For purposes of this question, reincarnation works in the following manner:

  • The transmissible personality, memories skills, knowledge and experiences of a person shall be referred to in this question as a Soul.

  • During life, the soul develops alongside the body. At death (the moment that the soul's body ceases to be a suitable vessel to hold it, some time after the cessation of all processes of life), the soul becomes separate from the body.

  • Some time after the death of its previous body, the soul may attach to a newborn member of the same species. As the newborn matures, the soul progressively integrates with the body and becomes fully integrated at physical maturity. The period of time between death and reincarnation may be as little as a second, or may be many lifetimes, though it is most commonly a relatively short period of time. It may be speculated that one or more 'bad' lives may delay reincarnation, however good lives may also delay reincarnation. There is some correlation between the reincarnation times of a particular soul. Reincarnation is not guaranteed.

  • While reincarnation often occurs in a region in which the reincarnated individual lived, it need not always do so, and could theoretically occur at interstellar distances (with lightspeed delays). Reincarnation may occur into a newborn of any race or gender of its species provided that the newborn has a reasonably normal brain.

  • A reincarnated individual gains the mental skills, knowledge and memories of the soul which attaches to it.

  • a newborn without a reincarnated soul is not born with a soul of its own, it develops one at some point during adolescence.

  • The effects of brain injury and/or illnesses such as dementia do not become part of the soul. A soul may be considered to function additively, and has no mechanism for externally applied subtractive processes.

  • A soul's content degrades slightly between reincarnations. A soul which has reincarnated many times may typically remember roughly ten previous lifetimes, more if the previous lives were short, or less if the previous lives were long. More recent previous lives are more clearly remembered than older lives.

  • Rarely (on the order of 1 in 100,000 reincarnations), a soul may reincarnate into two newborns simultaneously (taking lightspeed delays into consideration). Such duplicate reincarnations lead to two separate souls, the souls do not become one again after the death of both of the bodies.

  • Where the species' population numbers are rising, there will be many newborns who are not born with a reincarnated soul. Where the species' population numbers are constant or falling, more newborns will have reincarnated souls, to the point where a newborn without a reincarnated soul is very rare.

Obviously there would be profound social implications for this sort of reincarnation, but they are not the point of this question. However, the social implications will have to be addressed if they could affect the evolution of the species.

My question is, What effect would reincarnation have on a species?

As an example, humans do not demonstrably, frequently and reliably reincarnate. However, for purposes of this question, let us suppose that at the time that hominids speciated from chimpanzees and bonobos, roughly 5.5 million years ago, the hominids gained the ability to reincarnate. If all other selection pressures remained effectively identical and evolution progressed similarly wherever reincarnation has no evolutionary effect, what differences could we expect between real-life humans and reincarnating humans after a similar period of time since speciation 5.5mya?

These differences need not be purely physical, and may be differences in the basic psychology of the species.


There seems to be some confusion as to how souls work. They do not function as a metaphysical cloud storage that's accessible at any time, like a google drive account that's able to have its content downloaded into RAM relatively quickly.

Rather, a soul's content must be copied into the physiological memory of the body during maturation before it becomes accessible, and any content that is in excess to that which the body can retain is culled according to the usual physiological processes that discard or overwrite less useful memories.

After the body's brain is physiologically mature, the soul isn't able to add much more of its content to it, perhaps only 5% of that gained during maturation.

The process by which memories of former lives may be lost is that memories successfully written into the body are removed from the metaphysical soul. During life, a new soul is written from the body's brain. When the body dies, the remainder of the old soul is copied in a slightly lossy manner to the new soul before the new soul goes on to be reincarnated... when and if it does so at all.

Perhaps 1 in 20 souls do not reincarnate at all, and perhaps 1 in 6 do not reincarnate 'immediately' (within a single typical lifespan of a member of the species).

  • $\begingroup$ Do you have karma associated with your reincarnation? So better souls get better bodies, while the "evil" souls are only able to inhabit low-quality hosts? $\endgroup$
    – DWKraus
    Commented Oct 13, 2021 at 1:25
  • $\begingroup$ Won't work well interspecies, even less interplanetary. $\endgroup$ Commented Oct 13, 2021 at 1:49
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    $\begingroup$ Evolution is about increasing fitness for the environment as measured by surviving long enough to produce the maximum possible number of offspring to pass one one's genes. And the problem is that there's no feedback loop that affects the soul; a soul that produced no offspring or failed to raise them properly suffers no penalty for it and can continue to do so reincarnation after reincarnation. You need to specify some mechanisms for the body to affect the soul in order for evolution to have any effect. $\endgroup$ Commented Oct 13, 2021 at 2:04
  • $\begingroup$ @DWKraus There is no karma. Aside from reincarnation only functioning within a species, a given soul's chance of reincarnating into a body with particular traits is not significantly different to the natural distribution of bodies with those traits. $\endgroup$
    – Monty Wild
    Commented Oct 13, 2021 at 7:27
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    $\begingroup$ Extremism and social pressures (such as "Your body sucks, get rid of it already, don't make kids this time around") would probably be pretty common. $\endgroup$ Commented Oct 13, 2021 at 17:50

4 Answers 4


One thing to consider is that accumulation of experience as a creature ages tends to soldifiy past beliefs and patterns behavior to a degree that they no longer are changeable. "You can't teach an old dog new tricks" as they say and we've all met older people which we politely describe as "set in their ways". However, this can be highly deleterious; the learned behaviors of an individual who has a lifetime of experience for survival in a jungle will not be all that useful when transmigrated to the body of an individual born in a sub-arctic environment.

Its unclear how the body affects the soul in this scenario, but it seems likely that there will be strong evolutionary pressure toward the neuroplasticity of childhood to last into adulthood to allow the individual to learn new behaviors and overwrite old ones that are harmful in the soul's new environment as well as for increased propensity for risk-taking to help the individual overcome no longer appropriate learned behaviors.

  • $\begingroup$ Children learn faster as their hormones dictate not only when to grow taller or get facial hair, but also how fast connections are made and destroyed. As there's also new initially 'soulless' people and as you say they can go to a different environment, neuroplasticity is pretty much guaranteed imo. $\endgroup$
    – Trioxidane
    Commented Oct 13, 2021 at 5:23
  • $\begingroup$ @Trioxidane The question mentions that "...the soul progressively integrates with the body and becomes fully integrated at physical maturity..." so it's unclear whether the memory of previous lives are fully accessible during the childhood years, hence the need for neuroplasticity to last longer. $\endgroup$ Commented Oct 13, 2021 at 6:12
  • $\begingroup$ I agree with your second paragraph but I want to add that we don't know that "accumulation of experience solidify past beliefs". We know that your beliefs solidify as you get older and that you get experience as you get older. But it could be all linked to your physiology and nothing to the accumulation of experience itself. $\endgroup$
    – Jemox
    Commented Oct 13, 2021 at 13:54

Regression in both evolution and also society - and it may be quickly lost.

The important thing to remember about evolution is that it is the genes that are important, not the individual. The incremental improvement of genes relies upon the death and selecting out of non-beneficial genes.

So if an organism, say a flying insect such as a house fly, flies into a spiders web and is eaten - it is reborn and remembers now where the spiders web is, and avoids it.

This pattern repeats many times, meaning two things: The fly never actually dies (although eaten) and does not evolve a keen sense of sight to detect the web, and the spider has to also evolve and step up its game in order to catch the fly (perhaps using more and more larger webs, or creating a situation where the fly has no ability to evade the web even if it remembers where it is). Both organisms adapt gradually, however the fly regressively so.

Basically, the removal of 'survival' imperative from the genome means the genome does not have as much incentive to incrementally improve. The ability to transfer souls would actually over time would actually be lost.

This also happens to Homo Sapiens in society. If someone dies, but their soul appears in another, there is no real incentive to stay alive. Some of the worlds best inventions are based on necessity, and perhaps even the fear of death - remove this and the tenuous web of society may never have formed.

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    $\begingroup$ @DWKraus But the difference here is you are actually reborn - effectively with personality, memories and 'by extension' actually are the same person - just younger. So with the complete removal of Risk, there is absolutely no incentive to fear death, you are the same person as the guy that was born 10,000 years ago. This is different than religion, as when we are born we are still just speculating what would happen after we die. The finality of death is still very real to all of us, but if we instead know we were the same person - this is very very different. $\endgroup$
    – flox
    Commented Oct 13, 2021 at 2:10
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    $\begingroup$ I don't have an incentive to die playing a video game if there are no save points. I'll have to start all over at the beginning, spending all that time working my way through the levels. Plus, memory is lost over time, so eventually the current you WILL FADE away. But there would be different pressures on people, certainly. I just disagree it would lead to the destruction of the species involved. There might be negative cultural viruses that could spread, however (say, a fanatical cult that converts by brainwashing and destroys rivals or backsliders) that would harm the species. $\endgroup$
    – DWKraus
    Commented Oct 13, 2021 at 2:21
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    $\begingroup$ It's more like losing a D&D character you've been playing for years and starting over again at first level. Sure, you can do it, but it's not an upgrade unless the body you were in was kind of sucky. In fact, it may create a directed evolution for BETTER bodies, as those with flaws are weeded out by people killing themselves without reproducing. It's why I suggested a supportive culture and that reproduction would need to be more hormonal and less a choice - society cares for the children, less so the individuals birthing them. $\endgroup$
    – DWKraus
    Commented Oct 13, 2021 at 2:35
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    $\begingroup$ Hmmm... there's that nagging "Reincarnation is not guaranteed." in the body of the question. How does this play into your "... the fear of death - remove this and ...", @flox? $\endgroup$ Commented Oct 13, 2021 at 2:51
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    $\begingroup$ I don't think the memory aspect of this answer makes much sense. Flies don't avoid spiderwebs by memorizing where every one is - they avoid them by sight, which has biologically heritable component. A fly that flies into a spiderweb because it has poor eyesight won't get to pass on its genes, and won't do any better in the next lifetime given a similar body. The fly that has better eyesight genes can procreate and pass on its genes to the next generation. Also, there's still plenty of reason to fear death - I like my life, I don't want to start it over from scratch. $\endgroup$ Commented Oct 13, 2021 at 12:40

Clever, Strong, but with Poor Memory:

Your species are subject to the same forces that affect anything else. Evolution only drives the development of things that are rigorously reinforced, and leads to the degeneration of traits over time with low or no survival advantage. So what happens with your species with reincarnation?

Your individuals are able to survive despite having a poor memory, because your souls are providing multiple lifetimes of experience to the bodies they inhabit. An organism doesn't need a great memory, but it does benefit from being clever.

Culture would evolve somewhat independently from biology, and I think there would be a strong pressure for a more altruistic culture with collective child-rearing. There may be some social pressure to weed out souls with poor attitudes, but how this would manifest would depend greatly on choices and how your reincarnation system worked.

I'm assuming souls have some options in what bodies to inhabit, so whatever traits the souls prefer will create a pressure for bodies to match it. Everyone wants a clever host. And physical prowess disproportional to the needs of biology may be selected for due to the souls desiring to be strong. I'll assume the souls can somehow evaluate whom they inhabit as well (either through genetics or the ability to perceive the mother).

Reproduction may need to be a very hormone-driven process, as intelligent organisms will intellectually feel less tied to their bodies and will be less driven to have kids. Kids will grow much faster, and need a greatly reduced period of learning, since memories of past lives mean they are ready to be adults intellectually the moment they are born.

Then there are surely opinion pressures that will matter based on what the souls prefer. If souls remember being bonobos, then they may tend to select bodies that are bonobo-like. If they like variety, then the bodies will become dimorphic, so the souls can mix things up between lives. If there are standards of physical beauty, they may inhabit the offspring of pretty females.

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    $\begingroup$ Intelligent reincarnating species might find children incredibly important, as it allows themselves to be reincarnated and continue inparting their wisdom on society. $\endgroup$
    – Trioxidane
    Commented Oct 13, 2021 at 5:20
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    $\begingroup$ In an expanding population, there will be more kids being born than people dying to be reincarnated into them. Depending upon the rate of population growth, the first-timers being born may very well considerably outnumber the reincarnated. $\endgroup$
    – Monty Wild
    Commented Oct 13, 2021 at 7:22
  • $\begingroup$ Additionally, I probably didn't explain properly that a body must be able to physically retain the skills and memories it inherits from its soul in order to make use of them. The soul doesn't act as a metaphysical cloud storage mechanism that's accessible at any time, its knowledge is impressed upon the body during maturation. $\endgroup$
    – Monty Wild
    Commented Oct 14, 2021 at 0:14
  • $\begingroup$ @MontyWild Impressed knowledge wouldn't need to follow the standard mechanisms of memory and learning, so I don't see the problem. It's not like playing a movie, then remembering the movie, is it? If so, they would be more like the Neanderthals portrayed in Clan of the Cave Bear, where they have heritable memories from their ancestors. In that case, memory would be the important factor and intelligence less relevant. And I suggested collective child care for the dual reason of separating inheritance from parentage and also protecting those without the advantages of prior lives. $\endgroup$
    – DWKraus
    Commented Oct 14, 2021 at 0:32

Very little change

Since the memory is not immediately copied and can easily be over written, it is unlikely that reincarnated people will have much if any advantage. Therefore, not much change will be imparted upon the species in the form of beneficial mutations.

here lies andy; peperony and chease

In the Paleolithic Era, only around 60% of humans even made it to be 15 years old. So a person might not even know if they were reincarnated, since they are about as likely to die first as they are to survive.

I have no memory of this place.

One thing a prehistoric person might want to remember is the migration patterns of prey, the creation of basic tools, or which plants are poisonous. However, by the age the reincarnation memories kick in these people will probably already know this stuff. Furthermore, the migration patterns of arctic prey is of little concern to someone living in a desert. So unless they reincarnated very close to their last life, it is possible all this information will get overwritten, or at least ignored, since it isn't helpful. Also, if they remember that they were a tool maker in a previous life, but now they are a hunter, if hunting is the better profession, that information isn't very useful.

Intercontinental Tool design collaboration

One advantage would be remembering previous tool designs. This might have a minor effect on the effectiveness of tools over the entire population. This means the whole world would have the most effective bows, slings, knives, and fire starters for their region after a few cycles. This wouldn't require any changes to the genome, and any change that could be made won't help.

prehistoric Career fair

One of the few advantages this provides is if the previous life did a job that is difficult to learn or teach, that can also be done here, the reincarnated individual can do that job. If this person were a leader this also applies. However this has very little effect on the society, and therefore modifying everyone's brains to have more space to maybe remember more lives would have a negative instead of a positive effect.


While a few people in the prehistoric times could use this to their advantage, the effects are too small to make changes to the species beneficial enough to justify the potential cost. The ability to learn from your ancestors would help people, but many people during this time learned from their ancestors by talking to them, and were prepared for the world mostly by adolescence. Therefore expensive changes to the brain would not be worth it for a fairly circumstantial benefit.


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