History Analysis / IncestIsRelative

You can read about the commonly cited pair of phenomena: UsefulNotes/WestermarckEffect and Genetic Sexual Attraction. The short version, though, is that both are very poorly evidenced and often called pseudo-science.

Nowadays, UsefulNotes/{{genetics}} is the main explanation cited for the taboo against incest. But the taboo did not originate there, and before a modern understanding of genetics emerged, the taboo was explained in terms of religious prohibition and/or social roles.

Even now, the genetic explanation is patchy. Unrelated couples are not immune to having children with genetic disorders. In the case of an unrelated couple who both have sickle cell, it might be ''unwise'' for them to have biological children together, but it's not "gross" or "wrong".

!!Actual genetics
For reference, every relationship is technically incest since everyone is at least your 16th cousin. Individuals have to be ''somewhat'' related to each other to be able to reproduce--good luck producing children in an InterspeciesRomance.

What qualifies as "incest" varies culturally. When looked at cross-culturally:

* '''"Hard" incest:''' The prohibition of parent–child and sibling unions (sharing 50% of genes) are virtually a cultural universal.
* '''"Mid" incest:''' Unions between aunt/uncle–niece/nephew or half-siblings (sharing 25% of genes) are ''usually'' prohibited, but not so often that it's a cultural universal.

** Some cultures split cousins into [[https://en.wikipedia.org/wiki/Parallel_and_cross_cousins "parallel cousins" and "cross cousins"]], where cross cousins are marriable and parallel cousins are not. Genetically there is no distinction.

First cousins[[note]]people who share one pair of the same grandparents (ie. the children of siblings)[[/note]] share 12.5% of genes, and for them the risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s. Second cousins[[note]]people who share one pair of the same great-grandparents (ie. the children of first cousins)[[/note]] share 3.13% and third cousins[[note]]people who share one pair of the same great-great-grandparents (ie. the grandchildren of first cousins)[[/note]] share 0.78%--they're barely related at all.

When inbreeding ''actually'' becomes a problem is when it is ''repeated for generations''. The percentage of shared genes compounds or builds up over time. Let's say cousins Alice and Bob marry and have 2 kids, Carol and Dan. Carol and Dan are siblings (50%) but they're also second cousins (3.13%). So they share 53.13% of their genes. Rinse and repeat for several generations, and this eventually adds up to levels that becomes relevant.

The most infamously inbred royal of European history, UsefulNotes/CharlesIIOfSpain, was the result of [[https://www.discovermagazine.com/the-sciences/inbreeding-and-the-downfall-of-the-spanish-hapsburgs many generations]] of frequent marriage between cousins and uncles/nieces. Or for another example, the last surviving population of mammoths--the Wrangel Island mammoths--were an isolated population of about 300 individuals, where they were all defacto related. Fast-forward about 6,000 years, and the harmful mutations built up, until they eventually suffered a [[https://www.pbs.org/video/the-island-of-the-last-surviving-mammoths-fnpmr0/ mutational meltdown]] and went extinct.

!!The Genetic Explaiation

!!Genetics
For reference, every relationship is technically incest since everyone is at least your 16th cousin. Individuals have to be ''somewhat'' related to each other to be able to reproduce--good luck producing children in an InterspeciesRomance. However, we use the term "incest" for people who are at least 3rd cousins. In reality, marrying a 3rd cousin is taboo because of social reasons and not genetic as 3rd cousins are not known to produce mutant offspring.

Take a look at [[https://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_relationships how many genes various relations share]]:
* Identical twins, or clones -- 100%
* Full siblings[[note]]Siblings who have ''both'' the same parents as you. Includes fraternal twins.[[/note]] -- 50%
* Parent & child -- 50%
* Half siblings -- 25%
* Grandparent & grandchild -- 25%
* Aunt/uncle & niece/nephew -- 25%
* 1st cousins[[note]]First cousins have one pair of the same grandparents--they are the children of siblings[[/note]] -- 12.5%
* Double first cousins[[note]]Double first cousins are the children of two pairs of siblings who married each other[[/note]] -- 25%
* Half cousins[[note]]Half cousins have one grandparent in common--they are the children of half-siblings[[/note]] -- 6.25%
* 2nd cousins[[note]]Second cousins have one pair of the same great-grandparents--they are the children of cousins[[/note]] -- 3.13%
* 3rd cousins[[note]]Third cousins have one pair of the same great-great-grandparents--they are the grandchildren of cousins[[/note]] -- 0.78%
* 4th cousins[[note]]Fourth cousins have the same great-great-great-grandparents--they are the grandchildren of second cousins[[/note]] -- 0.20%

For 1st cousins, the risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s. 2nd and 3rd cousins are barely related at all.

Unrelated couples are not immune to having children with genetic disorders. Think of a couple who both have sickle cell; it might be ''unwise'' for them to have biological children, as they'd have a high risk of a genetic disorder--but because their genetic risk isn't due to inbreeding, it's not "gross".

When inbreeding ''actually'' becomes a problem is when it is ''repeated for generations'', because the percentage of shared genes compounds or builds up over time. Let's say cousins Alice and Bob marry and have 2 kids, Carol and Dan. Carol and Dan are siblings (50%) but they're also second cousins (3.13%). So they share 53.13% of their genes--not much of a difference from normal siblings. But rinse and repeat for several generations, and this eventually adds up to levels that becomes relevant.

The most infamously inbred royal of European history--Carlos II of Spain, of the Habsburg family--was the result of [[https://www.discovermagazine.com/the-sciences/inbreeding-and-the-downfall-of-the-spanish-hapsburgs many generations]] of frequent marriage between cousins and uncles/nieces. Or for another example, the last surviving population of mammoths--the Wrangel Island mammoths--were an isolated population of about 300 individuals, where they were all defacto related. Fast-forward about 6,000 years, and the harmful mutations built up, until they eventually suffered a [[https://www.pbs.org/video/the-island-of-the-last-surviving-mammoths-fnpmr0/ mutational meltdown]] and went extinct.

!!Theroies
You can read about the commonly cited pair of phenomena: UsefulNotes/WestermarckEffect and UsefulNotes/GeneticSexualAttraction. The short version, though, is that both are very poorly evidenced and often called pseudo-science.

!!Westermarck Effect
!!! For the Westermarck Effect hypothesis, see '''[[UsefulNotes/WestermarckEffect here]]'''

For 1st cousins, the risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s. 2rd and 3nd cousins are barely related at all.

3rd--and even 2nd--cousins are barely related. For 1st cousins, the risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s.

When inbreeding ''actually'' becomes a problem is when it's ''repeated for generations'', because the percentage of shared genes can compound or build up over time. The most infamously inbred royal of European history--Carlos II of Spain, of the Habsburg family--was the result of many generations of frequent marriage between cousins and uncles/nieces. Or for another example, the last surviving population of mammoths--the Wrangel Island mammoths--were an isolated population of about 300 individuals, where they were all defacto related. Fast-forward about 6,000 years, and the harmful mutations built up, until they eventually suffered a [[https://www.pbs.org/video/the-island-of-the-last-surviving-mammoths-fnpmr0/ mutational meltdown]] and went extinct.

* Parent/Child -- 50%

* Grandparent/Grandchild -- 25%
* Aunt/Uncle/Niece/Nephew -- 25%

3rd--and even 2nd--cousins are barely related. For first cousins, the risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s.

The UsefulNotes/WestermarckEffect hypothesizes that growing up in proximity at a young age (through approximately six years of age) usually prevents kids from thinking about each other sexually. It has to due with upbringing, not genetics--so NotBloodSiblings who were raised together really shouldn't be thinking like that, SurpriseIncest is all too plausible, and {{Unlucky Childhood Friend}}s often fall victim of Westermarck effect too.

Two people who grow up together are unlikely to engage in sexual activity with each other because their instincts tell them it's wrong. That's why most people who are an incestuous relationship seem odd: Somehow their natural instinct was broken, which is usually done through some traumatic experience(s).

However, it is important to remember that the Westermarck Effect is a ''hypothesis'', and a contested one at that--''not'' a proven phenomenon.

For reference, every relationship is technically incest since everyone is at least your 16th cousin. However, we use the term incest for people who are at least 3rd cousins. In reality, marrying a 3rd cousin is taboo because of social reasons and not genetic as 3rd cousins are not known to produce mutant offspring.

The UsefulNotes/WestermarckEffect means that growing up in proximity at a young age (through approximately six years of age) usually prevents kids from thinking about each other sexually. It has to due with upbringing--not genetics--so NotBloodSiblings who were raised together really shouldn't be thinking like that, SurpriseIncest is all too plausible, and {{Unlucky Childhood Friend}}s often fall victim of Westermarck effect too.

Take a look at [[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy how many genes various relations share]]:

Take a look at how many genes various relations share:
* Identical twins -- 100%

3rd--and even 2nd--cousins are barely related. The risk of a couple having babies with genetic disorders is present, but minor--comparable to a woman having a baby in her 40s.

The UsefulNotes/WestermarckEffect means that that growing up in proximity at a young age (through approximately six years of age) usually prevents kids from thinking about each other sexually. It has to due with upbringing--not genetics--so NotBloodSiblings who were raised together really shouldn't be thinking like that, SurpriseIncest is all too plausible, and {{Unlucky Childhood Friend}}s often fall victim of Westermarck effect too.

The Westermarck effect means that that growing up in proximity at a young age (through approximately six years of age) usually prevents kids from thinking about each other sexually. It has to due with upbringing--not genetics--so NotBloodSiblings who were raised together really shouldn't be thinking like that, SurpriseIncest is all too plausible, and {{Unlucky Childhood Friend}}s often fall victim of Westermarck effect too.

Two people who grow up together are unlikely to engage in sexual activity with each other because their instincts tell them it's wrong. That's why most people who are an incestuous relationship seem odd: Somehow their natural instinct was broken, which is usually done through some traumatic experience(s).

* Identical twins -- 100%
* Full siblings[[note]]Siblings who have ''both'' the same parents as you. Includes fraternal twins.[[/note]] -- 50%
* Parent/Child -- 50%
* Half siblings -- 25%
* Grandparent/Grandchild -- 25%
* Aunt/Uncle/Niece/Nephew -- 25%
* 1st cousins[[note]]First cousins have one pair of the same grandparents--they are the children of siblings[[/note]] -- 12.5%
* Half cousins[[note]]Half cousins have one grandparent in common--they are the children of half-siblings[[/note]] -- 6.25%
* 2nd cousins[[note]]Second cousins have one pair of the same great-grandparents--they are the children of cousins[[/note]] -- 3.13%
* 3rd cousins[[note]]Third cousins have one pair of the same great-great-grandparents--they are the grandchildren of cousins[[/note]] -- 0.78%
* 4th cousins[[note]]Fourth cousins have the same great-great-great-grandparents--they are the grandchildren of second cousins[[/note]] -- 0.20%
** (And occasionally you have relations like double cousins ((a pair of brothers marries a pair of sisters; their kids are ''double cousins''--cousins on both sides--and 25% genes shared)) or sibling-cousins ((a man has a child with each of a pair of sisters; the kids are ''sibling-cousins''--cousins on one side and cousins on the other--and 37.5% related))

!!Why Incestuous Relationships are Banned and Why it is Rare for Them to Occur Outside of Media

For reference, every relationship is technically incest since everyone is at least your 16th cousin. However, we use the term incest for people who are at least 3rd cousins. In reality, marrying a 3rd cousin is taboo because of social reasons and not genetic as 3rd cousins are not known to produce mutant offspring. Relationships ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy when graphed]]) show that third cousins only have an "r" rating of .78% where as half siblings have an "r" rating of 25% which more than 32 times greater. Even if two people are greater than 3rd cousins away, they can still be both missing a copy of one of their genes which isn't a problem since they have a copy. However, the offspring has a 25% of having no copies of the gene and still dying (happened to two of my cousins). Legislation in certain areas allows great grand parents and their great grand children to marry (which is on the same level as 3rd cousins), but in the USA any relationship of 3rd cousins or more related is illegal. All this means is that in media, if a character says "well, they're only half-siblings" it doesn't matter as that relationship still has an "r" rating of 25% which is dangerous to potential offspring. Even first cousins once removed, still has an "r" rating of 6.25%. Lastly, since identical twins are basically clones, they are counted as the same person (genetically) and the offspring they have (with two different people) are still technically half-siblings. If they have a kid with the same person, then the offspring can be considered full offspring. Most incestuous relationships are prevented by nature as two people who grow up together (usually in the same house) are unlikely to engage in sexual activity with each other because their instincts tell them it's wrong. This is true even if both people are not related (3rd or more). That's why most people who are an incestuous relationship seem odd. Somehow their natural instinct was broken which is usually done through some traumatic experience(s).