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Consider a society that is both matrilineal and non-monogamous, such as Patrick Rothfuss' Adem from The Kingkiller Chronicles.

For those unfamiliar with the work, there are basically two factors at play:

No Cultural Aversion to Sex

When an Adem is feeling frisky, they seek to fulfill that need as a matter of course, and have few aversions as to whom they use to that end. One of the Adem's nearby friends might even offer to "help"!

This invariably leads to lots of non-monogamous sex. (Perhaps a tendency towards monogamy later in life, but irrelevant for the example and ultimate question)

The Myth of the Man-Mother

Women naturally ripen like fruit, and give birth to children. Men have "no" effect on whether or not a Woman ripens.

This means that any given Adem has no firm knowledge of whom their father is (though they don't care and don't believe in "man-mothers" anyway).

Conclusions

It is quite possible that Adem could mate with their own half-siblings, cousins, aunts, uncles, etc.

Is this likely to cause their inhabitants any harm due to accidental inbreeding? Or is the sort of thing where as long as it doesn't happen too much (and since the culture isn't intentionally inbreeding), it's a moot point?

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    $\begingroup$ That's not how statistics works. (And, by the way, marriage between cousins it considered perfectly normal in very large parts of the world. The Fount of All Knowledge says that up to 10% of marriages worldwide are between first or second cousins -- and we don't see any bad effects.) $\endgroup$ – AlexP Mar 2 '20 at 23:11
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    $\begingroup$ @AlexP Or, to clarify that, what you're saying is that over a large population, a small amount of inbreeding will occur, and thus it will suffer from a small level of inbreeding that scales inversely with their population, assuming complete randomness. $\endgroup$ – Halfthawed Mar 2 '20 at 23:15
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    $\begingroup$ @Halfthawed: Almost. What I'm saying is that for inbreeding to be inbreeding, it must be continued for some time in a small genetic population. As long as genes flow in a large population, no inbreeding effects will be manifest. $\endgroup$ – AlexP Mar 2 '20 at 23:17
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    $\begingroup$ @TCooper - As per the source material, it's that the Adem, culturally, think men have no input, but are regular humans. (So of course they actually do!) $\endgroup$ – Raven Dreamer Mar 2 '20 at 23:53
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    $\begingroup$ @jamesqf: There was nothing wrong biologically with European aristocrats, as a group. There were some congenital diseases in some families at some times, just like with any other families. But by and large, European aristocrats were (and still are) biologically just fine. Queen Elizabeth of Canada is 93 and doing quite well for her age. Her mother died at 101. $\endgroup$ – AlexP Mar 3 '20 at 1:10
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It all depends on the size of the population, and the diversity of people's sexual partners.

"Accidental inbreeding" (specifically excluding purposeful incest like what the Habsburgs did) happens to mankind only when there's a handful of people in a secluded situation: commonly a mountain village, but it can happen to sufficiently xenophobic social, economic or ethnic communities as well. A few centuries pass, and everyone is triple cousins. The defects start building up soon.

But you need a tiny society with very little outside contact to achieve that. In regular medieval villages, many people never moved away, but enough people did that the diversity of the population was high enough to prevent inbreeding. Villages of a few hundred people have thrived, as long as they were on a road and connected to the outside world.

Your species has a handicap that they don't know their fathers, but they do know their mothers, so half the risk is eliminated. Communal raising of children is also done by many societies, such as the indigenous population of the Amazon basin. Specific parents matter less to them, and while I do not know if they tracked them and made genealogy charts to avoid inbreeding, they too lived in villages of up to a few thousand, and they have flourished, as long as they were in contact with other villages.

There's a reason for this: inbreeding is hard. In a village of a thousand people, picking a partner by random chance gives you only a fraction of a percent odds that they are closely related to you. At best, one in a hundred might have children with their half sibling. But the odds of that happening twice in a row (for any one person) are one in ten thousand, and you need a couple of generations for real defects to start building up.

So, as long as populations of your species do not live totally isolated from one another, they are not going to become inbred by chance.

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    $\begingroup$ Inbreeding and triple cousins can also happen in the midst of a large population if you are from an insular group within that population and you choose mates only from your own small group. $\endgroup$ – Willk Mar 2 '20 at 23:39
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    $\begingroup$ Your probability analysis is wrong. It's like the standard problem of figuring out the probability that two people in a group will have the same birthday. While the probability that someone will have the same birthday as you is 1:365 (neglecting leap years), you need only 23 people to have a 50% chance that some pair will have the same birthday: en.wikipedia.org/wiki/Birthday_problem $\endgroup$ – jamesqf Mar 3 '20 at 0:33
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    $\begingroup$ @jamesqf I was not calculating the probability that any two villagers would engage in incest; I was specifically estimating the fraction of the entire village that would. That's no more than one in a hundred, and it is relevant because you need to be part of that fraction over several generations for birth defects to spring up. $\endgroup$ – KeizerHarm Mar 3 '20 at 8:11
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    $\begingroup$ there is also the fact aversion to sex with relatives may not require conscious knowledge of who is a relative. en.wikipedia.org/wiki/Westermarck_effect so most ways of inbreeding are just as unlikely as in other cultures. $\endgroup$ – John Mar 3 '20 at 11:12
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    $\begingroup$ Also don’t forget transient populations. ‘Oh, that traveller’s blond hair looks cute’ -time passes- ‘What do you mean you can’t understand why our son has blond hair when we’re both brown haired? Will of the gods, innit?’ $\endgroup$ – Joe Bloggs Mar 3 '20 at 12:43
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This statement always ends up being taken as controversial but I will state it nevertheless: If we for a minute put morality and social norms aside, the negative effects of inbreeding are severely exaggerated among general population.

  • yes, the possibility of recessive genes ending up paired increases, but there's nothing intrinsically bad about recessive genes.
  • the always-used example of Habsburgs, which I do see as an example of negative effects of inbreeding, is even more an example of what happens when natural selection and Darwinism no longer applies for a certain line. Individuals that by themselves were unlikely to reach adulthood, let alone have offsprings, would reproduce their genetic material as a result of their surname. Their flaws were then accentuated (but not caused) by inbreeding.

So to answer your question - as long as rules of Darwinism are in effect in your world - the occasional accidental inbreeding won't cause harm to general population.

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    $\begingroup$ This is true. In the anthropology class where I learned that 10% of the world's marriages are between what our culture would consider first cousins, the professor also said that first cousin marriage doubles the chance of severe birth defects. This sounds terrible until you realize it's only going from 1% to 2%. $\endgroup$ – StackOverthrow Mar 3 '20 at 23:04
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Pheromones (possibly)

There have been studies (this is one) where blind tests have been used to determine how attractive a person will find the odour of their near relatives compared to strangers. The data suggests that in general a person will find the odour of a near relative less attractive, thus reducing the chance of accidental incest.

Note that this research contradicts the earlier assertions of the proponents of Genetic Sexual Attraction, a pseudoscience proposed to justify attraction to close relatives.

While not a perfect protection from accidental incest, this provides a further factor to reduce the statistical likelihood of occurrences as described in KeizerHarm's answer.

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In Iceland it certainly seems possible. No offence meant.

https://www.independent.co.uk/news/world/europe/icelandic-anti-incest-app-aims-to-stop-families-getting-too-close-8578404.html

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  • $\begingroup$ Incest might very well happen in Iceland, but I don't think anyone would claim they're inbred. A few cases does not make a public health hazard. $\endgroup$ – bendl Mar 3 '20 at 19:12
  • $\begingroup$ @bendl I focussed on the accidental part $\endgroup$ – Thomas Mar 4 '20 at 7:42
  • $\begingroup$ I understand, but there is a difference between incest and inbreeding. What's happening in Iceland is rare enough that it is not really dangerous to the wider population' $\endgroup$ – bendl Mar 4 '20 at 13:28
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Is this likely to cause their inhabitants any harm due to accidental inbreeding?

probably not. Well, at first it would, but since it would be a widespread problem the body begins to adapt.

Pheromones were already mentioned and are a good preventative protection, but if you truly want them to have sex with anyone , then that is not a way.

However, a reactive protection that already exists in humans could be extended. Because a mothers womb aborts almost all birth defects completely by itself. [Which is why you see the "down syndrome" only with a specific chromosome: because all fetuses with a tripling of any other chromosome get killed off very early on]

This mechanism could sharpen, which would reduce birth defects generally (but also lower overall fertility due to "false positives"

Alternatively, a process could evolve that specifically checks for a "minimum difference" between egg and sperm cell DNA during the initial fertilization and completely stopping the process if the similarity is too high. (Which would also lower fertility due to false positives).

HOWEVER these lowered fertility rates could also explain how the liberal attitude towards sex comes to be:

There are few enough children already, so when a female gets pregnant it is a cause for celebration and everyone is happily helping with the upbringing of the child.

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