https://www.scientificamerican.com/article/when-incest-is-best-kissi/
Interestingly, one evolutionary argument for mating with a relative is that it might reduce a woman's chance of having a miscarriage caused by immunological incompatibility between a mother and her child. Some individuals have an antigen (a protein that can launch an immune response) on the surface of their red blood cells called a rhesus factor—commonly abbreviated "Rh." In some cases—typically during a second pregnancy—when a woman gets pregnant, she and her fetus may have incompatible blood cells, which could trigger the mother's immune system to treat the fetus as a foreign intruder, causing a miscarriage. This occurrence is less probable if the parents are closely related, because their blood makeup is more likely to match.
"It may well be that the enhanced reproductive success observed in the Iceland study at the level of third [and] fourth cousins, who on average would be expected to have inherited 0.8 percent to 0.2 percent of their genes from a common ancestor," Bittles says, "represents this point of balance between the competing advantages and disadvantages of inbreeding and outbreeding."
In many societies cousin marriage is decidedly not considered incestuous (10% of marriages in the world are between first and second cousins; in India it is close to 40%; in Pakistan it is around 50%3), and in many cultural settings it is very functional—it eases tensions amongst in-laws, it allows spouses to accommodate to their new home, it lowers the price of dowry, etc. Yet, somehow, Americans are overwhelmed by its yuckiness and remain scandalized at the thought of first cousins marrying. Indeed, the United States is one of the few countries in the world (along with not-so-democratic countries such as China and North Korea) to outlaw cousin marriage (in 24 states it is illegal).4 It is thus necessary to set the record straight and ask: how truly dangerous is cousin marriage, and should it be legalized?
Therefore, it can be safely said that natural selection pressured against incest. But not necessarily against mating with more distant relatives, or as some may call it, consanguinity. In fact, although consanguinity may also carry the risk of proliferating recessive genes, this may be outweighed by other selective advantages.
Similarity of traits in parents may turn out to be advantageous for various reasons. For example, if a woman with Rh- factor pairs with a Rh+ man, their child may be Rh+, and this can create complications in pregnancy and delivery. Instead, if that woman pairs with, say, a first cousin, there are higher probabilities that such a man is also Rh-, and therefore that also increases the chance that the child will be Rh-, thus avoiding the pregnancy and delivery complications. Renowned geneticist Patrick Bateson offers a more colorful example:
The size and shape of teeth are strongly inherited characteristics. So too are jaw size and shape… The potential problem arising from too much outbreeding is that the inheritance of teeth and jaw sizes are not correlated. A woman with small jaws and small teeth who had a child by a man with big jaws and big teeth lays down trouble for her grandchildren, some of whom may inherit small jaws and big teeth. In a world without dentists, ill-fitting teeth were probably a serious cause of mortality. This example of mismatching, which is one of many that may arise in the complex integration of the body, simply illustrates the more general cost of outbreeding too much.12
Anthropologist Robin Fox adequately sums up the argument: “So nature aims for a middle ground: organisms breed out to avoid losing variability, but not so far out that they dissipate genetic advantages. In human terms this means that the immediate family is taboo, but that marriage with cousins should be preferred.”13
There may be an additional reason why natural selection favored consanguinity. The renowned evolutionary biologist W.D. Hamilton proposed that the intensity of altruism correlates with the degree of genetic proximity. This argument has been expanded by some theoreticians into saying that if an animal mates with a genetically close partner, their offspring will also be genetically closer to the parents, and this will ensure greater intensity of altruism and care from the parents to the offspring, which is a significant evolutionary advantage. This mechanism includes humans, and therefore, we evolved a preference for mating with partners that may be genetically closer to us.14
In fact, it has long been noticed amongst human and non-human animals that individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern. This phenomenon, known as assortative mating, may be behind preferences for consanguinity. Cousins may not necessarily look similar (i.e., have similar phenotypes), but they often do, and in that regard, they may feel more attracted to each other.
So, we may expect that, given our natural disposition, we feel sexual repulsion for parents and siblings, but sexual attraction for cousins and other more distant relatives. Perhaps Elvis was right when he sang about Kissin’ cousins. Indeed, he was. It has been estimated that about 80 percent of all marriages in human history have been between cousins.15 If natural selection primed us for cousin marriage, and if it offers some evolutionary advantage, then the biological arguments against its legality (appealing to its dangerous biological consequences, as it is often done) lose strength.
| Degree of relationship | Relationship | Coefficient ofrelationship (r) |
|---|---|---|
| 0 | identical twins; clones | 100% |
| 1 | parent-offspring | 50% (2−1) |
| 2 | full siblings | 50% (2−2+2−2) |
| 2 | 3/4 siblings or sibling-cousins | 37.5% (2−2+2⋅2−4) |
| 2 | grandparent-grandchild | 25% (2−2) |
| 2 | half siblings | 25% (2−2) |
| 3 | aunt/uncle-nephew/niece | 25% (2⋅2−3) |
| 4 | double first cousins | 25% (2−3+2−3) |
| 3 | great grandparent-great grandchild | 12.5% (2−3) |
| 4 | first cousins | 12.5% (2⋅2−4) |
| 6 | quadruple second cousins | 12.5% (8⋅2−6) |
| 6 | triple second cousins | 9.38% (6⋅2−6) |
| 4 | half-first cousins | 6.25% (2−4) |
| 5 | first cousins once removed | 6.25% (2⋅2−5) |
| 6 | double second cousins | 6.25% (4⋅2−6) |
| 6 | second cousins | 3.13% (2−6+2−6) |
| 8 | third cousins | 0.78% (2⋅2−8) |
| 10 | fourth cousins | 0.20% (2⋅2−10) |
*This Is A 2nd Cousin Of Mine. She's Been Attracted To ME For A Very Long Time! She Happens To Be Hawaiian As Well! (There Are Other Female Cousins That Are Attracted To ME As Well!)
But it also explains why the professors’ kids are a disproportionate fraction of the National Merit Finalists in a college town – their folks, particularly their fathers, are smarter than average – and so are they. Reminds me of the fact that Los Alamos High School has the highest scores in New Mexico. Our local high school tried copying their schedule, in search of the secret. Didn’t work. I know of an approach that would, but it takes about 15 years.
Salter’s defense of self-sacrifice for one’s people gives a rational foundation for racial activism. A person who foregoes having children of his own to serve the larger interests of his race—one who lives or dies for his race—may serve his genetic interests better than someone who leaves a large family but works against the interests of his race as a whole.
Furthermore, many people who already have children are intimidated from open racial activism because they fear for their children. But Salter shows that our genetic interests are broader than our immediate families. Thus, from a genetic standpoint, “It would appear to be more adaptive for an Englishman to risk life or property resisting the immigration of two Bantu immigrants to England than his taking the same risk to rescue one of his own children from drowning.”
Salter also gives hope to those who are unable to have children of their own: people who are unlucky in love, people unable to have children because of a physical defect like sterility or a psychological aberration like homosexuality, women who ran out their biological clocks pursuing lifestyles that stymie motherhood, and so forth. Such people can still salvage their genetic interests by devoting themselves to the good of their people.
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