<p>This is not my field, but I gave it a quick search. <a href="https://www.ncbi.nlm.nih.gov/pubmed/21364180" rel="nofollow noreferrer">This</a> article seems to speak directly to this question, summarizing and comparing multiple theories to each other. In light of these theories, the article also seems to propose a synthesized theory of the avoidance. Formal citation below. Hope this helps!</p>

<p><strong>APA Citation</strong></p>

<p>Paul, R. A. (2011). Incest Avoidance: Oedipal and Preoedipal, Natural and Cultural. Journal of the American Psychoanalytic Association, 58(6), 1087–1112. <a href="https://doi.org/10.1177/0003065110395759" rel="nofollow noreferrer">https://doi.org/10.1177/0003065110395759</a></p>

CC BY-SA 3.0
2017-06-10T21:59:47.473Z
2017-06-28T22:50:15.563Z
2017-06-28T22:50:15.563Z
mflo-ByeSE
<p>The answer is that we don't really know.  It is understandably difficult to conduct experiments on humans in this field, so many theories remain speculative.</p>
<p>The two competing forces (nature and nurture), known as <a href="https://en.wikipedia.org/wiki/Inbreeding_avoidance" rel="nofollow noreferrer">the inbreeding avoidance hypothesis</a>, and the <a href="https://en.wikipedia.org/wiki/Incest_taboo" rel="nofollow noreferrer">incest taboo</a>, both have mixed support with respect to humans.  However, as is typical of human behaviour, there is probably a mix of both at play.</p>
<p><strong>The Westermarck Effect</strong></p>
<p>The <a href="https://en.wikipedia.org/wiki/Inbreeding_depression" rel="nofollow noreferrer">negative effect of inbreeding</a> on genetic fitness is well supported in animal studies, though there is less confidence about humans.  This suggests that a cognitive mechanism for inbreeding avoidance might be adaptive, but it remains to be determined how <a href="https://en.wikipedia.org/wiki/Kin_recognition" rel="nofollow noreferrer">kin detection</a> actually works.</p>
<p><a href="https://en.wikipedia.org/wiki/Westermarck_effect" rel="nofollow noreferrer">The Westermarck Effect</a> was proposed as early as 1891 as the instinctual mechanism in humans that might implement kin detection.  The effect is typically assumed to involve a genetic predisposition to learn to identify kin through early childhood experience.  Supportive studies demonstrate cases where genetically unrelated individuals who are raised together in their early childhood are less likely to find each other sexually attractive, less likely to find similar individuals attractive, and less likely to marry such individuals.  There are also counterpart studies on genetic siblings who are not raised together, and hence do not experience this effect.</p>
<p>This effect is found in animals, and is often implemented through olfactory detection - animals tend to avoid mating with others whose smell resembles those they have been raised with - and recently, the sense of smell has been proposed to account for the <a href="https://en.wikipedia.org/wiki/Inbreeding_avoidance#Human_kin_recognition" rel="nofollow noreferrer">Westermarck effect in humans</a> as well.  Conclusive evidence is elusive however, as classical conditioning would spread avoidance to other senses.  Furthermore, much of the evidence for this effect in both humans and other <a href="https://doi.org/10.1038/s41559-021-01453-9" rel="nofollow noreferrer">animals</a> has been called into question over the years.</p>
<p><strong>Alliance Theory</strong></p>
<p>While the apparent universality of the incest taboo suggests a genetic predisposition, the exact way in which it is implemented varies significantly across cultures (for example, how different cultures treat marriages between first-cousins).  As such, a cultural origin has been surmised.</p>
<p><a href="https://en.wikipedia.org/wiki/Exogamy" rel="nofollow noreferrer">Exogamy</a> was proposed in 1949 as the mechanism behind the anthropological <a href="https://en.wikipedia.org/wiki/Alliance_theory" rel="nofollow noreferrer">Alliance Theory</a>.  This theory suggests that the incest taboo is socially adaptive, as it encourages marriages between families, increasing social stability.  This was a common practice for maintaining peace among the ruling class of many monarchic societies, for example.  Accordingly, when <em>intra</em>-family relations (endogamy) were more socially adaptive (because of isolation or protection from enemies), then the incest taboo was not present, as was the case in a few ancient societies such as Egypt, Inca, Hawaii, and China.</p>
<p>While a cultural origin may explain biologically counter-productive exceptions, it does not eliminate a biological impetus - a selective preference for societies that adopt the taboo over ones that don't.</p>

CC BY-SA 4.0
2017-06-28T22:32:10.883Z
2021-05-10T23:54:46.610Z
2021-05-10T23:54:46.610Z
Arnon Weinberg
<p>Before I can answer this question, I would like to draw your attention to <a href="https://itunes.apple.com/gb/podcast/why-does-law-recognize-marriage-restrict-it-as-it-does/id385577095?i=1000085423181&amp;mt=2" rel="nofollow noreferrer">an iTunesU talk</a> hosted by Professor <a href="https://law.stanford.edu/directory/ralph-richard-banks/" rel="nofollow noreferrer">Ralph Richard (Rick) Banks</a> at Stanford Law about marriage restriction, which covers incestual relationships and the arguements for legal restriction in incestual marriage and relationships.</p>

<p>In this talk, Rick is not talking about incest between an adult and child as this is obviously wrong and covered by child sexual abuse laws.  What he is referring to is incestual relationships between 2 consenting adults.</p>

<p>One argument he has is that society and the law generally prohibits relationships between people related by marriage - step relatives such as step-brother and step-sister or step-mother/father and step-son/daughter such as <a href="http://www.nationnews.com/nationnews/news/32470/siblings-love-sex" rel="nofollow noreferrer">Nick Cameron and Danielle Heaney</a>, and these cases have no bearing on genetics which others have put forward.  And on that subject, Rick argues that the risk of genetic problems occurring does not legally stop anyone having a sexual relationship with someone with genetic abnormalities, so why should the law stop consensual incest between adults?</p>

<p>This question would come into the <a href="https://www.simplypsychology.org/naturevsnurture.html" rel="nofollow noreferrer">nature vs nurture debate</a> (Are behaviours inherently biologically or environmentally determined?) And cases such as Nick and Danielle, above, highlight some of the reasons why this sort of question pops up on occasions.</p>

<p>There are papers and articles which argue that <a href="https://www.psychologytoday.com/blog/animals-and-us/201210/the-problem-incest" rel="nofollow noreferrer">incest causes genetic abnormalities which can weaken the gene pool within populations</a>, and there are arguments which counter-act those arguments because there is the phenomenon of <a href="https://en.wikipedia.org/wiki/Genetic_purging" rel="nofollow noreferrer">Genetic Purging</a>.</p>

<p>In fact, Rick Banks puts forward a valid argument which is that the main reason for restricting incest is that it can be socially destructive for individuals in the family context.  However, is that because of social barriers to incest, or some other psychological reason? — I am digressing as that is another question.</p>

<p>I would say that the answer to your question on learnt or instinctive avoidance of incest is a matter of opinion at the moment.  Some people say that the taboo comes from some sort of anti-incest mechanism within the biological framework of the person.  However, from looking at things on balance through articles and papers I have read, including some of those linked in the genetic purging link above, <strong>I, along with <a href="https://www.psychologytoday.com/blog/animals-and-us/201210/the-problem-incest" rel="nofollow noreferrer">some anthropologists, say</a> that incest taboos are learnt social conventions</strong>.  Look at how it was generally believed that homosexuality for example is down to genetic faults or mental illness.</p>

CC BY-SA 3.0
2017-06-13T16:45:29.860Z
+1 Very interesting!
mflo-ByeSE
CC BY-SA 3.0
2017-06-13T16:34:59.033Z
2017-06-13T16:50:56.217Z
2017-06-13T16:50:56.217Z
Chris Rogers
<p>It's selected against due to negative recessive traits/phenotypes/diseases being more pronounced and more prevalent in the gene pool of the species or population. </p>

<p>Individuals who do are less fit and don't fare as well on average and become less numerous in the population. </p>

<p>There is a social component too, as some comments say. To what degree is social but it is taboo in most species, certainly for mammals. </p>

<p>Example: We breed K9s to different phenotypes. We inbred to get the desired traits such as body size, color, temperament, etc. The further one diverges from the ancestral populations' phenotypes, the more health issues and generic disease are expressed. For instance, hip dysplasia. A wild individual would probably die and not get to pass any genes along if they were suffering from an ailment due to close inbreeding. In this case the selection is artificial, but the point is the same. </p>

<p>A good source to reference is Richard Dawkins' book <em>The greatest show on earth</em></p>

CC BY-SA 3.0
2017-06-11T19:57:33.627Z
This does not answer the question
Charlie
CC BY-SA 3.0
2017-06-11T20:39:04.390Z
@Charlie I believe it does. The ultimate cause is selection. If you are looking for a proximate cause you can read into the social aspects further. But selection is key. It shapes out brains and phyches, "programming" them to have proclivities for certain behaviors and not for others. Incest is one of these. It's untimately physical.
ebrohman
CC BY-SA 3.0
2017-06-12T16:03:16.947Z
You're failing to grasp the nature of the question
Charlie
CC BY-SA 3.0
2017-06-12T16:42:46.500Z
@Charlie I'm tracking, I don't think you're grasping the answer. It's programmed, by natural selection. If you are looking for something else please clarify the question. If you just don't like the answer, I can't help you any further.
ebrohman
CC BY-SA 3.0
2017-06-13T01:53:56.383Z
I'm also a bit confused... It seems you are saying that all those with inbreeding inclinations die off (due to bad traits). But dogs/cats/fish don't seem to avoid inbreeding, even if the results die off. Or are you assuming this is only true for humans, and different for other species? Perhaps I'm misunderstanding your argument?
mflo-ByeSE
CC BY-SA 3.0
2017-06-13T17:06:22.637Z
@mfloren the K9 breeding is just an extreme example of how closely inbred populations are less fit than their respective wild populatuons. A chihuhua would not survive and reproduce in a population of wolves. Evolution by natural selection will lessen the numbers of said individuals, the ones whose ancestors had a proclivity to in breed. In the example I gave, the selection is artificial, but the principle and mechanism of action is the same.
ebrohman
CC BY-SA 3.0
2017-06-13T18:06:29.077Z
This seems to imply that natural selection "breeds out" the tenancy to inbreed, which seems [anecdotally untrue](http://boards.straightdope.com/sdmb/showthread.php?t=335069).
mflo-ByeSE
CC BY-SA 3.0
2017-06-13T18:13:46.177Z
@mfloren No, Natural selection selects against unfit phenotypes. The fact that those phenotypes were caused by inbreeding is not known to nature, nor does nature care. It just does what it does. Again, this is the ultimate cause. If the OP is lookingfor a proximate cause, it should be clarified.
ebrohman
CC BY-SA 3.0
2017-06-15T16:34:34.613Z
Ebrohman, it would be appreciated if you could add any credible sources to back up specific claims. That may take away much of the questions and discussions I see in the comments. In fact, sources are expected in answers here on CogSci
Robin Kramer-ten Have
CC BY-SA 3.0
2017-06-16T12:59:47.163Z
@RobinKramer see the last sentence of my answer for a great source on the subject. Also check out On The Origin of Species by Charles Darwin.
ebrohman
CC BY-SA 3.0
2017-06-16T13:52:32.150Z
@ebrohman I saw the reference to the book indeed. The downside of citing an entire book is that people have to spit through 100s of pages to find the statements that support your claims. The more specific you are, e.g. chapters, pages or even specific quotes, the clearer your answer will get (and likely the more upvotes you'll receive :)  )
Robin Kramer-ten Have
CC BY-SA 3.0
2017-06-16T18:32:18.080Z
@RobinKramer while I agree with you, I have provided an answer that is legitimate and didn't do so for upvotes. If one is unfamiliar with the mechanisms of natural selection, one should familiarize oneself. I have provided a resource for one to do so. It's not on me to write a book summary.
ebrohman
CC BY-SA 3.0
2017-06-11T19:31:52.123Z
2017-06-11T19:31:52.123Z
ebrohman
<p>Is the natural avoidance of incest something that is learned or is the human brain programmed by instinct to have a negative response to incest? This of course would have an evolutionary advantage and would hence make perfect sense.</p>

CC BY-SA 3.0
2017-06-10T21:18:38.463Z
Definition of incest may be important here... What does it mean to you? Specifically, would you consider [this](http://www.floridasunpost.com/miami-man-discovers-bride-granddaughter-no-plans-divorce/) relationship to be incestuous under your definition (or, more generally, relationships where the partners were ignorant about their kinship)?
mflo-ByeSE
CC BY-SA 3.0
2017-06-10T22:34:20.783Z
I think it's important to the question that the people involved know they are related
Charlie
<learning><sexuality><evolution><behavior><anthropology>
Is an avoidance of incest/inbreeding learned or instinctive?
Charlie
# Is an avoidance of incest/inbreeding learned or instinctive?

(Asked by: username_0 on 2017-06-10)

Is the natural avoidance of incest something that is learned or is the human brain programmed by instinct to have a negative response to incest? This of course would have an evolutionary advantage and would hence make perfect sense.

**Tags:** learning, sexuality, evolution, behavior, anthropology

> **Comments:**
> **username_1**: Definition of incest may be important here... What does it mean to you? Specifically, would you consider [this](http://www.floridasunpost.com/miami-man-discovers-bride-granddaughter-no-plans-divorce/) relationship to be incestuous under your definition (or, more generally, relationships where the partners were ignorant about their kinship)?  
> **username_0**: I think it's important to the question that the people involved know they are related  

---

## 4 answers

### Answer by username_1 on 2017-06-10. Score: 5

This is not my field, but I gave it a quick search. [This](https://www.ncbi.nlm.nih.gov/pubmed/21364180) article seems to speak directly to this question, summarizing and comparing multiple theories to each other. In light of these theories, the article also seems to propose a synthesized theory of the avoidance. Formal citation below. Hope this helps!

**APA Citation**

Paul, R. A. (2011). Incest Avoidance: Oedipal and Preoedipal, Natural and Cultural. Journal of the American Psychoanalytic Association, 58(6), 1087–1112. <https://doi.org/10.1177/0003065110395759>


---

### Answer by username_2 on 2017-06-28. Score: 3

The answer is that we don't really know. It is understandably difficult to conduct experiments on humans in this field, so many theories remain speculative.

The two competing forces (nature and nurture), known as [the inbreeding avoidance hypothesis](https://en.wikipedia.org/wiki/Inbreeding_avoidance), and the [incest taboo](https://en.wikipedia.org/wiki/Incest_taboo), both have mixed support with respect to humans. However, as is typical of human behaviour, there is probably a mix of both at play.

**The Westermarck Effect**

The [negative effect of inbreeding](https://en.wikipedia.org/wiki/Inbreeding_depression) on genetic fitness is well supported in animal studies, though there is less confidence about humans. This suggests that a cognitive mechanism for inbreeding avoidance might be adaptive, but it remains to be determined how [kin detection](https://en.wikipedia.org/wiki/Kin_recognition) actually works.

[The Westermarck Effect](https://en.wikipedia.org/wiki/Westermarck_effect) was proposed as early as 1891 as the instinctual mechanism in humans that might implement kin detection. The effect is typically assumed to involve a genetic predisposition to learn to identify kin through early childhood experience. Supportive studies demonstrate cases where genetically unrelated individuals who are raised together in their early childhood are less likely to find each other sexually attractive, less likely to find similar individuals attractive, and less likely to marry such individuals. There are also counterpart studies on genetic siblings who are not raised together, and hence do not experience this effect.

This effect is found in animals, and is often implemented through olfactory detection - animals tend to avoid mating with others whose smell resembles those they have been raised with - and recently, the sense of smell has been proposed to account for the [Westermarck effect in humans](https://en.wikipedia.org/wiki/Inbreeding_avoidance#Human_kin_recognition) as well. Conclusive evidence is elusive however, as classical conditioning would spread avoidance to other senses. Furthermore, much of the evidence for this effect in both humans and other [animals](https://doi.org/10.1038/s41559-021-01453-9) has been called into question over the years.

**Alliance Theory**

While the apparent universality of the incest taboo suggests a genetic predisposition, the exact way in which it is implemented varies significantly across cultures (for example, how different cultures treat marriages between first-cousins). As such, a cultural origin has been surmised.

[Exogamy](https://en.wikipedia.org/wiki/Exogamy) was proposed in 1949 as the mechanism behind the anthropological [Alliance Theory](https://en.wikipedia.org/wiki/Alliance_theory). This theory suggests that the incest taboo is socially adaptive, as it encourages marriages between families, increasing social stability. This was a common practice for maintaining peace among the ruling class of many monarchic societies, for example. Accordingly, when *intra*-family relations (endogamy) were more socially adaptive (because of isolation or protection from enemies), then the incest taboo was not present, as was the case in a few ancient societies such as Egypt, Inca, Hawaii, and China.

While a cultural origin may explain biologically counter-productive exceptions, it does not eliminate a biological impetus - a selective preference for societies that adopt the taboo over ones that don't.


---

### Answer by username_3 on 2017-06-13. Score: 2

Before I can answer this question, I would like to draw your attention to [an iTunesU talk](https://itunes.apple.com/gb/podcast/why-does-law-recognize-marriage-restrict-it-as-it-does/id385577095?i=1000085423181&mt=2) hosted by Professor [Ralph Richard (Rick) Banks](https://law.stanford.edu/directory/ralph-richard-banks/) at Stanford Law about marriage restriction, which covers incestual relationships and the arguements for legal restriction in incestual marriage and relationships.

In this talk, Rick is not talking about incest between an adult and child as this is obviously wrong and covered by child sexual abuse laws. What he is referring to is incestual relationships between 2 consenting adults.

One argument he has is that society and the law generally prohibits relationships between people related by marriage - step relatives such as step-brother and step-sister or step-mother/father and step-son/daughter such as [Nick Cameron and Danielle Heaney](http://www.nationnews.com/nationnews/news/32470/siblings-love-sex), and these cases have no bearing on genetics which others have put forward. And on that subject, Rick argues that the risk of genetic problems occurring does not legally stop anyone having a sexual relationship with someone with genetic abnormalities, so why should the law stop consensual incest between adults?

This question would come into the [nature vs nurture debate](https://www.simplypsychology.org/naturevsnurture.html) (Are behaviours inherently biologically or environmentally determined?) And cases such as Nick and Danielle, above, highlight some of the reasons why this sort of question pops up on occasions.

There are papers and articles which argue that [incest causes genetic abnormalities which can weaken the gene pool within populations](https://www.psychologytoday.com/blog/animals-and-us/201210/the-problem-incest), and there are arguments which counter-act those arguments because there is the phenomenon of [Genetic Purging](https://en.wikipedia.org/wiki/Genetic_purging).

In fact, Rick Banks puts forward a valid argument which is that the main reason for restricting incest is that it can be socially destructive for individuals in the family context. However, is that because of social barriers to incest, or some other psychological reason? — I am digressing as that is another question.

I would say that the answer to your question on learnt or instinctive avoidance of incest is a matter of opinion at the moment. Some people say that the taboo comes from some sort of anti-incest mechanism within the biological framework of the person. However, from looking at things on balance through articles and papers I have read, including some of those linked in the genetic purging link above, **I, along with [some anthropologists, say](https://www.psychologytoday.com/blog/animals-and-us/201210/the-problem-incest) that incest taboos are learnt social conventions**. Look at how it was generally believed that homosexuality for example is down to genetic faults or mental illness.

> **Comments:**
> **username_1**: +1 Very interesting!  

---

### Answer by username_4 on 2017-06-11. Score: 0

It's selected against due to negative recessive traits/phenotypes/diseases being more pronounced and more prevalent in the gene pool of the species or population.

Individuals who do are less fit and don't fare as well on average and become less numerous in the population.

There is a social component too, as some comments say. To what degree is social but it is taboo in most species, certainly for mammals.

Example: We breed K9s to different phenotypes. We inbred to get the desired traits such as body size, color, temperament, etc. The further one diverges from the ancestral populations' phenotypes, the more health issues and generic disease are expressed. For instance, hip dysplasia. A wild individual would probably die and not get to pass any genes along if they were suffering from an ailment due to close inbreeding. In this case the selection is artificial, but the point is the same.

A good source to reference is Richard Dawkins' book *The greatest show on earth*

> **Comments:**
> **username_0**: This does not answer the question  
> **username_4**: @username_0 I believe it does. The ultimate cause is selection. If you are looking for a proximate cause you can read into the social aspects further. But selection is key. It shapes out brains and phyches, "programming" them to have proclivities for certain behaviors and not for others. Incest is one of these. It's untimately physical.  
> **username_0**: You're failing to grasp the nature of the question  
> **username_4**: @username_0 I'm tracking, I don't think you're grasping the answer. It's programmed, by natural selection. If you are looking for something else please clarify the question. If you just don't like the answer, I can't help you any further.  
> **username_1**: I'm also a bit confused... It seems you are saying that all those with inbreeding inclinations die off (due to bad traits). But dogs/cats/fish don't seem to avoid inbreeding, even if the results die off. Or are you assuming this is only true for humans, and different for other species? Perhaps I'm misunderstanding your argument?  
> **username_4**: @mfloren the K9 breeding is just an extreme example of how closely inbred populations are less fit than their respective wild populatuons. A chihuhua would not survive and reproduce in a population of wolves. Evolution by natural selection will lessen the numbers of said individuals, the ones whose ancestors had a proclivity to in breed. In the example I gave, the selection is artificial, but the principle and mechanism of action is the same.  
> **username_1**: This seems to imply that natural selection "breeds out" the tenancy to inbreed, which seems [anecdotally untrue](http://boards.straightdope.com/sdmb/showthread.php?t=335069).  
> **username_4**: @mfloren No, Natural selection selects against unfit phenotypes. The fact that those phenotypes were caused by inbreeding is not known to nature, nor does nature care. It just does what it does. Again, this is the ultimate cause. If the OP is lookingfor a proximate cause, it should be clarified.  
> **username_5**: Ebrohman, it would be appreciated if you could add any credible sources to back up specific claims. That may take away much of the questions and discussions I see in the comments. In fact, sources are expected in answers here on CogSci  
> **username_4**: @RobinKramer see the last sentence of my answer for a great source on the subject. Also check out On The Origin of Species by Charles Darwin.  
> **username_5**: @username_4 I saw the reference to the book indeed. The downside of citing an entire book is that people have to spit through 100s of pages to find the statements that support your claims. The more specific you are, e.g. chapters, pages or even specific quotes, the clearer your answer will get (and likely the more upvotes you'll receive :)  )  
> **username_4**: @RobinKramer while I agree with you, I have provided an answer that is legitimate and didn't do so for upvotes. If one is unfamiliar with the mechanisms of natural selection, one should familiarize oneself. I have provided a resource for one to do so. It's not on me to write a book summary.  

---

