vanished' you wouldn't immediately notice in the US, but then China and India would not be able to produce massive amounts of cheap labor. That would be noticed. – David Grinberg Feb 8 at 21:52

Effects that occur to me:

One: As yo mention -- so let's deal with that right off the bat -- in many cultures it is advantageous to have sons. In China, the custom is that your sons provide for you when you get old. Your daughter doesn't: she'll be working to support her husband's parents, not her own. So if you don't have a son, you can never retire. You have to work until the day you die. In India, the parents of a girl are expected to pay a large dowry to her husband and his family when she marries. I think it's much less of an issue in Western cultures, but still, fathers often want to have a son to carry on the family business. In general, it is sons who carry on the family name. Etc.

So if you're going to make this work, you have to someone bring about social changes so that baby girls are equally desirable. It's easy to describe social changes that would make this work. Getting people to actually accept them is another story. And, I might add, it's easy for some technocrat sitting in his office to dream up social or economic change that he thinks will be an improvement. But putting it into practice often brings up all sorts of issues that he never considered.

Two: You also mentioned supporting the elderly. Yup, that's another problem. In the U.S., social security was originally designed when there were, well, in a quick search I couldn't find the numbers when it started, but in the 1950s there were 16 people paying in for every 1 receiving benefits. Today it's less than 3 to 1 and falling. Taxes to support it have risen from 2% to 12.4%. With a declining population, you'd have to either force people to retire at older ages or increase taxes even more. At some point the burden on the young becomes unsustainable. This of course applies whether you rely on a system like social security, the young person directly supporting his parents, or any other system. The money to support the retirees has to come from somewhere.

Three: Economic diversity. As technology advances, people become more specialized. A couple of thousand years ago most people were very literally self-sufficient. Most people were farmers, and a farmer built his own house, made his own tools, then used those tools to grow his own food, etc. But as the economy becomes more complex, people specialize. I develop software for a living. I have never made more than minimal efforts to grow my own food -- a few attempts at a vegetable garden -- and those have been total failures. I have no idea how to build a house, or 90% of the other things I use every day. Even farmers today aren't self-sufficient. They don't build their own tractors and drill for oil to run them, etc.

If the population is shrinking, it becomes more difficult for people to specialize. Sure, there are enough store clerks in the world that if population was cut by 80%, we could say fine, there will now be 80% fewer store clerks. But some jobs are specialized enough that there aren't millions of them in the world. If we cut population by 80%, can we just cut the number of coal slurry pipeline workers by 80%, and have the system still work? The number of agronomists? Etc.

Three: Some things only work on a sufficiently large scale. If the population was reduced by 80%, presumably we wouldn't need as many roads. But could we abandon 80% of the roads? Not if we still want to be able to travel around like we do today. Could we abandon 80% of the cell phone towers? Not without losing coverage to large areas. Etc.

Four: How do you manage the depopulation? I live near Detroit, a city that has lost 60% of its population since its peak in the 1950s. This has caused huge problems. For example, there are now neighborhoods that used to have hundreds of people, but now only 1 or 2 houses are still occupied. How do you continue to provide utilities to those 1 or 2 families? Miles of water pipe and power lines and phone lines were built with the intent that they would support hundreds of customers. Now the maintenance cost is almost the same but they are supplying 1. Who pays for that? In some cases the city has paid people to move out of abandoned areas just so that they can shut off the utilities and stop maintaining the infrastructure. People find themselves living in areas where all the houses around them are abandoned and slowly decaying. Crime is rising. Wild dogs and other animals are becoming a problem. Etc.

Five: Science and technology advance through the contributions of a small number of geniuses. But we all benefit. For example, the microwave oven was basically invented by one person, Percy Spencer. I don't know how many microwave ovens have been produced since then, but say it was one billion. How many people would it have taken to invent it if two billion had been made? Or only one million? Clearly the answer is, Still just one inventor. It doesn't matter how many people benefit from his invention, the work and creativity to invent it are the same. If you double the population, then all else being equal you should have twice as many inventors. But the benefit to the average person for each invention is not halved because there are twice as many people. It's the same. So the more scientists and inventors, the more we all benefit. Cut the population by 80%, and presumably there will be 80% fewer scientists and inventors. Scientific and technological progress will slow by 80%.

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Re "have to either force people to retire at older ages", you really need to understand than not everyone wants to retire at 60-something even now, but are effectively forced to by actual (e.g. airline pilots) or de facto age discrimination. Since for most of us increased longevity also means better health at whatever age, there is no fundamental reason why 85 shouldn't be the new 65. (Do a search on "60 is the new 40", for instance.) – jamesqf Feb 8 at 19:09
    
"60 is new 40" for white collar workers. Not so for blue collars and some other jobs. 60 years old firefighter or coal miner? – Peter Masiar Feb 8 at 19:40
    
"60 is the new 40" Sure, people staying healthier longer allows them to work longer. That could help. But I doubt it solves the problem. Yes, some are forced to retire at 65. But plenty of people retire earlier than that, or would if they could. Lots of people in their 20s and 30s say that they'll never retire, they couldn't imagine sitting around doing nothing. I said that when I was 30. Now I'm in my 50s and counting down days to retirement. – Jay Feb 8 at 20:50
    
@PeterMasiar 60 year old football player? 60 year old exotic dancer? Probably not. – Jay Feb 8 at 20:50
    
Re the specialization: what if you not only cut the number of store clerks by 80%, but also the least-profitable (say) 10% of remaining stores go out of business, and those people get highly-specialized jobs instead? (Doesn't have to be the exact same people, just the same number of people) – immibis Feb 9 at 9:30

If timescale isn't that much of a problem then you can effect a 'by percentage' sterility lottery. Essentially you take 1 of every x children from each gender and sterilise them at birth. It's a bit draconian, but neatly sidesteps a few of the issues as you can then tailor the sterilisation rate to balance the need for young workers etc. If you also let it be known that the sterilisation rate will be balanced to the male/female ratios then people will not want to force one gender or the other, as an overabundance of one gender will lead to that gender being sterilised more often (which isn't ideal if you want to pass your genes on).

Employing Population Engineers to watch over the census of any given area and tweak the lottery rates accordingly would be a very sound strategy, requiring some bureaucracy to support it but not needing any form of 'enforcement' to take punitive action on couples having children illegally.

If a particular person proves that they should have their genes persisted then options can still be open for providing surrogacy/IVF treatments, and adoption or surrogacy are still options available for parents with no option for natural childbirth. Both of these things can be presided over by licenced Population Engineers to ensure that no particular area is exceeding the population constraints.

Once you're down to your ideal population you can still use this method to control the population, albeit with a greatly reduced chance of being picked in the lottery.

One last note: In this society nobody would ever want to get the Powerball.

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"... would be a very sound strategy" - Until you get sterilization gerrymandering. – immibis Feb 9 at 9:31
    
@immibis: Rule of Government No 4. : There is no policy that survives contact with live politicians. – Joe Bloggs Feb 9 at 9:32

So far Cyrus has given an outstanding answer, and current trends seem to support the idea that depopulation will happen more or less on it own as human societies become wealthier.

The real problem isn't so much how this can happen (already answered), or if we can maintain our economy (robots and automation are already increasing productivity with high capital as opposed to labour inputs), but how we maintain our society.

Society and culture are products of social and mental interactions among and between people. More people and more interactions mean more ideas, access to more resources in the form of social capital and larger markets to try ideas and reap rewards for successful ones.

For example, few people have all the skills needed to become successful in creating a business. Many people have ideas, but little understanding about finance, management or production. In a lot of cases, would be business tycoons can't get started unless they are introduced to someone else who has a complimentary skill, or is willing to invest capital into the idea. In fact, it is most likely that you need to assemble at least three people to really get a business off the ground to ensure you have enough skill sets. Smaller social networks will ale that increasingly difficult.

Now you have your business up and running, you need to sell your product or service. In a declining market with smaller populations, there will be fewer people to sell to (and probably more competition for the remaining market). Fringe ideas which can find niche markets today will have an even smaller slice of the market in this future, which is smaller in absolute terms as well. I can also envision people and groups trying to maintain their power by colluding or using governmental regulations to raise barriers to entry and deny new products, services and business access to the markets. In today's world, the City of London Ontario has onerous restrictions of "food truck" vendors, including such frivolities as ordering the vendors to have GPS trackers on their trucks. The net result is no one wants to apply for a licence, existing restaurant chains are protected from competition and you can't suddenly decide to buy a hot dog as you walk down a street. This also means for each truck there are also dozens of potential jobs never created for supply, service, advertising and other support for the truck owner. Now expand this to a global scale.

This stifling effect on business will also gradually happen in other fields. There will be fewer new scientists, musicians, artists, philosophers or even gadflies and crazy people to explore new ideas or stir things up. The world will gradually become smaller, poorer and more "conservative" and ossified. This effect will take centuries to manifest itself (a world of 3 billion people in 2300AD will still have the accumulated wealth and history of the past to mine), and the effect won't even rely be noticeable to the people living at the time, except perhaps when grandparents tell stories of the "good old days" or a perceptive person really takes a close look at the past.

So the far smaller world of 2500AD may resemble a small town, with people in everyone's business and carrying out the same routines, the way their fathers and father's father's did before them.

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Then again, with less competition it seems like it should be easier to start a business. – immibis Feb 9 at 9:32
    
But also for the already established to put up barriers – Thucydides Feb 9 at 18:56

Yay, technocracy, don't we all love technocracy!

So the people in charge think the know what's best for everyone, (don't we all?) and they've decided that there are too many of us, but how do you dispose of a full 80% of the population without utter chaos.


The quickest solution is to allow utter chaos, a good plague should do the trick. It'll mostly kill off the unhealthy, elderly and poor in overcrowded areas and the inability to dispose of the bodies fast enough will cause another bout of disease to kill off the rest of those overcrowded people. Problem solved. The rich and healthy (and western socialist countries) will survive, the people too poor to pay for healthcare die off.

You're now living in a world without an underclass, everyone is rich educated and healthy, that's really cool! Until you see the cost of good go through the roof, but never mind that, by the way did the cleaners come through the office last night...oh right.


There's a balance to the world population that hasn't really shifted since Roman times (probably longer but I haven't really looked further). There's a small wealthy class and a large underclass and occasionally a moderate middle class. In the current age we've outsourced our underclass to the far east but the same balance still holds. To maintain your culture you can't keep one group and remove the other the way you might think, you have to maintain the balance somehow. To remove 80% of the population you'll have to do it across the board, you can lift people out of absolute poverty but relative poverty will remain.

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I didn't say some people ought to be preferred over others. Although giving special privileges to a very small amount of people is probably a good idea (people with exceptional genetic traits). I don't think a plague is a good way because it is uncontrollable and will bring too much misery and eventually disagreement with the idea of depopulation. A structured, peaceful, process has a better chance to succeed and be accepted by everyone, as well as a better chance to remain in effect in order to prevent future overpopulation. – DraxDomax Feb 8 at 13:06
    
@DraxDomax, you're apparently a lot nicer than I am – Separatrix Feb 8 at 13:50
    
I am not a nice person. If I could initiate a plague that will wipe out 80% of the population, excluding people that I like, I would totally go for it. – DraxDomax Feb 8 at 13:54
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Well, our system is definitely a lot more effective, when not replaced by illegal mandates by a certain cough black cough president who has no regard for his nation's laws. Coug America's cough laws cough. Which means we have it easier when it comes to cures and serving people, because government is one of the most inefficient things ever created. – XandarTheZenon Feb 8 at 14:39
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@XandarTheZenon: nope, it's the worst and most expensive ;) – Separatrix Feb 8 at 14:41

China's one-child policy didn't result in a different demographic development than a country like Thailand without such a policy. As a result it wouldn't be the policy to focus on.

Current trends mean that many developed countries achieve a children/woman ratio substantially lower than 2. That means over the long-term the population would shrink.

A population engineer would therefore do whatever they can to repeat to develop other countries as well. Reducing childhood deaths is especially important.

Which trends could still produce long-term population growth? Growing religions such as the Mormons demand it's adherents to have many children. Taking actions to deconvert those groups and let them shrink instead of grow over time could be another area for policies.

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The demographics in China, like much of the Western world, is starting to resemble a spindle rather than a pyramid because of the one child policy. This is the source of the saying "China will get old before it gets rich", there will be more elderly people to take care of than young people to fill the workforce, PLA/PLAN etc. – Thucydides Feb 8 at 22:23
    
@Thucydides : The Western world didn't have a one-child policy and had that development. Thailand had similar demographics at similar times then China, so from where did you take the information that the one-child policy caused anything? Not forgetting that the the fall or the birthrate was was a lot faster in China before the introduction of the one-child policy. China rate of woman/child was the same in the year of the introduction of the one-child policy (1979) as it was a decade afterwards. On the other hand it was it halved in the decade before the introduction of the one-child policy. – Christian Feb 9 at 14:17

One way to do so would be to sterilise everyone, but retain the ability to reverse that when needed. This added biological limitation should be able to be passed on to the next generations ideally. If a pair is given a permit by the population engineer/government to have kids (and reversal of their sterility) for their merits to the society the kids automatically at birth are sterile or made sterile shortly after. Same goes for parents after producing offspring, unless further permits were issued. People are made fertile for a certain number of kids and not indefinitely.

To decrease the population without causing anarchy and letting the markets and social structures adjust to new reality, the fertile population should be adjusted to decrease population at no lower rate than 0.96, ideally 0.98-0.99 every year. At this rate in 100 years population will drop from 8 billion to 2 billion. Since there are not time limits you could further bring this number closer to 1 and extend time it takes to reach desired population. By allowing only certain individuals of proven merits to have an offspring, it will encourage people to earn their right to have children and will prevent TV shows such as Benefit Streets or Kate Plus 8. Society will place far more importance on science and engineering and jobs will replaced at much faster rates by robots.

This concept centre of the plot of British TV show UTOPIA, where of course things don't go as smoothly. I also assumed that permits would be issued fairly to the people deserving them and not to the highest bidder.

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Starting process is easy. Managing it is impossible.

1) Decrease significantly arable land, i.e. by melting glaciers to rising oceans by say 200 feet = 70 m. Food will become much more expensive.

2) Decrease usable land even more by making sub-tropical and tropical areas non-habitable, say by increasing temperature by say 4 degree Celsius, 8 F. Conveniently, hotter water (less oxygen) decreases food production in seas.

3) Cause global chaos as nations fight wars over now scarce resources like water and arable land. Add more chaos by spreading new infectious diseases.

4) Cause even more chaos when refugees from countries which are barely habitable (and not capable to feed own population) are trying to escape to more fortunate parts of the globe. Bonus if they can bring some new rare infectious diseases with them.

5) Now add even more political chaos. Political parties in habitable countries refuse to accept those eco-refugees, because accepting them will strain their own resources even more, and they need resources for own citizens (who can vote and BTW did paid taxes) as expenses to protect coastal areas (where most capitals are) go through the roof.

Politicians will promote good reasons why migration should be limited to "right kind of refugees" to preserve own culture. They WILL get elected, and nativist policies WILL get enforced. New migrants will be required to accept majority culture, which they will refuse, and civil unrest in habitable countries will do more damage to infrastructure and culture.

6) For a win, add ecoterorists from nations destroyed by climate change attacking infrastructure in habitable countries as revenge.

Is such chaos, preserving culture would be hard to impossible. Maybe some chances in "Fortress North America", with some luck and saner policies. Which will not be pleasant or nice.

Wait, that is exactly what we are doing! That "science" is called "global profit-based finance system"! We will get there in some 300 years! So we even do have a deadline!

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You've had some pessimistic responses, here's something close to utopian:

As you've suggested that most people are already convinced, and that you have no deadlines, social norms will reduce the number of large-family holdouts still further over time, until their numbers are small enough to be irrelevant. If 1-2 children is seen as normal, 0 commonplace, and 3 very strange (per woman for simplicity). It would take a couple of decades for a significant demographic shift to have any impact; actually reducing the rate at which people are convinced wouldn't be a bad thing, softening this blow.

In the past, education and healthcare have reduced birthrates, so heavy investment in those would be worthwhile; universal access to contraceptives would of course be a requirement.

A global population that could agree on cutting population would be able to agree on many other issues too, meaning that large armed forces wouldn't be needed. This frees up young healthy people to care for the ageing population (as well as money). The research effort that currently goes into military hardware could be diverted to robotics for everything from farming the land that hasn't yet been returned to nature, to care for the sick/elderly.

A goal would be to have a society in which most people had culturally-fulfilling roles ("jobs" if you like) while labour was provided by robots (sustainably powered of course). People who have such jobs tend to have fewer children anyway.

I suspect that the hardest part would be convincing a large enough number of people to start with. Or maybe stopping the decline after many generations.

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The problem with the one child = all boys is a cultural one.
So, in order for your cult to make it work, they'll probably push at least for the usual random game that gives us roughly 1000 female birth for 1020 male birth (out-of-the-hat number, from long ago).
In most western societies/culture, this wouldn't be too much of a problem. Where woman are considered a drain on their family income, that would be the main problem. Then you have to make it so that society has the means to take care of those who can't work (and maybe of those who don't want to).
So, "universal income", "social security",... with a globalized approach (meaning that you can get all the governments in the world to do it at the same time) that might be done. - That's utopia we're talking about.
And those things are the easy part.

Once everything is set up, you have to convince people of giving up on having children. In most culture, it mean giving up on the Future. And this is heavily depressing.

You can either go the nihilist way : you have no future and there is no need to have children who will all die !
The other way is through a second demographic transition : people will have to agree that their future does not lay in their own children, but in a collective humanity.

So, supposing you don't go the Mad Max way... You have to convince every one that the only thing worth fighting for is this "2 billion" greater good. Because people are the main resource in resolving a dispute and as soon as two groups will have a disagreement your population regulation policies, enforcement and so on will shatter, either under the reality or against popular rebellion.

So...
Create a "Maslow" security.
Create World Peace on every and all subject.
Get people to give up on conflicts of any form.
Make people give up on their future and posterity (except for the few who will get to keep on having kids).
Get people to agree on who gets to keep on having kids.

The other solution is contraceptive/poison in the water.
You'll get two end of the population : those who can drink only bottled water and those who have to drink water from rivers and home made well.
And probably a Mad-Max-esque end.

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You don't need to make people give up on having children; you just need to make them have less children. – immibis Feb 9 at 9:34
    
Fair, but to a lot of people of today, "less children" or "no children" are basicaly the same. Don't forget that reproduction is a biological imperative for any life-form. It's only advance in medecine and society that allows us to overcome this. – MakorDal Feb 9 at 9:46
    
I would say there's a pretty big difference between 2 children and 0 children. – immibis Feb 9 at 19:11
    
So do I. But for a lot of people across the world it's the same. Both for cultural (see how many children I have !) and economical (who will pay for my old age ?) reasons. Plus a couple of reasons far less nice (my kids are my soldiers !). – MakorDal Feb 9 at 21:13

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