# Humanely reducing the human population?

Imagine quite a way into the future, the Earth becomes overrun with humans. Birthrates are similar to where they are now, and the advancement of medical science means that people live a lot longer (say average life expectancy is 150 for the developed world). We are, at this point, in frequent contact with species from other worlds, who (with us as a member) have formed an intergalactic federation. The Federation has decided that the human population of Earth is too great, and have given Earth an ultimatum that we are to reduce our population from 120 billion to 8 billion (if this is too large, the Federation may be open to negotiation) in the next 20 years. The means we use to do this are up to us.

Presumptions:

• The technology level of humans is assumed to have moved at a realistic rate
• All major political powers are open to working together towards this requirement
• Most governments still care about ethics, and aren't up for just randomly choosing people and shooting them.
• The Federation will take action if we do not attempt to carry out their ultimatum.

So, what would be the most humane way to select people to die for such a good cause, and how would it be done?

• The idea of my children dying for this cause makes my blood boil - I'm sorry, but this question simply assumes too much about human nature. This would put the entire planet into outright rebellion. Take my baby and rest assured that what happens to me no longer matters - I'm coming for you. – Sean Boddy Oct 4 '15 at 15:25
• Has exporting people been ruled out? If there's a federation, are there other planets they can live on? Or are you saying that there are too many humans period, whether they live on Earth or elsewhere? – Monica Cellio Oct 4 '15 at 20:09
• "The Federation Council has determined the population of Earth is too large. It's crowded and dangerous. For the safety of your population... kill 94% of the population." Federation morality is weird. Why is this Federation concerned about how many people are on Earth again? If we've reached 120 billion without total collapse, we've probably overcome the ecological and social problems of overpopulation. – Schwern Oct 5 '15 at 8:48
• What's the Federation's leverage here? They're literally saying "suffer 94% casualties, or...". There is no rational reason to comply with this request; complying is already essentially a worst-case scenario. – Leushenko Oct 5 '15 at 18:54
• What rationale does the Federation have for adding the time requirement? What is 20 years vs 200 years on a planet you don't control? Given 200 years, it could be done reasonably ethically - birth control and deaths from natural causes would have time to work their magic. If, given the choice between patience and genocide, the Federation chooses genocide then I'd say the Federation is EEEEEEEEEVIL and as a matter of principle humanity must resist. – chucksmash Oct 5 '15 at 20:49

## War

Clearly the most humane way to choose the people is to get volunteers. It's likely to be hard to get volunteers for the problem as stated. So instead ask for volunteers to attack those members of the Federation who voted for this proposal.

This could be combined with a draft. People who do not wish to be subject to the draft can accept a one-in-seven chance of living and mandatory sterilization. Those who fail the one-in-seven chance will be humanely terminated.

If we lose the war, our population should be drastically reduced. Combined with birth control at or below replacement level, this should solve the problem.

And of course if we win the war, we've gotten rid of the Federation and are no longer subject to population controls.

This is the only humane way to proceed. Anything else is simply capitulating to tyranny. This also has the side benefit of improving leverage in negotiations for a longer term of population reduction and/or an increase in the allowed population.

It also gets around a big problem. Assuming current fertility rates continue, how would a country like Japan react? They currently have a shrinking population. How would they feel about having to give up around 90% of their population to cover for population gained in other countries, like India?

## Unrealistic

This whole thing seems unrealistic though. First world countries like Japan or those in Western Europe already have declining native populations. If the entire world was rich enough to have a life expectancy of 150 years, it seems likely that this would be even more of a problem. The world population may continue to grow for a while, but in a hundred years, I'd expect the concern to be switching to the possibility that we're going to die out due to lack of children.

This would be more realistic if set soon enough in the future that our population is still growing, say 2050. Obviously our population won't be 120 billion then. Perhaps twelve billion with a reduction to one billion. And of course our life expectancy won't be 150 years at that point without some major changes.

• War as only humane way ... baffled – Hagen von Eitzen Oct 4 '15 at 14:12
• @HagenvonEitzen Given the unreasonable requirements put forward in the question, it may well be the most reasonable answer. – kasperd Oct 4 '15 at 15:22
• +1 Having a committee to choose who to kill or abandon isn't humane at all. They only make it looks humane. And anything else other than a war is not likely able to reduce the population that much. Note that it isn't the war being inhumane in this case, but the need of reducing population. But the cost of a war might be huge if happened in undesirable ways. If it is in a story, maybe they can be lucky and invent another dangerous job in the right time, but it is still likely going wrong for economical reasons. – user23013 Oct 4 '15 at 18:10
• @user23013, there is no concept of good and evil where an act of genocide falls on the side of good. Being given the order to kill ourselves would be met with an ultimatum telling the Federation to either take their best shot or shut up. – Sean Boddy Oct 5 '15 at 4:10
• I think this is what would happen no matter what you choose (unless you're very, very sneaky). Any government that agreed to a foreign demand to kill 93% of its own citizens just wouldn't last long. – Deolater Oct 5 '15 at 16:16

I only see one way to do it humanely: Negotiate with the Federation to get the technology to quickly build lots of huge space habitats/habitats on other planets and transport the humans there.

Now if you manage to negotiate a longer time frame, comparable to the life time of a human, then you have one more option at your disposal: Implement very strict birth control.

• I agree, this is the only humane option: all the others involve killing people or rendering them infertile. – Max Williams Oct 6 '15 at 10:53
• Dyson ring, anyone? – Jon Story Oct 6 '15 at 13:40
• A Dyson ring would be a most excellent power source, perhaps, but less good as a generation ship. – Isaac Woods Oct 8 '15 at 16:48
• The problem is that voluntary birth control is a kind of selective breeding - we are engaged in a massive genetics experiment where people who want (and are able to get) children gradually replace those who don't. Birth control is going to be harder and harder to enforce as time goes on, as the people who can be prevented from having large numbers of children get weeded out. – Ebonair Dec 20 '17 at 19:00

The 120 billion of humans fight for their survival against federation, if won we colonise whatever planet/spaceship they live in. Earth (considering vastly superior future technology) can easily sustain 50bil people, rest are sent as colonists.

• I vote this up because this is what we tell the people. If we win the war it was true, and if we lose the population is reduced sufficiently to meet demand. – Joshua Oct 5 '15 at 0:55
• Earth cannot support 50 bil people even with advanced tech, unless gmo food in future grows like 4x quicker – P.Lord Jul 15 '19 at 18:54

I believe the most "humane" reaction would be to flip this Federation off.

120 billions of people would most likely not appreciate being bullied like this by some abstract "Federation", and would most likely throw the politicians which are okay with this out the nearest window.

It would take probably less than a day for any politician which is okay with this to not be okay with this anymore.

Everything goes on the table then: 20 years ? Make that 2000, and we can talk business. 120 Or: to 8 billion ? Make that 120 to 80, and make that 1 billion decrease every 100 years.

The point is that as far as people would most likely be concerned, cutting even 1 person by decree will be unacceptable (the "by decree" part is the point). A planet-wide armed riot / civil war is very possible.

How can politicians prevent the potential self-annihilation of the civilization on the planet ? They can either:

• Enforce compliance, starting to weed out people and turning the planet into an armed dictatorship (people will resist and will fight).

• Ignore the directive, accepting the repercussions. We're back to the population feeling bullied.

• Flip the federation off, dropping out from it. People would be happy-ish (no more "cut back on the population" nonsense) until the inevitable consequences (I foresee import/export difficulties and frantic attempts to keep friends with neighbour civilizations and/or the federation itself)

My point being: politicians might be okay with it, but the other 120 billion people won't be. This kind of demand can't possibily be open to negotiations enough to become likeable. Either party has to fold to prevent open conflict.

• Hi Alex, welcome to Worldbuilding SE. If your answer is correct, here on Worldbuilding like slightly more expanded answers... – clem steredenn Oct 5 '15 at 11:25
• I actually intended to post a comment, something went wrong along the way... Well, I'll take the chance to write up some more – Alex Oct 5 '15 at 11:56
• Very good answer, i agree with it – veryRandomMe Oct 7 '15 at 15:45

Assuming negotiation is not an option:

1. Strict birth control. You do not have to kill what is not (yet) born. It will have little effect in scale but let's be honest every single life counts.
2. Ask for volunteers. Will make some little difference I suppose.
3. Promote extreme sports. Same as above.
4. Stop treating the terminally ill. Harsh but it will save some lives.
5. Set up a Survival Determination Project. Should be the most popular URL instantly.

The Survival Determination Project
What is needed is a fool proof mechanism to select survivors from among the total population. To ensure popular support and compliance, and to eliminate political wrangling this needs to be thought out and created publicly, with open and probably vigorous discussion about the why’s, how’s, and when’s. Initially I used RFC (Request for comments) for this concept but it is indeed more along the line of an open source project, as @celtschk rightly pointed out.

The project should provide for:

1. Some kind of raffle. In the end you need some random way to spread survival chances among the to-be-reduced living population. Better be absolute foolproof.
2. Some kind of life gift method. Can only be given, not asked to avoid mass coercion. So must stay secret until day zero for the recipient. This way parents are able to give up their meagre percentage to their children.
3. Possibly gladiator games can be introduced for those who want to fight and/or believe in survival of the fittest. Gives the rest something to watch while it all plays out.

Interesting times indeed.

• Birth control isn't too effective in a 20 year span, especially with a life expectancy of 150 years. What do you mean with "RFC"? The only meaning I know for that abbreviation is "request for comments", but I'm pretty sure that's not what you mean. – celtschk Oct 4 '15 at 12:17
• Not within 20 years as asked for, but birth control can be very effective as you can see in China. Their birthrate is droping for decades and by 2035-2040 more people will get to old to work than young people get old enough to work. (At least they said that in the french/german polit show >Mit Offenen Karten<) – jawo Oct 5 '15 at 11:56
• @celtschk birth control does not have great scale but it cannot be skipped as every life counts; currently some fast growing states have half their population under 20. I've also edited the text around RFC as clearly the intent does not come across. My bad. Hopefully now the explanation has become more useful. – Bookeater Oct 6 '15 at 14:30
• An exam would be good too, in order to ensure the remaining people are the best and brightest, not just the strongest and most merciless. – stephen Oct 6 '15 at 16:44
• Birth control is so obvious that it seems that the 20 year ultimatum is cruel--they are just toying with us. If they gave us 150 years we could set the population to exactly any number (relatively) painlessly through birth control, therefore I would reach the conclusion that there are other motives for the ultimatum--possibly to see how brutal we are or if (how?) we choose to make the decision to kill others to save ourselves. – Bill K Oct 6 '15 at 17:03

Mass hybernation. People are frozen and stored in underground storages. Then they can be awakened at some shedule, then hybernated again. The technology exists today but currently used only for recently dead people in hope the medicine of the future could cure their deseases. It was also tested on animals.

• The technology does not provenly exist today. – gerrit Oct 4 '15 at 19:06
• @gerrit It does provenly exist. We just don't have a way of bringing the frozen people back, yet. Not that much of a problem if the bigger problem is overpopulation, if you ask me. – timuzhti Oct 5 '15 at 2:22
• @Alpha3031 Without proving that they could be brought back, this technique becomes tantamount to mass murder by hypothermia. Nonetheless, +1, because this could definitely be possible wiith near-future tech. – ApproachingDarknessFish Oct 5 '15 at 7:27
• @Alpha3031 That's like saying that you can fly down a cliff or high building, and that it's just the landing you need to figure out. – gerrit Oct 5 '15 at 11:28
• I think this answer works considering this is set far enough in the future really. – Tim B Oct 5 '15 at 12:03

In Dan Brown's Inferno a mad scientist releases a rapidly spreading virus that causes infertility in about 50% of all people.

It cuts the population by a drastic amount without actually causing anyone harm, though it is delayed by about 1-2 generations.

• This is definitely the most humane and fair way to do things. – Varrick Oct 5 '15 at 11:02
• This may reduce the population in the long run, but even if you halted all births immediately, 20 years is too short for this to make any significant difference. – pluckedkiwi Oct 5 '15 at 18:21
• without actually causing anyone harm With respect, I think you vastly underestimate the social and emotional harm that people suffer even now from infertility. It might be possible to argue that this is the least inhumane way to do things, but forced sterility is anything but humane. – GrandOpener Oct 7 '15 at 19:54
• Infertility is a lot more fun than starvation. – Peter Wone Oct 9 '15 at 6:09
• Sounds a bit like the Sterile Insect Technique and related inherited sterility in insects – Kelly Thomas Oct 9 '15 at 11:33

Set Baby Rights

Allocate every woman the amount of 1/2 of a baby allowed. A couple or a woman can sell or buy the rights to a full baby and give birth to one.

Allow the Free Market to Take Over

A woman (single or as part of a couple) that has the means to buy the allocation of others, can have a full baby or more, if they have the cash. This way, a woman who, for example, may be impoverished, can sell her allocation (probably for a lot of money). This evens the playing field a bit.

In a lesbian couple, one of them can have a baby (two half-allocations) or they can 'buy' more baby. Women who are unable to conceive can sell their allocation and can pay a couple to adopt, but that's a different story.

Two men who want to adopt will just pay whatever it costs to adopt someone who has had a baby; this will be more expensive, because unlike a heterosexual couple they do not start with 1/2 allocation.

• How do you actually plan to police this? Without dystopian levels of surveillance, policing this is near impossible. – March Ho Oct 5 '15 at 9:59
• And what do you do if someone conceives? Or if a couple is sold a fake birth right? – Davidmh Oct 5 '15 at 12:52
• This will still not have significant effects on total population in 20 years. – Paŭlo Ebermann Oct 5 '15 at 13:23
• @Mikey the problem is that even if no new kids are born at all, with a life expectance of 150 years, only after about that time most of the currently living people are dead. – Paŭlo Ebermann Oct 5 '15 at 18:45
• @DJMethaneMan The solution is that everyone gets 1/4th of a baby regardless of gender. Yay, equality! – DoubleDouble Oct 6 '15 at 22:01

Computer Avatars

2045 Initiative comes to mind. This organization says that by 2045, we will have analyzed the brains complexity and will be able to upload our self into a virtual reality world. Instead of a body we would have an avatar (and in the future, a real-life "robot" avatar).

If this technology exists, it might not be hard to convince the old and the young to live in this simulator.

• The problem is, that's a simulator. You still die if you are killed. You have no real connection to the simulation when dead. – veryRandomMe Oct 7 '15 at 15:47
• @NoviceInDisguise I don't understand. The body will be dead (which is what they want) but the conscious will be in the simulator. You can see family/friends that are in the simulation. People from the real world could go "play" and visit like in modern MMORPG. If you define someone from the conscious then nobody actually dies. – the_lotus Oct 7 '15 at 18:31
• That is all virtual. You yourself are not actually within the simulation. When you die, the simulation continues, yes. However that simulation is not you. You will not be able to connect with it. It will appear the same to people still alive, the simulation of the other person will be realistic, however that person will indeed be dead and unable to think, act, or perceive. – veryRandomMe Oct 7 '15 at 18:43
• That's an interesting philosophical question of what constitutes the "self." If I were able to "upload" my consciousness into a simulation with memories and emotions intact, while my meat-body died, I would consider that simulation to be myself. – GrandOpener Oct 7 '15 at 19:54
• @GrandOpener Searle has an interesting analogy in his Chinese Room argument. If you can simulate a human brain, you can surely simulate a thunderstorm at the same level of complexity. Would you be worried about water damage when you run the thunderstorm simulation? If, for a thunderstorm, a simulation is obviously not the real thing, how come, for a human brain, a simulation is the exact same thing with the exact same causal powers such as creating phenomenal conscioussness? – Solanacea Apr 14 '17 at 19:53

The federation sounds pretty unintelligent to think that you can remove 85% of a population in 20 years without any adverse side-effects and I'd wager we could outwit them but let's assume they're just mean.

Assuming there's a roughly equal amount of every age, you can prevent 7.5% of the population via birth control, and 7.5% would die in those 20 years so we can safely ignore any new births and expect 7.5% to die right from the top.

The moon is roughly a quarter the size of earth, and assuming 120b people are living on earth, 30b of those people could live on the moon since it's very likely the technology to do so is entirely there.

• With this knowledge we can strategically place 25% of the population (under 130 years of age, so as to not have any die) on the moon. (30b safely away)
• 7.5% of the total population will still die, but as they're all on earth and this will make sure the moon stays at full capacity upon inspection day. (9b from natural death)

At this point we have 73b people left to work with to meet the demands fully via humane means. It's not looking too good.

Hopefully they can be made to reason with us as we've almost halved our population in a matter of 20 earth years without a single shot fired. All good things take time and hopefully they may see the value in allowing time to take its course and we can be left to continue our means of population control by sending the old to earth and keeping a persistent 30b people on the moon until 8b remain on earth.

or

The Federation eliminates all humans on earth and we persist via the strategically placed youngest humans on the moon. (Sorry gramps)

• If we can move 30 billion to the Moon, maybe we can move 73 billion to Mars? The problem with either though, is that the aliens would just tell us we're restricted to something like 2 billion on the Moon and 6 billion on Mars. It's doubtful we'd be able to hide those massive populations somewhere else in the solar system. – MichaelS Oct 6 '15 at 2:36
• 1 quarter the size does not mean 1 quarter surface ;) the living space on the moon is not 1/4 of earth. Also earth is covered by oceans. – CoffeDeveloper Oct 6 '15 at 15:32
• If we assume the same population density as Tokyo, we can easily fit all 120 billion people on the Moon :) There's also Mars, with 4x the surface area. We just need enough space farms for food. – timuzhti Oct 12 '15 at 3:46

# Colonoize another planet, or live in space

You mentioned that technology will be growing at a good rate. We're already have probes on multiple planets, and the commercial space industry is exploding, so its reasonable to assume that by the time your scenario comes around we'll have much better inter-planetary transportation abilities.

The bigger question is, why does this federation of aliens care how many people are on Earth so much that they are threatening us?

• Welcome to Worldbuilding. While your answer is not bad as such, it is generally encouraged to elaborate a bit. – Burki Oct 6 '15 at 7:03
• Best solution from all answers. Why they care - just wish to see how we will solve that, or they find earth ecosystem more valuable resource then entry human population - because it takes 100000 to calculate all that live for their supercomp - so way to save resources for other calculations. So moving to spacehabitats is good solution, if they did't mean population in solar system. If they did - anyway move to space and prepare to war. – MolbOrg Jun 30 '16 at 2:08

To reduce the number of humans on earth we will use our advanced biological science to mutate 112 billion humans on the planet to become lizard men.

Thus by definition we have reduced the human population. If the federation is not amused by our trick then we will send the lizard men after them (who of course are naturally well suited for war).

A most (hu)man(e?)ly method would be to start a war with The Federation.
(Given their demands it should be easy to get global support for it)

War will always cause casualties, and as such will reduce the population at a fast rate.

Whether Earth wins or looses, the population objective will be met in the end.

• Sidenote: Don't attempt this when The Federation has spaceships that can destroy planets and is willing to use them. – LukStorms Oct 6 '15 at 12:39
• If I am one of the 112 BILLION people, I could hardly care less to be honest. – veryRandomMe Oct 7 '15 at 15:48
• If the Federation demands that 93% of the human race be killed, it doesn't take much chance of success to increase the expected number of human survivors. There's also the possibility of sending combination armies and colonies to hit other Federation worlds. Or, alternatively, inform the Federation of how many worlds humanity will attempt to destroy. – David Thornley Sep 20 '18 at 19:49

Negotiate over the time-line, we will need about a human lifespan (according to the comments possibly a lot longer). Then make everyone rich.

If we have achieved space travel it seems probable that a post-scarcity society would be technological possible. The only reason it hasn’t happened already is we still have a class system imposed by our free market. Free markets tend to favour people who start with wealth to invest, this enables them to generate more wealth with greater ease than people who cannot make an investment.

However with this ultimatum from the Federation we suddenly have a very strong incentive to stop this nonsense. The governments will redistribute wealth right across the globe. When families are wealthy they tend to have fewer kids or none at all. Japan and Germany both experience population decline for this reason.

The truth is that we could do most of this today. If we redistributed wealth to places that currently have very high birth rates, then their child mortality would fall, and their economic prospects would rise. It wouldn't take much to greatly increase the quality of life of many of us. Lets not wait for an alien ultimatum, lets do this today.

• Are you sure you're not confusing cause and effect? What proof do you have to back the claim that "when families are wealthy they tend to have fewer kids"? Is it possible that families who have fewer kids just tend to have more money available? In that case, distributing wealth would do nothing to change the growth of population in cultures where having many children is common. – Patrick Roberts Oct 5 '15 at 0:52
• @PatrickRoberts If less children where the cause then we would exspect to see the effect vary from family to family. It should be possible for a family in a lower economic strata to have no kids and climb. Instead we see low social mobility, tinyurl.com/o6zjyzj. Yet when the entire contry becomes more economicly sucessful, its birth rate sees a sharp decline, tinyurl.com/nvvuy5g. – Jekowl Oct 5 '15 at 6:34
• Since the article points to (albeit uncommon) successes, this does suggest the effect varies, and is not conclusive proof on the differences between successful and unsuccessful families. Secondly, the graph seems convenient in that it fails to label the 3 countries that don't follow the trend, nor does it imply causality of the trend. There is also no indication of whether the birth rate and GDP are country-wide or per family, so please provide context for the graph. – Patrick Roberts Oct 5 '15 at 10:14
• This is the most humane solution, I think - although the time frame would probably have to be quite big. If each woman has 1.5 children on average, you'd need about 23 generations - that's maybe 500years (Assuming the population is split about 50% female). – Jost Oct 7 '15 at 6:03
• Reason for this being the most humane thing: a) You solve two problems in one go (poverty and the federation threat), b) There is no force involved - no one-child laws, no economical reasons to not get a child.Some reasons why it will work: It just removes a bunch of economical reasons for getting children: They are no longer needed as workforce to feed the family, and there will be fewer "accidential" pregnancies, because people have the means for birth control. – Jost Oct 7 '15 at 6:11

"Birthrates are similar to where they are now"

In lots of developed countries, birthrates is lower than 2 ! Which means that without immigration from poor countries population would decline (in fact Japan could even lose all its population within some hundred years given its current birthrate).

So the answer is sample : make underdeveloped countries developed so that they undergo a demographic shift.

• Stabilizing the population growth to levels slightly below replacement will slightly reduce the size of each subsequent generation, but does not even begin to address the need to kill off nearly everyone within a mere 20 years. Even preventing all births will not significantly reduce the population within that timescale. – pluckedkiwi Oct 5 '15 at 18:25
• Indeed, but it would be stupid not to use a peaceful solution for decades, then suddenly decide decide that 120 billion is too much and that we should decrease to a few billions. But I agree that given the setup only murder/displacement could work – agemO Oct 7 '15 at 6:58

There is no way to 'humanely' reduce the population by that much in twenty years. You can see this if you picture yourself at somewhere between 20-30 years old in the here and now, and then imagine that only 6% of the people you know at that point in your life are alive 20 years later. Even taking someone who is 60, you're still describing mass slaughter.

This is assuming that you aren't using the meat industry's definition of "humane" I suppose. But even then, the idea of "humane death" is one that decreases suffering as much as possible. If you're going to announce that 94% of humanity needs to die in the next 20 years, you're pretty much announcing an inhumane outcome.

So yeah - war with Federation it is, like other people said. There's no sense in trying to reason with a bunch of genocidal maniacs.

Well if you are George Lucas you have some really cool options.

You file a Form 382-G with the intergalactic courts to get them to stop the process. (Get them tied up with that for a while.)

You're going to also probably want to file for a HIQ9 restraining order at the same time. But that's going to cost because you're going to have to hire process servers to serve papers to each federation world leader, AND you're going to have to take out a full page legal notice ad in the intergalactic news beacon next month.

Then you file a Z-91 notice of foreclosure (with a NV-56.121 notice to vacate attached) against each member of the federation home worlds. You don't really have a case but they're still going to have to prove it. Now that's going to buy you a good 60 years or so.

Of course you mustn't forget to file a bunch of motions with the senate subcommittee on population control. It would be ideal if these were as confusing as possible as you want to tie up the process as long as possible.

Then you haul over to the interstellar transportation board and file to have a toll zone put in between Mars and the ort cloud. You'll probably have to pay a few bribes for that one. So you give them Pluto (Jokes on them, it's not eve a planet anymore.)

Of course, the toll zone isn't going to keep them out forever, but everyone knows they aren't going be paying that toll just come check on you.

So that bought you some time. Now what?....

Now you just wait about 20 years, green screen the whole planet and digitally alter it to make it appear that people that were there aren't there anymore.

Just to round it out you fill in every empty space with random creatures for no good reason.

Then make a nice film about how barren your planet is and send it back to the federation senate, where they watch all of about 20 minutes of it and silently all vow to pretend like the entire thing never happened.

• Actually I had a real legit point with this... There obviously is some assumption of some sort of "rule of law" in this federation and as members the earth would be entitled to some legal recourses inside the system. Armed conflict is not automatically the only way to fight back. – Justin Ohms Oct 6 '15 at 21:21
• Very interesting! The Federation prides itself on its legal process, and I wondered if anyone would pick this line of thought! – Isaac Woods Oct 10 '15 at 8:50

The way I see it, nobody would take the risk to accept this ultimatum in front of the populations. Since a democratic choice would lead to chaos all over the world, I think the only realistic way would be to do it without the consent of peoples.

Some deadly viruses/diseases could be successively unleashed like the 1918 flu or Ebola and that could decrease the population a bit. Keeping only 1 percent of a human population seems really a difficult task.

Some nuclear war may be efficient enough to kill a huge amount of population but who would pull the trigger ?

Freeze them

Freeze the excessive population (choose who to freeze by lottery, age, geographical area ...) and implement strict birth control to prevent new people being created. When somebody dies, unfreeze someone to replace them.

• What does this add that wasn't present in previous answers? – zeta Oct 5 '15 at 20:58

According to the WHO 56 million people die each year. With the population at 120 billion, even though people are living older and the fact that medicine would have presumably improved, I don't think this would entail the mortality rate being proportionally lower per capita, as the state of being a lot older actually would nullify the improved medical treatment. So I'm going to assume a mortality rate in proportion to today's, that is (56 x 17) 952 million people a year.

Assuming reproductive rights were strictly suspended by the powers that be, and this injunction was adhered to by the citizens, or successfully enforced by the government(s), to drop down to a population of 8 billion from 120 billion would take 118 years. That is IF no extra person were born in that time.

After this period of 118 years, those who were babies at the start of the reproductive ban would be 118 years old, and eight billion humans would be between the ages of 118 and 150. The 118-year-olds would be the youngest generation.

If you assume the mortality rate per capita to be LOWER because of improved medicine, then this 118 year span would become longer. Even if you quadrupled the mortality rate, so that it were just under 4 billion people dying per year, it would still take, all other factors held constant, about 28 years to accomplish an 8 billion population goal.

This is assuming NO new births, and no voluntary or involuntary euthanasia.

The most destructive war in terms of lives was WW2, which claimed (higher estimates) around 80 million people. And in my opinion, a great portion of these were non-combatants dying from famine, disease and genocides.

So that's 80 million in a 6 year period, that's 13 million per year. Well it's a start. The deadliest earthquake on record killed about 800,000 Chinese. The Black Death of medieval Europe wiped out between a quarter to half of it's population (50 mil to 200 mil). This is a measly 10 to 40 million people a year. The Spanish flu of 1918 killed between 25 and 50 million people a year. So with extra mortalities thrown in we'd be getting there.

But don't forget this is with a complete ban on reproduction. Assuming a complete baby ban, and a mortality rate per capita four times what it is today, we'd be sitting around 28 years to achieve the goal. To reduce that time to 16 years about 900 million people extra would need to die per year. Likewise, you could assume a mortality rate of 8 times what it is today, per capita, and the natural number of attrition would drop the population down to 8 billion after 16 years, that's about 8 billion people dying per year.

Did I mention with a complete baby ban.

I think we can hardly better Johnathon Swift's Modest Proposal in 1729. "I have been assured by a very knowing American of my acquaintance in London, that a young healthy child well nursed, is, at a year old, a most delicious nourishing and wholesome food, whether stewed, roasted, baked, or boiled; and I make no doubt that it will equally serve in a fricasie, or a ragoust. I do therefore humbly offer it to publick consideration, that of the hundred and twenty thousand children, already computed, twenty thousand may be reserved for breed, whereof only one fourth part to be males; which is more than we allow to sheep, black cattle, or swine, and my reason is, that these children are seldom the fruits of marriage, a circumstance not much regarded by our savages, therefore, one male will be sufficient to serve four females. That the remaining hundred thousand may, at a year old, be offered in sale to the persons of quality and fortune, through the kingdom, always advising the mother to let them suck plentifully in the last month, so as to render them plump, and fat for a good table. A child will make two dishes at an entertainment for friends, and when the family dines alone, the fore or hind quarter will make a reasonable dish, and seasoned with a little pepper or salt, will be very good boiled on the fourth day, especially in winter." Just scale up the numbers and sell to the Federation

To reduce population you need to focus on two things, decrease birth rate, and increase death rate.

Fastest way to increase death rate, will be start conflict. It will be very easy to construct it. If federation create that kind of demand, in next step people will split into two groups. one which agreed with those terms, and another which will against. Now few "terrorist" attacks, and large anti-terrorist operation should successfully help to decrease population to required limit.

Other solution, is much simpler, just increase costs of health service and costs of living and promote unhealthy life style.

To decrease birthrate, I see two solutions. One was described in Dan Brown book, Inferno, which was virus which made 1/3 humanity infertile. Other one, will be to introduce Laws which allows only some people to breed.

An "intergalactic federation" spans between galaxies. Why are we worried about population on one planet? The implication of FTL travel and significantly vast energy scales means people should colonize new worlds and artificial habitats. With the technology of hundreds of billions of worlds, and substantially higher technology implied by FTL etc and because some worlds will be much older, the carrying capacity of one planet will be significantly higher, too, not limited by the energy of the sun.

So your premise needs to be at least quantified better, or certainly explained and justified better.

First someone should ask in which frame of reference are those 20 years defined.

If the federation people are constrained by the speed of light and moving at relativistic speeds, then it is quite possible that those 20 years are actually a significantly longer time. Even if time dilation is not a factor, the distances involved are.

If they do have faster-than-light travel, then Earth could kindly ask to borrow one of the time machines they certainly have and a) destroy or undermine the Federation in their past or b) teach our Bronze age ancestors abstinence. I'll leave the various paradoxes and ways to avoid overshooting the target as an exercise...

• +1! This is an almost perfect answer, but there is a paradox free use for the time machines which I will outline in my answer below. Great answer! – Henry Taylor Jun 23 '17 at 4:27

Personally I agree with the folks suggesting war. But, if for the sake of argument that isn't an option, there is perhaps an actual humane way if the Humans can negotiate two things - a longer time period and for the 'federation' to cough up some money.

The solution would be to pay people not to have children.

If the federation is powerful enough that it feels it can issue a mandate to humanity, then it clearly has incredible resources. If they are willing to part with some, they can offer humans large payments in exchange for accepting permanent birth control. It is a win-win transaction for everyone involved with minimal coercion. If payments are large enough, then enough people will accept them. Over the course of an average human life-time, you'd see a significant drop in the population. If its not dropping fast enough, the federation ups the payment. If its dropping too fast, they lower the payment.

There could of course be cheating (freezing eggs, using surrogates), but DNA tests at birth should stamp out the majority of it.

• Very interesting! Thanks for adding this answer. This also opens a potential for negotiation with the Federation to extend the timespan to a human life's worth as well! – Isaac Woods Oct 10 '15 at 8:47

@thkala's answer was almost perfect, but missed one implication of the Federation having FTL capabilities...

FORWARD TIME TRAVEL

1. First of all, sterilize all but 8 billion people and hold those still fertile humans in reserve for the last step of the process.
2. Acquire several of the Federation's Time Machines.
3. Divide the remaining 112 billion people into groups of 8 billion people each; carefully balancing skills and capabilities so that every group has all the knowledge and experience needed to thrive on their own.
4. Send each group of 8 billion people into the future, advancing each group forward 150 years farther than the group before. Each of those groups can then live out their lives in total before the next group arrives to take their place.
5. After all of the other groups have gone forward, send the 8 billion fertile people to the empty world which follows the death of the furthest sent group. It is their job to carefully propagate the human race into the future.

Once that fertile group has left, the Federation can come pick up their time machines so that nobody is tempted to misuse them.

Note: This could also be done without Time Machines if the Federation could supply enough star ships to contain the 112 billion (otherwise time-travelling) people. Those ships would travel out on vast circular routes at heavy time-dilating speeds in such a way that enough ships to offload 8 billion humans, would return to Earth every 150 years.

Note: It could also be done with cryogenics, but that might not satisfy the Federation because the 112 billion frozen humans might still be considered alive, making the entire effort moot.

Late to the party, but I had a shower thought (literally)

Choosing people at random is the only way that will be viewed as fair and acceptable. But of course those chosen to die will go to war against both galactic federation and human leaders that serve them.

Instead, the rulers of the earth will need to reframe the situation. Evil empire has infected the Earth with a virus, but Galactic Federation has generously provided billions of doses of vaccine. So we randomly choose those who will receive it.

Ofc, the virus would need to be made and distributed in secret, but earth government or the Federation. Federation's own weapon is is probably a virus anyway. Any other weapon of mass distraction will mess up the planet, and if they do not mind messing up the planet, they would just blow it up with exhaust from their intergalactic drive.

In Mass Effect the solution was spreading the disease that makes most victims sterile. But again, it needs about 0.75 times of a lifespan to bring result. The real humanely way is to send everybody to another planet, may be cheat the aliens with classic "genie wishes" trick.

First I want to say that it's impossible to have 120 billion humans on the Earth but well, it doesn't matter right now, sooner or later all of them will be dead in some way.

# Birth Control Rate

It's an slower rate but it's quite ethical.
By this way it's impossible to kill 120 billion humans, but if you prohibit them having any children (even if they want children you can sterilise them, I don't know if it's possible in humans but it's possible to permeate with gamma rays in mosquitoes and is very fast and cheap). You could also make a deal to increase the human survival or increase the time limit you have to do it.
Personally I think it's impossible but if humanity shows that they are trying to complete the deal (even out of time) the Federation could give them a second chance.

Obviously you can not stelize useful and smart people, it is your chooice.

Also, you can make an aleatory process (or selected) where people who lose (14 out of 15 people :)) have 2 options:

• Be executed.
• Be sterilize.

# Controlled Plague

A faster way could be using a similar method used in Utopia.

• Bassicaly they make an Alpha protein which is given to humans secretly in cereals (and other types of food I think).
• Then 7 secret persons fly 7 planes (planes used to irrigate field crops) but instead of irrigating with water or insecticide they irrigate the cities (yes, the 7 most important capitals, not farms) with a lethal sickness (Black Death).
• Finally, the US government gives free vaccines to all the American citizens and also gives vaccines to other countries. These vaccines contain the Beta protein which causes a chemical reaction with Alpha proteins:
• This sterilises randomly 95% of the human population. This "attacks" humans bodies only if they have a certain Z (I would say X but maybe it would bring confusion) gene, this gene is almost random in the human population.
• Also (it wasn't totally planned in the series) this Alpha-Beta combination can destroy the human immunity to the black plague, making it more lethal.
• They said that in 100 years the human population would be stabilized to 500 million humans. In your case, it would be 7.5 - 6 billion humans.

Important people could have the real vacine (without the Alpha or Beta protein).
Basically you can poison (or a sterilising drug), water, food or even a person (like a plague?).

# Reduce Medical Care

• Terminal and non-terminal but lethal diseases are not allowed to be healed by medics.
• Suicides and bloody accidents aren't treated by medics.
• Vaccines aren't given to humans.
• People in comas, vegetative states, intensive care, etc... disconnected.
• Medical drugs replaced by placebos.

If you don't want to make a revolution you can instead make it very expensive. Rich and smart people (who often are rich or at least have friends) won't die.

# War

War is very useful, it's waged mostly by volunteers and there are a lot of deaths.

• Countries could make wars to kill people, maybe the country who loses would have to "sacrifice" more people for the Federation.
• The world's population could make a war against the Federation.
• If they lose, human overpopulation would be almost resolved.
• If they win, there is no more human cap.

# Other methods, quite... dangerous.

• Poison Water: you can poison the drinkable water (or at least make it sterilise humans) when humans reach the 8 billion you can make a vaccine and also stop poisoning the water. (You can distribute fresh water or vacines to VIPs).
• Controlled Droughts: secretly governments could destroy crop fields (drought by removal of water, controlled insect plagues, fire) to make citiziens die by starvation. VIPs generally are rich or smart (and with rich or other VIPs friends), they won't have any problems buying food.
• Melt Ice Poles: yes, it's a crazy idea, but if you melt the poles a lot of people would die from drowning, then if you stop heating them they would slowy freeze again (right???).
• Nuclear War: obviously.
• Biological War: like the controlled plague but on a bigger scale (this would be in your enemies countries not on your own).