A mutation or virus (we do not know) has suddenly modified humanity by making people live indefinitely though cycles of aging (to 70 yo) and then rejuvenating (to 3 yo).

  • This does not make people immortal: they are still vulnerable to typical accidents, we just rule out the death by aging and illnesses. The birth rate had homogeneously dropped worldwide so that it compensates these accidental deaths. This way the world stays at today's ~7 billion inhabitants.

  • People are spontaneously aware of the change (an alternative could be to have them discover this as they go, I believe this would just be an initial perturbation leading to stability equivalent to them having been made aware of the change spontaneously).

  • People keep their memories through the cycles (their experience is cumulative across the cycles).

How would that change our world?

This is an idea I had under the shower so feel free to make reasonable assumptions on what I could have missed

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    $\begingroup$ Related (definitely not duplicate): worldbuilding.stackexchange.com/questions/25242/… $\endgroup$ – Green Oct 19 '15 at 22:12
  • $\begingroup$ Could you provide some extra detail on precisely how much of a person's memory survives and how much of their mind survives the regression from 70 to 3? Does their mind stay the same in that regression or does it regress back to a 3yo? $\endgroup$ – Green Oct 19 '15 at 22:14
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    $\begingroup$ @Green: sorry for not having been clear: their body gets older / younger but their mind evolves as if they were aging linearly (they retain their memories, their experience, etc.). There is no senility - they get more experienced with time. Their brain basically learns the same way we do from 0 to death. $\endgroup$ – WoJ Oct 19 '15 at 22:30
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    $\begingroup$ may I suggest a undulating from age 70 to age 18 instead of age 3? There are a lot of morphological changes to the human head between 3 and 18 that will need to be reversed if going from 70 back to 3. I suggest 18 because the majority of bone growth is finished by that age. It simplifies any physical changes. $\endgroup$ – Green Oct 19 '15 at 22:34
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    $\begingroup$ Also, this question is fascinating but it's really broad. Trying to figure out how all of human society will change makes for some very long answers. $\endgroup$ – Green Oct 19 '15 at 22:36

People would be a lot more careful!

There's a lot of things that you can't regrow at 3yr old. Many anatomical structures grow in the womb, and are very hard to reproduce after the fact. Everything you develop before you are 3 would be considered remarkably essential to preserve, so people would be very careful not to lose it.

Assuming they magically are brought back to "what they were at 3 yrs old," with perfect regeneration, the pattern of lifecycles may be dwarfed by the mere ability to live as long as you like. Depending on how you manage that one in your storyline, the actual lifecycle may matter little at all, or you may be able to make it an essential part of how people go about their existence.

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    $\begingroup$ There's a lot of things I can't regrow at 30-something, too. I don't understand your point at all. Are you suggesting that because people will have to live with scars/burns/amputations for multiple life cycles, they'll behave in a way to minimize the risk? I don't buy it - I'm already minimizing my risk for the one life I've got. $\endgroup$ – Nuclear Wang May 24 '17 at 12:55
  • $\begingroup$ @NuclearWang You're right. It's the same game, new odds. You're minimizing risk for the life you've got. Statistically speaking, its 78 years long, so you can do 1/78th of the total damage your body can endure every year, statistically speaking. Now add regeneration into the mix. Now let's say you make it an average of 500 years before making a fatal mistake. Now you can only do 1/500th of the total damage your body can endure every year, but you get to ignore the damage that is regeneratable. People will have to pay attention to the long game much more. $\endgroup$ – Cort Ammon May 24 '17 at 15:03
  • $\begingroup$ It's not like the body is something that slowly wears down, eroding until it just falls apart - you talk about "total damage" like it's HP in a video game. I can endure scrapes and bruises on a daily basis with no long-term effects, and that doesn't change if I live 78 years or 780. I take good care to avoid things that do have long-term effects, like losing a leg or a hand, and that also won't change no matter how long I live. It's not like I'm thinking, I'll do this thing that might cost me an arm, but it's OK since I'll only have to live with one arm for another 40 years instead of 400. $\endgroup$ – Nuclear Wang May 24 '17 at 15:33
  • $\begingroup$ @NuclearWang That is true for the class of effects which are repairable. I'm focusing on the ones that won't be repaired. And you do in fact think like "I'll do this thing that might cost me an arm, but it's OK"... we call it driving. Every day we risk life and limb, but the risk is worth the cost. $\endgroup$ – Cort Ammon May 24 '17 at 15:38

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