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We’ve all read those Isekai mangas... or at least I have... but how realistic would they really be put into practice?

Say a normal average dude suddenly gets caught in some weird temporal event that sends his mind back in time to his five year old body. He is nothing special intelligence wise, but he’s also not stupid. He did well in high school, and went to college for something science based and graduated in the top five of his class. He’s smart, but not in any inherent way, rather just from hard work.

  1. How much of his memories and knowledge could his brain feasibly retain? I’ve read that children’s brains are like sponges, but that they also are still developing so might not have the capacity to retain a whole life’s worth of memories? Also, he was not a genius child growing up, so his brain would not look like the brain of a gifted child which could affect things, especially considering:

When analyzing the working memories of the prodigies, scientists came across an amazing result. Each of the eight prodigies had a working memory which put them in the top 1% of the population and each of them made it to the top 99th percentile in terms of working memory capacity.

Additionally, I’d assume there would be psychological issues that would stem from such an extreme influx of information to the brain. Perhaps the brain would simply lock away parts of his memory or knowledge until he’s older and his brain is more able to deal with it? I’m not really sure if there’s anything comparable in real life to this sort of ‘brain info dump’ but it would surely be comparable to traumatic events in childhood being forcibly forgotten.

  1. Dependent on the answer to 1, how much of a genius would he be considered? It’s not like he has any increased ability, but rather that he just already knew more than a child ever would. So once he reaches the limit of said knowledge would he cease being considered a genius?
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    $\begingroup$ What little scientific knowledge I ever had is now several decades out of date, but last time I checked, we had no idea how memory worked biochemically and the theories which did exist couldn't be tested empirically. We also had no numerically accurate way of measuring the contents of a brain of any age. So "facts" about the capacity of young brains were unsubstantiated then and probably are now. Add to that the cognitive capacity that is packed into a crow's miniscule brain and arguments about brain size relativity to intellect fly out the window. Let your time traveler remember everything! $\endgroup$ Aug 24, 2021 at 2:19
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    $\begingroup$ A few more comments, also unworthy of an answer... Your traveler would be considered a genius for as long as they stayed ahead of the normal learning curve and since at 5 years old, they don't need to be investing their time in 5 year old studies, they could invest them instead in whatever level studies their adult self was ready for. With this much a lead, they could stay ahead of the pack for a very long time. However my use of the word invest is not accidental. Your traveler would not need to wow academia to grow powerful, they already have enough future knowledge to own the stock market. $\endgroup$ Aug 24, 2021 at 2:44
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    $\begingroup$ It would be nice (and proper) to at least link the source for your blockquote. As for your question, isekai worlds usually have magic. Most isekai authors also assume that souls exist and transmigration does not affect the soul, it just 'places' it into a new body. Another assumption common to this genre is that souls and bodies are fully independent entities and the body does not affect cognitive functions. Could you please state what kind of assumptions are you making for your world? The answers may differ depending on the existence of magic and your solution to body-soul dualism. $\endgroup$
    – Otkin
    Aug 24, 2021 at 3:20
  • $\begingroup$ To clarify my point about magic: In most isekai universes transmigration is powered by magic if the process is explained or it is left completely unexplained, but we can safely assume that the process is magical rather than technological in its nature (based on available descriptions of technology). $\endgroup$
    – Otkin
    Aug 24, 2021 at 3:25
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    $\begingroup$ "realistically" you would die instantly, or at least be reduced to a vegetative state. People tend to think of their bodies as a fixed, if growing, thing, but that's not really true--bodies are constantly shifting and shedding/replacing millions and millions of cells. On a per-neuron level, a human's brain is probably nigh unrecognizable with 20+ years of growth and learning compared to the 5 y/o brain. Human memories are not like computer files that you can simply 'copy over', they're more like printed books: you can't just scrape letters off one book and sprinkle them on another to transfer $\endgroup$
    – Dragongeek
    Aug 24, 2021 at 7:04

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Five year old brains are about as big as adult brains.

According to a site I found with a quick Google search, a five year old's brain has approximately the same mass as an adult brain.

While the structure of that brain would undergo considerable refinement through the development and pruning of neural connections during childhood and puberty, if an adult's mind was dropped into the brain of a five year old, presumably the five year old's brain would have the mass to contain the adult mind.

However, it may not remain similar for too long, since the natural processes of growth would likely result in unused memories and skills being pruned away.

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    $\begingroup$ But size is not the only thing that matters when it comes to brains. Another quick Google search showed that the lateralization strength for visuospatial memory continued to develop in children between 6 and 16, but not much for language production. I'm not a neurologist, but if I understand this correctly this would mean that the adult will find little impairment of their language skills, but they may find e.g. orientation more difficult than when they still had their adult brains. This might impair their genius status quite a bit. $\endgroup$
    – Schmuddi
    Aug 24, 2021 at 10:53
  • $\begingroup$ @Schmuddi the only way to transfer a mind is to overwrite the existing neural connections, so as long as there is enough room most of a Childs neural connections won't matter, you are basically making a copy of the adult neural connection pattern with fresh cells. much of the learning children do is taking unspecialized cells and making them specialized. $\endgroup$
    – John
    Aug 24, 2021 at 20:23
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The real difficulty will be with other people. All of a sudden, this five year old won't want to play with other kids or play like they do. The discussions with adults will leave him very dissatisfied. Most adults will discount whatever he says. When we look at underaged people (12 yr old's, etc.) in college, they have a lot of trouble being taken seriously by those older. The "shortest person in the army" had to be twice as tough to be taken seriously.

Anytime someone who has grown to expect certain reactions from others, when that suddenly goes away, there is a period of "disconnect" and real discomfort. If he were to try to demand adult respect, he would receive strong and likely physical reactions. There would be a real chance that this person would be committed for being insane. (See the book, Black like Me, and how the disconnect between being white one day and black the next started to be a real burden.)

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It should mostly fit just fine.

this is just to add to Nick012000's answer, much of a child's learning is taking unspecialized cells and turning them into specialized networks via pruning and myelination. We can actually see that in things like learning to read vs not. Existing specialized networks expand to use the space unless they are used for something first (like learning to read)

So as long a there is enough cells a adult brain should just overwrite a child's brain just fine. BUT by 5 years old only 90% of the brain mass is there, so some losses may occur but what if anything would be lost is difficult to say.

But lets try a little wild ass speculation, most of the rest of the mass develops during puberty so much of it may be sexual dealing with preference and hormonal production since the brain has not hit puberty yet. so they may use some emotional connection to previous sexual partners and preferences.

Other considerations

Their motor control will not be great, their limbs and balance is different so they have to relearn some of that.

What they know will be more flexible than an adults, some new neurons can migrate in to place so learning should be a little more flexible and fast than an adult.

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Regarding the capacity of the brain to hold memories and knowledge ... unless there have been some radical new discoveries in the last few years that I haven't read about, I don't think anyone really know how 99% of this works. Is a child's brain still developing so that it is incapable of holding all the knowledge of an adult? Or is what is "developing" the accumulation of knowledge and experience, so that the capacity is there but is just unfilled? To the best of my knowledge, no one knows. The workings of the brain are still very mysterious.

In any case ... I have no idea what "Isekai manga" is. But if you're going to postulate that there is some technology or some magic that can send a person's mind decades into the past, why not simply also postulate that any problems with transplanting this knowledge and memories into a child's mind are solved? Like, if I was going to write a science fiction story where the hero can read minds, and I simply declare that he can do this without any explanation of how it works, I can't imagine that I would suddenly be caught up short worrying about, "But would whatever signals travel from mind to mind be blocked if the subject is wearing metal earrings?" (Not unless I was going to use such a thing as a plot point.) I mean, if you're going to assume that some big technical problem is solved for the sake of a story, I think it is fair to assume that little technical problems are also solved.

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  • $\begingroup$ "I have no idea what "Isekai manga" is." Manga are Japanese comics. "Isekai" is a genre whose name means "another world", that involves the protagonist being sent to another world (usually some sort of fantasy world) from our world. $\endgroup$
    – nick012000
    Aug 24, 2021 at 21:56
  • $\begingroup$ "I have no idea what "Isekai manga"... or search engines, apparently. The first result from Google is the Wikipedia article on them. I have added the link to said article to the question. $\endgroup$
    – Matthew
    Aug 25, 2021 at 15:57
  • $\begingroup$ Saying "I have no idea what 'Isekai manga' is." is a valid frame challenge to "We’ve all read those Isekai mangas". Though I would have personally put that in the OP's comment section rather than in the answer since it is trivial to the actual answer. $\endgroup$
    – Nosajimiki
    Aug 25, 2021 at 16:01
  • $\begingroup$ @Nosajimiki It appeared to me that the question was complete and answerable without knowing what "Isekai manga" is, so I wrote an answer despite that point of ignorance. If I missed some key point because of that ... sorry. $\endgroup$
    – Jay
    Aug 26, 2021 at 2:43
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We don't know the limit of human memory.

There's billions of neurones, and trillions of connections. Presumably there's an upper limit for memory. But, it's not reached in human life, and child brains aren't much smaller than adult brains. They could probably store everything.

That said, memories get altered a lot in childhood because children's brains are very plastic. They'd lose a lot of knowledge over time.

They would probably forget most of the stuff they didn't focus on.

Memories aren't locked away, or removed. If you don't access a memory for a very long time, the connection can fade, or be reused for something else. They'd presumably keep the stuff that was useful or especially interesting and prominent, and forget the stuff that didn't matter.

He would look very smart, but he'd not do well in elite genius classes.

There's no shortage of adults who do their children's work. There's lots of smart professionals who don't want their children to do meaningless work and so do their homework for them or tell them what to do in exams.

They tend to get a B or a low A grade at best. They don't know the teachers or the exams that well, and exams don't grade on pure science knowledge, they grade on key word usage and fluent concept usage and other obscure things that require knowing the exams well.

They'd look like a bright kid, who knew a lot of stuff, and they'd be able to pull ahead of their peers, but they wouldn't look like an elite genius, or like an experienced tutor who has memorized the exam curriculum and past papers.

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Your brain is not done being built by the age of 5

Though many brain patterning processes are complete at birth, the human brain exhibits further dramatic biological development during the preschool years, and roughly quadruples in weight before the age of six.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3511633/

The physical layout of a 5 year old's brain is different than an adult brain; so, you can't just put a adult into a child's brain any more than you can install iOS on an Android phone. It's not just a matter of size, but these brains fundamentally function differently. It would be best to wait until your younger self is at least 10-12 years old, since this is when your brain reaches its full size, and all of your vital structures will more-or-less be in place.

Also... it would kill you

The bigger concern though of cramming 20+ years of memories and experience into the 5 year old's brain would be that the cranium is too small. When you go to convert all those unspecialized brain cells into specialized ones, they get bigger. While other answers have dismissed this 10% size difference as not too big of a deal, it is actually a huge deal. This would result in your brain swelling which would expose it to fatal pressures that would closely resemble a very severe case of Meningitis/Hydrocephalus.

It may also be necessary to slowly build up your past brain ahead of schedule rather than all at once. Your brain is not a programmable computer, so much as network of physical connections. This means that any sudden dramatic changes to it require physically disconnecting and re-attaching a bunch of stuff which leaves tons of room for things to go wrong. Think of it like installing incremental patches. If you have an old OS, and want a newer version of it, the safest thing to do is to install each incremental patch in the order it was released. The only way to go straight from Windows 10.0 to Windows 10.21H2 is to completely delete Windows 10 and start with a fresh install of the fully patched OS, but in the case of a human brain, you can't just delete the old brain and put a new one in your little 5yr old scull. So, the safest approach would be to start with the youngest memories of you, and add them in the order you learned them... this way each introduced memory causes the mind to reshape in the right order using manageably small changes to build up your neural network without damaging it or having older you memories forming in shapes that prevent younger you memories from being able to be made.

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Instantaneous insanity?
Or possibly catastrophic misfire manifesting as unending epileptic seizure?

A very large part of the brain's storage and organization is physical, depending on a specific pattern of nerve cells and dendrites to have died off and atrophied in the correct pattern. Imposing the informational structure of a mature brain on the physical structure of a young brain (or even on another person's mature brain), will cause a catastrophic mismatch between the hardware and software aspects of the brain.

I strongly advise that the approach turn away from "dumping" one mind into another brain, and rather focus on rapidly teaching the new brain all the knowledge and habits of the old brain. That way the new brain has at least a chance of adapting to the flood of new content before performing a BSOD in panic.

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There are a lot of things we don't know about the brain. The idea could work, but taking some assumptions even though evidence would be scant. First of all the size of the brain considering a rough count of the cells number would be similar, but a child brain has much less connections, how much the extra connection matter for the memories we don't know, but they may have a role, and probably they have a role also in the reasoning capabilities. Let's assume that by transferring the memories the brain also quickly rewires itself, would it be enough? Probably not, all our memories are bound together in a network of logical references, often to recall something first you have to recall something else that is related.

With the above considerations I don't think that any transfer of memory would have an immediate effect, the child would have to rethink those memories over and over again to become fully aware. The process would be slow and rather than appearing a genius for a while the child would just appear absent minded. But going forward things would change, the possibility to redo a second time the entire learning process with a lot of hindsight might be a great opportunity. The downside is that with a lot of things to think about he will never appear the bright child with the quick answer. If he had the chance to become a genius that would happen slowly.

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