The setting is as follow. In a planet like earth, one day, 10% of the population suddenly becomes immortal. The "immortals" are not physiologically different from the "mortals" and initially it is hard to differentiate the mortals from Immortals. The only way to differentiate in the early stage is to kill a person. If he revives, he's an "immortal".

Now, this power of immortality is transferable. Through copulation. The rules are as follows:

  1. If an "immortal" copulates with a "mortal", the "mortal" becomes "immortal" and vice-versa.

  2. If both parties are "mortal"/"immortal" the status effect does not change.

  3. In a scenario where more than two parties are involved (in the act of copulation) status effect does not change.

  4. If a "mortal" and an "immortal" has a child, the child has 50% probability to be "immortal". If the child turns out to be "immortal", the parent with immortality loses his status effect. The status effect loss happens AFTER the baby is born (i.e. one of the parent is still "immortal" during pregnancy).

  5. If two "immortal" have a child, the child will also be "immortal". Parents would retain their status effect.

  6. "Immortals" age normally upto age 25. After that they only age once the status effect is removed.

  7. A "mortal" above the age 25 stops ageing whenever they recieve the status effect.

  8. Am "immortal" loses his/her immortality, if he/she does not copulate with another "immortal" in 50 years.

At the beginning, no one is aware of their status effect. They may come to know later (once they revive after death or once they realise they are not getting old). Also, the rules about the transfer of immortality is not known to the population a priori (though someone down the line might figure it out).


  1. Given the rules, constrains and the transferable nature of immortality, how would the population of the "immortals" evolve over the years?

  2. The "immortals" could start selective "breeding" to increase the population of "immortals" once ALL the immortality transfer rules have been discovered. What is the timeframe for this to happen?

Assume humanity (i.e. most people won't be willing to burn their babies to check if they are "immortal").

Edit: added rule 8 (was previously missed).

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    $\begingroup$ 2. The immortals simply create a worldwide genocidal pogrom and reduce the population to immortals only. Timeframe: near instantenous compared to selective breeding option. $\endgroup$ Sep 14 '19 at 14:37
  • $\begingroup$ Q2. is asking for the timeframe to discover all the laws of immortality transfer. Should I rephrase the question? $\endgroup$ Sep 14 '19 at 14:46
  • $\begingroup$ Ahh. I see where I read wrong there. Yeah. "How long would it take to figure out all the immortality transfer rules?" would be less prone to confusion. However I reckon that both questions would be opinon based and based on story requirements. Stack exchange has a one question per question rule ;). How would you judge one answer as better than another? $\endgroup$ Sep 14 '19 at 14:53
  • $\begingroup$ Out of curiosity, would rule 1 apply in the case of non-consensual copulation, how about necrophilia - and for how long after death of the mortal. Are you referring to specifically heterosexual acts, exclusively penetrative acts (in which case whither lesbians?) or is it more along the lines of heavy petting does the job? Depending on your answer, the whole immortality thing could end-up in the hands of criminals gangs and unsavory very powerfull people. $\endgroup$ Sep 14 '19 at 15:37
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    $\begingroup$ This question seems to be asking "what would happen if most people don't really know all the rules?" The answer, of course, is "it varies, so whatever you want to happen." Once folks figure out that immortality is sexually transmitted, they will have a lot of fun exploring it. Once they learn that they can lose immortality through sex, most of that that fun will suddenly stop. $\endgroup$
    – user535733
    Sep 14 '19 at 16:18

I'm going to take as a given what the other answers have indicated: that the immortals will form a naturally dominant caste which will either be oppressed or (most likely) come to rule society. But for hints of what that society would actually look like, note that you've introduced some seriously twisted relationship dynamics which are going to aggravate a lot of existing issues:

If I am mortal, I can steal immortality from an immortal by having sex with them

Immortals are constant rape targets; never mind what they're wearing, merely the fact that they are ageless is enough to mark them out. Expect society to institute draconian laws to control such behaviour. Also expect issues within stable bi-immortal partnerships when one partner loses their immortality through infidelity: they are then tempted to try to steal their partner's immortality before their newly-mortal status becomes apparent.

Less adversarially, note that a stable 'hetro-mortal' couple (one mortal and one immortal) will naturally have a lifespan double that of a bi-mortal couple simply by maintaining a regular sex life and passing the immortality back and forth between them. Makes amicable divorce essentially impossible, however, if the couple know who is leaving with the immortal conch.

If I am immortal, the only people I can safely have sex with are other immortals.

Fortunately there is an easy and reliable way to prove you are immortal: kill yourself and revive. Expect the development of various rituals and tests among the immortal caste to prove their continued membership.

An immortal-only brothel would be a highly coveted, but extremely expensive-to-maintain, institution. It would be interesting to explore the dynamic which forces such an establishment to be populated by people from the highest, rather than lower, class of society. On the other hand, given rule #3...

As an immortal, threesomes are the only truly safe sex

If by only engaging in multi-person sex an immortal can protect their immortality indefinitely (ie it is sufficient to satisfy rule #8, while also allowing for procreation) then you can expect that that's what they will do. It would probably make for a more interesting storytelling dynamic to at least say that it is not sufficient, ie immortals have to expose themselves to risk at least once every half-century.

As an immortal, having children is highly risky

Although by a slightly twisted mechanism: although one might be part of a happy, safe, bi-immortal partnership at the start of the pregnancy, there is then a period of nine months in which both parents have to stay immortal, otherwise the faithful parent is also at risk of losing their immortality to the child.

There is a nondestructive test for immortality

Although it's not pretty:

  1. Capture an immortal.
  2. Kill them, check that they revive. If not, go to 1.
  3. Have the testee rape them.
  4. Kill them again.

If they die the second time, the testee is now immortal but wasn't before. If they do not die, the testee was immortal before (and still is now).

Given those dynamics, then what you've basically created at first is a cult with what looks from the outside like some fairly outlandish sexual practices: orgies, masochism, incest, and not a lot of childbearing. On the development/evolution side, as noted the presence of half a billion immortals will be noticed within a matter of days (expect a massive spike in idiotic suicides in the immediate aftermath!). Working out that the immortality is transferred by intercourse will be discovered within a few months, although it will take a long time to work out all the rules (a large number of newly-emergent immortals will become celibate and will get hit by rule #8 after 50 years).

In the years after the 'emergence' immortals will probably be oppressed and targeted, mostly in the underworld. Immortality will be the ultimate thing for criminals to take by force or deceit, either for themselves or to order. Immortals in 'respectable' society will probably remain incognito as far as possible, while forming secret networks - the real illuminati. Expect ritualised sacrificial tests to start developing here.

I suspect that the 'takeover' of society by the immortal caste will be quite slow and insidious, as immortality doesn't confer any physical or mental advantage apart from indefinitely increasing experience and power base.


Assuming that people aren't initially aware that they are immortal after the event that caused it, i.e they have some sort of sense/feeling that they are immortal, then it is going to take an incredibly long time for people to realise the immortality is there and even longer to work out the rules.

With these rules and percentages the only people who would remain immortal for any length of time are monogamous couples, who decide not to have children, where both parties are immortal. Given that 10% of the population is immortal, the chance of a couple both being immortal is 1%. In the rest of the population immortality would be passed back and forth, depending on how active each individuals sex life is (apparently on average people have sex once a week, but that is obviously very dependant on a lot of factors) and you would not know whether you are currently immortal or not. (In the case of celibate immortals, they would notice after some time, but if they remain celebate then their powers eventually are gone).

So now considering that 1% of monogamous couples are immortal, the question of how long do the remain monogamous comes into play. Apparently 50% of marriages end in divorce in the US, and that is with a normal lifespan, and as the lifespan increases chances of developing irreconcilable differences increases. Plus as soon as one party cheats (which is probably more likely if you stay 25 forever, and factoring in cheating doesn't always end in divorce) they are likely to lose their immortality. So in the same way that an immortal person is statistically likely to end up trapped somewhere, I think an immortal couple is statistically likely to end up mortal, either due to cheating or eventually getting bored of sex with each other.

So you have a society where most people are normal, with some periods of extended life after they unknowingly have sex with an immortal person and become immortal for a bit until they next have sex, and a few immortal couples (who statistically eventually lose it) and some immortal (for 50 years) celibate people. So, realistically, it would be a society much like ours, but with an extended lifespan. As the generations go on, as there's only 50% chance that immortality gets passed on in mixed immortality couples, the population percentage of immortal people would get lower each generation. It is unlikely people would fully work out the rules until immortality is so rare in the population that it becomes a moot point.

If however, somehow people are aware they are immortal, and are aware of the rules, then it may be a different case.

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    $\begingroup$ Some good reasoning, but suggest you reconsider how short some of the timeframes will be. With the record-keeping and information sharing in today's world, the existence of inexplicable survival will be headline news within 48 hours after 10% of all deaths... aren't. The phenomenon will then be studied intensively - rules 1, 6 and 4 will be inferred in less than a year. Rules 2, 5 and 7 will take about a decade to confirm. Rule 3... hmm. Rule 8 is the kicker, it will take at least 60 years to work that one out. $\endgroup$ Sep 15 '19 at 14:19
  • $\begingroup$ @KerrAvon2055 - I agree with your point to some extent, but its still only 10% of people who inexplicably survive, and only would be noticable in situations where any chance of survival would be completely surprising. Remember that even in things like plane crashes, you sometimes get a few survivors, but the vast majority of deaths are caused by things like heart attacks, and someone not having a heart attack wouldn't be noticed. $\endgroup$
    – L Mason
    Sep 16 '19 at 19:07

Do not underestimate the power of immortality

Our lifespans have steadily grown longer over the last two centuries. We have so far observed the following effects of our current long-lived status:

  • We tend to now have children at a much later age (to the degree we are dangerously now having them close to menopause, or even beyond)
  • We tend to concentrate now on career, before children (to the point many live today with no desire to have children at all)
  • Our family size has decreased, to now even below replacement levels

Extend this trend now to being actually immortal (as in real immortality, not simply life extension):

  • You can expect us to seriously consider not have children at all. For whatever reason, perhaps we think we can always have them later, or other priorities seem more important, or we don't want to jeopardise our lifestyle
  • Careers become paramount- unlike other aspects of life, careers grow and we become expert at them the longer we do them. They also have the added benefit of providing more wealth and power over time to accumulate.

So, in your society, I would actually think the 10% would just become immortal, and stay that way. Not have children. Even keep copulation to a minimum (or at least, copulate a lot but with 'protection'). They would tend to find diversion in career, accumulate wealth as they go, until they have vast quantities of wealth. In time, they may even see wealth as uninteresting, and swap careers when they saturate their own, or 'travel'.

You may find, after exhausting aspects of career (political or otherwise), immortals simply become travellers, become nomadic and you won't really notice them much at all (after, of course, noticing very much their wealth in their 'early years').


I think immortality will be regarded by most immortals as infinitely precious and something that must be protected and preserved at all cost. Even at the expense of life itself. Let's assume the transition will be peacefull, because there is no detection and people will only notice when they don't age.. or when one of their parents starts to age. I'll focus on 2 rules,

Quote opening #1: If two "immortal" have a child, the child will also be "immortal". Parents would retain their status effect.

Quote opening #2: An "immortal" loses his/her immortality, if he/she does not copulate with another "immortal" in 50 years.

This will cause many immortals to isolate themselves in communities with very harsh religious rules, that guard against any contact with mortals and also against unplanned sexual intercourse between eachother. Sex is free, but actual procreation among immortals will become an assigned privilege, earned once every 50 years by good deeds, as viewed within the religious doctrine. Some of these immortal communities could become famous among the mortal population for their good wine, beer or cheese. But immortals won't become rich.. they will not take part in economic society, because that would involve contact with mortals. Immortals in these communities will become very old, very wise and very conservative.

And very bored.

Therefore, every few months, these communities will send a voluntary group of immortals into the outside world, individuals that want to escape this eternal riskless tranquility and travel elsewhere. Immortal people who choose to live elsewhere among other humans, and subject themselves to the risk of loosing their immortality. They may get rich or part of the elite, because they now have their immortality for sale. Some immortals will choose that road, because they prefer freedom. When they loose their immortality, a new immortal could be born. It depends on the cultural habits of the mortals what happens at that point. It could become tradition, to insist people leave society to join the immortals. After ca 20 years it becomes very clear to near ones, a person is immortal.. When it is a modern society, the immortal would be able to choose to join an immortal community.. and after 50-100 years they will probably do that, or found one of their own. Tired of all the loss of friends in the mortal world.

Cite opening question Given the rules, constrains and the transferable nature of immortality, how would the population of the "immortals" evolve over the years?

Quickly in the first 250 years, after that slowly. Humanity will have at least 25 years to prepare, before any effect kicks in. They will have 50 years to study the thing, before immortals realise they are mortal without having sex. Hoards of immortals will die from that, at first. Population rise really kicks in after about 100 years. Within that period, with now 6 generations surviving, the immortals will find that restraining their population is beneficial. Keep in mind according to Freud and others, an immortal person does not actually have the urge to procreate. In my imaginary, peaceful world based on your rules, procreation among immortals will become an arranged ritual.

Cite opening question The "immortals" could start selective "breeding" to increase the population of "immortals" once ALL the immortality transfer rules have been discovered. What is the timeframe for this to happen?

In our real world, if the conscious intent would arise to do this, racial wars would follow. Land disputes. There should be an incentive for a rise of animosity. Among the mortals, conspiracy theories could arise around immortals.

In my imaginary world, the immortal communities are not accessible for mortals from the beginning. The immortals would probably be prosecuted and expelled from mortal society. I don't believe mortals would resort in actual hostilities against the immortals, because they cannot kill the enemy. And for the immortals, there is never any reason for revenge, or excessive breeding to conquer the world. The immortals know they cannot overpopulate, they'll try to avoid that, lessons learned in the first 250 years. Immortals will isolate, avoid conflict, prefer segregation. During thousands of years, both societies will grow completely apart, and become totally different.


Them and Us

Eventually, once the rules are known, immortals will become the ruling class and the non immortal will be the disposable lower class.

A test will be developed to see who is and isn't then immortality will be traded. Some people will not be able to handle immortality and the rich would be willing to pay for it. Religion will be an issue. The immortals will be either revered or seen as cursed (as they will never receive reward in heaven)

Immortals will cause financial problems as they can accumulate wealth and never need to spend it. Eventually immortals will own all land and mortals will be excluded and forced to pay rent to their immortal landlords.

You could quite easily see war to address the inequality and immortals imprisoned for all time and forgotten.

Eventually, since immortals can breed more immortals, the world will become over populated with immortals and collapse. For the sake of your story, immortals shouldn't be able to breed with immortals. the population of immortals will be stable then.


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