The older a memory is the fewer details we remember. The importance or impact of an event can also drastically affect how much we remember about it, but even the most impactful will, presumably, eventually fade.

Most fictional races with immortality or extreme longevity seem to recall events from the distant past quite vividly, as though the last thousand years feels like a decade. Many of these races also possess superior intellect, which might help explain this.

If a being (or society) can live indefinitely, but has the same mental capacity as humans, how might the forgetting curve affect the them?

  • $\begingroup$ I think you could focus better the problem statement. This sounds like "how would X affect Y?" $\endgroup$
    – L.Dutch
    Jan 11, 2020 at 7:17

2 Answers 2


Psychology has a pretty well supported model for this called Interference Theory. There are several caveats to Interference Theory but the 3 key parts are:

  • Proactive interference: As you refine a memory through rehearsal, recalling the older version of it becomes harder.
  • Retroactive interference: As you learn new things, it becomes harder to recall things that you previously learned but have not rehearsed.
  • Output interference: As details are recalled from a memory with many details, your ability to recall all the details decreases.

In short, there is no real way to hold onto a memory forever. The more you ignore it, the more it fades, the more you recall it, the more it blurs. Eventually, even the strongest and most recalled memories become so distorted that there is no longer something tangible to remember. You are just recalling remembering something you remember remembering, and eventually it will just not come up in so long that the core of the memory is forgotten as well.

That said, the things we remember longest are the things that can not be grouped with other memories that we occasionally think about. An isolated event, like the 1 time you saw a clown perform in 1st grade can be remembered much longer and in more detail than any repeated pattern of happy holidays or traumatic abuse.

So, if your 500 year old elf has lived through a dozen wars, he will probably not remember the time he slaughtered women and children 300 years ago, but if he's only ever flown on a dragon once, he may be able to recall that he did that decades or even centuries later because it is so unique it is harder to interfere with.

Surrounding yourself with keepsakes are also a good way to preserve a memory. If he never wants to forget his war crime, he might keep a doll of one of the murdered children displayed in his great hall that he sees every so often reminding him to remember what he did. If he wants to remember the dragon, he might keep the saddle he wore. In time, the memory will blur away from output & proactive interference, but the key detail that he commited the crime or rode the dragon can be reinforced every time he sees for a very very long time.

This means if you want to make a believable scene where an elder being recalls something from centuries ago, you should put a keepsake in his displayed personal belongings that would remind him of that event.

Some Counter Intuitive Side Notes:

Emotionally powerful events that are not unique in your experience actually do not form stronger memories, they create stronger feedback loops. Psychologists who work in the field of trauma understand that the best treatment for PTSD is to "remember what really happened". When repeated trauma happens we often experience what is referred to as blacking it out. Basically we forget most of the details of the actual experiences as our minds try to compress the grouping of memories together, but because we try to think about it and remember it so often, we create false associations that generalize and blur much further into other areas of our life than it should. By analyzing what did happen you can make your brain re-evaluate the false associations which is how you keep every day things from triggering trauma inspired reactions. But here is the kicker: what you remember in PTSD treatment is often not accurate, nor does that matter. The treatment is to create a contained narrative that you can believe in so that your mind can create boundaries between when you were in danger and how you now are safe.

Also, memory is not infinite. While many neurologists will claim that the brain can have a virtually unlimited number of patterns, that does not make the amount of useful information it can store unlimited. Think of it like this: A comment on SE can contain 600 characters. If you store that in a UTF-8 format, that is about 8.8x10^1444 possible data configuration... however, that space can only be used to contain a single, relatively simple concept because almost every possible combination of values would cause a text parsing error, of those that don't almost every combination results in gibberish, and of those that don't almost everything you could say would be meaningless to the conversation, and once that set of 600 characters is consumed, you can not use it to store anything else without changing the meaning of what is already stored there. We know from studying brain structures like the hypothalamus that the mind uses many many techniques to minify how much it stores, and that failure to do so causes retardation because our brains can not function when you remember too much. In recent years, many neurologists and computer scientists have been studying each other's work and finding that most of the 'illogical' things that people do are consistent with highly optimized usages of limited resources.

  • $\begingroup$ Really nice Answer. Can you include or add sources especially for the last claim? I would like to read into that. $\endgroup$
    – Soan
    Jan 14, 2020 at 20:32
  • $\begingroup$ For the last claim, read: amazon.com/Algorithms-Live-Computer-Science-Decisions/dp/…. Most of the other information is derived from various psychology classes I took in college: General Psychology, Abnormal Psychology, & Neuropsychology; though, I do not recall the exact text-books we used. $\endgroup$
    – Nosajimiki
    Jan 15, 2020 at 3:51

Memory doesn't work that way (we think). Mostly because (we think) memory doesn't actually exist. By "we think" I mean "the current scientific understanding available to us in the literature". The truth is that memory is still a mystery. Absolutely nobody understands what memory is or how it works. As such, we don't understand memory nearly well enough to answer your question with any certainty.

Short Answer

However, working off current theories of memory, I'll give a broad answer: If humans lived forever (or a being lived forever but had a human-like brain), then they would be able to remember the most important and/or emotionally-impacting events of their lives forever.

Long Answer

Memory likely doesn't exist. It's not a physical "thing" that can be measured. From what I have available to me, memory is actually a series of firing neurons in the brain. That is, when you remember something, you're just convincing your brain to re-fire that neural pathway that was fired once before. But if memory is a mystery, then willful recollection of past events is basically an enigmatic black-magic box. Whenever we think or feel, neurons push closer together or pull further apart - very slightly. This allows the "signals" between the neurons to be stronger or weaker. Rinse and repeat with the estimated 100,000,000 (hundred million) neurons in the human brain, and you get an absurdly complex pattern of near-infinite pathways. When we say a "memory fades", we mean "that pathway no longer exists", or rather "those neurons are no longer in the right order to be fired again".

As such your question begins with a false assumption: memory does NOT fade across time. It seems like that to us, but it's not true. What's happening is we have more thoughts, experiences, and emotions which pushes and pulls those neurons closer or further away, and the firing patterns change. It's these changes which make it "more difficult to recall" a memory. But that's not the memory fading, it's the brain adapting to new experiences. It's changing neural pathways, not a "fading memory".

But people who experience emotionally-engaging (that is, "ferociously active neural experiences") events are more likely to remember them because those neural pathways were set hard. This can be positive or negative: falling in love or a vicious rape; getting a promotion or being mugged; getting married or getting divorced; a first kiss or losing a friend. Whatever is intense neurally is going to "cement a pathway" - ie, push and pull those neurons in a way that the firing signals are strong. When these "neural pathways" are set, (a) it takes way more effort to change them (either an equally intense experience, like head trauma, or a crap ton of mundane experiences), and (b) people are more likely to continually recall (re-fire and re-cement) those pathways.

By "continually recall" I don't mean "directly think about the experience", but "build upon those pathways by re-firing parts of them". For example, after falling in love, you're more likely to keep kissing, hugging, holding, cuddling, talk with, go on dates/share experiences with that person. So those pathways in your brain are building upon the prior experience. So it is on the negative side: the abused child runs from trusting others, the raped woman avoids mutually shared experiences with men, the PTSD victim has panic attacks, etc. To fall out of love takes a lot (either a "strong" experience, like catching them cheating on you with your best friend, or a lot of weak experiences, like "we just kinda drifted away.... for the last decade"). Similarly, to overcome trauma also takes a lot (either a "strong" experience, such as religious or medically-intervened solution, or a lot of weak experiences, like working day-in and day-out to control and push back the PTSD until it finally disappears).

So, you see, if a human-like brain were to continue indefinitely, so long as that neural pathway holds, the memory will hold. As such the "important things" (ie things you keep thinking / reciting to yourself), and "emotional things" (ie strong experiences) will last indefinitely. It takes new experiences to slowly pull those neurons closer together or further apart.

That's how you can forget what you ate for breakfast but remember that douchebag from 2nd grade like it just happened five minutes ago.

Final Note - you'll never run out of capacity.

If this theory of memory is correct, the brain can hold about 100,000,000! memories (about one hundred million factorial). Most calculators will give you an error or just say "infinity" if you try to calculate that.

A few years ago, for the novelty of it, I did an estimated calculation of this value. I forget what it was precisely, but I was able to estimate that if you formed a trillion memories for every atom in the universe at a rate of a trillion times per second, then after a trillion years you would finally accomplish 1% of the total number of neural pathways your brain could handle at one time.

So no, you'll never "run out of space". There's always room to hold onto what's important while adapting to new experiences ... even if you keep living until the end of the universe :)

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Jan 12, 2020 at 22:39

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