If close cousin marriage is so bad, why do people do it?

[Angela]

It's usually about property and wealth and the concentration of it, but also, seems that you get a lot more support from a lot more people, which is certainly a big plus.

However, the negatives are extreme, if not for the couple, for their progeny and for society.

Razib Khan talks about a recent paper examining the issue.

See:
https://www.gnxp.com/WordPress/2019/...medium=twitter

The paper is here:
"Associations of autozygosity with a broad range of human phenotypes"

https://www.nature.com/articles/s41467-019-12283-6

"n many species, the offspring of related parents suffer reduced reproductive success, a phenomenon known as inbreeding depression. In humans, the importance of this effect has remained unclear, partly because reproduction between close relatives is both rare and frequently associated with confounding social factors. Here, using genomic inbreeding coefficients (FROH) for >1.4 million individuals, we show that FROH is significantly associated (p < 0.0005) with apparently deleterious changes in 32 out of 100 traits analysed. These changes are associated with runs of homozygosity (ROH), but not with common variant homozygosity, suggesting that genetic variants associated with inbreeding depression are predominantly rare. The effect on fertility is striking: FROH equivalent to the offspring of first cousins is associated with a 55% decrease [95% CI 44–66%] in the odds of having children. Finally, the effects of FROH are confirmed within full-sibling pairs, where the variation in FROH is independent of all environmental confounding."

We're talking 400 authors, and a sample size of over one million, so I doubt this will be retracted!


From Khan:
"Rather, I want to ask: if inbreeding is so bad genetically and biologically, why is it so common? One of the consequences of the Protestant Reformation is that the Roman Catholic Church’s strict enforcement of consanguinity rules were dropped, and cousin marriage became much more common among elites (such as the Darwin-Wedgewood family). The material rationale for cousin marriage is actually rather straightforward, in that it keeps accumulated property and power within the extended lineage. Marriages between children of brothers may cement alliances, while matrilocality and marriages between cross-cousins in South India have been associated with lower domestic abuse rates (in contrast, in North India strongly enforced exogamy has been associated with the idea that women marry into an alien household).I would suggest perhaps that though marriages between relatives are biologically disfavored, there are many cases where it is culturally beneficial. In societies where collective family units engage in inter-group competition, some level of consanguinity may benefit cohesion. Other societies where individualism is more operative may exhibit no such incentives."
I think he gives the Church a bit more credit than it deserves in this case. It's true that the Church saw the problem and passed laws to curtail it, contrary to "pagan" practices.


However, depending on the political considerations, you could get it. Look at the poor Spanish monarchy; we know how that turned out.




Also, even for poorer people, it depended on the particular priest. Third cousins of my maternal grandmother, who were first cousins of each other, were still given leave to marry. I think it may have been a case of "anticipation" of marriage vows, if you understand me. The woman was pregnant innumerable times, but only three of her children survived, and of those children one could never have children, one had one, and one had two.

I can't even count the number of times they were held up to me as an object lesson never to let my eyes stray to any of my male first cousins.

Strangely, I read in another recent paper that fertility is very high when third cousins marry, and the biological closeness of couples generally who have high fertility is about at the third cousin level. That makes sense to me. If the two people have very different immunological systems, they're not going to have high fertility.

However, if third cousins continuously intermarry, in the end you're going to have "virtual" first cousins.

[Regio X]

@Angela
Interesting how things change. Check out what was said almost 20 years ago:
https://www.nytimes.com/2002/04/03/h...udy-finds.html

Apart that, when it comes to fertility, abt. 10 years ago, it was suggested the optimal mating would be between 3rd cousins:
https://www.sciencedaily.com/release...0207140855.htm

https://www.livescience.com/amp/2271...sins-kids.html

[Angela]

@Angela
Interesting how things change. Check out what was said almost 20 years ago:
https://www.nytimes.com/2002/04/03/h...udy-finds.html

Apart that, when it comes to fertility, abt. 10 years ago, it was suggested the optimal mating would be between 3rd cousins:
https://www.sciencedaily.com/release...0207140855.htm

https://www.livescience.com/amp/2271...sins-kids.html

Current research seems to reaffirm that 3rd cousin marriage may be advantageous, as I mentioned, but first cousin marriage, especially repeated first cousin marriage?

I don't know what they were thinking.

Cavalli Sforza did a whole book on Consanguinity in Italy more than a decade ago, and some of his original research around Parma was much older than that, and he mapped out the deleterious side-effects. The only thing that seemed to surprise him in the Val Parma mountainous areas of Italy where he first began his research is that there didn't seem to be depression of intellect, which was found in other instances.

My father was still alive when some of Cavalli Sforza's papers first came out, and we discussed it. He said Cavalli Sforza didn't seem to realize that what happened was that the children in some families were born either very bright, or with something obviously wrong with them. The latter were never allowed to marry. It was a big price to pay, but after living in the same spot for 500 years, everyone was related, and with no roads in or out there just wasn't much choice.

Of course, there are confounding factors like the particular alleles in the founding population, and whatever denovo mutation occurred.

The full title is "Consanguinity, Inbreeding and Genetic Drift in Italy". Copies are very expensive nowadays.

[Regio X]

Current research seems to reaffirm that 3rd cousin marriage may be advantageous, as I mentioned, but first cousin marriage, especially repeated first cousin marriage?
I don't know what they were thinking.
Cavalli Sforza did a whole book on Consanguinity in Italy more than a decade ago, and some of his original research around Parma was much older than that, and he mapped out the deleterious side-effects. The only thing that seemed to surprise him in the Val Parma mountainous areas of Italy where he first began his research is that there didn't seem to be depression of intellect, which was found in other instances.
My father was still alive when some of Cavalli Sforza's papers first came out, and we discussed it. He said Cavalli Sforza didn't seem to realize that what happened was that the children in some families were born either very bright, or with something obviously wrong with them. The latter were never allowed to marry. It was a big price to pay, but after living in the same spot for 500 years, everyone was related, and with no roads in or out there just wasn't much choice.
Of course, there are confounding factors like the particular alleles in the founding population, and whatever denovo mutation occurred.
The full title is "Consanguinity, Inbreeding and Genetic Drift in Italy". Copies are very expensive nowadays.

Thanks. It's now in my list. My little son is causing accumulation of reading to do, btw. ah ah

In my family, I know of just one case. The maternal grandparents of my paternal grandmother were 3rd cousins. Not big deal. :) But my mother's paternal grandmother was born in a small and relatively isolated place, and my guess is that many people there were "cousins".

One of these articles mentions RH factor. Coincidently, I just read this comment in Eurogenes, about farmers rich in RH+ vs. HGs rich in RH- (Tomenable also posted something about it in Eupedia time ago):
http://eurogenes.blogspot.com/2019/1...28140411504253

While it may help to explain, I just don't think it "completely" explains sex biases. Additionally, there could be also wars, climate, plague/diseases in general etc., as we know. Very complex factors/interactions must have evolved over time.
The impact was likely more relevant in the first contacts between such far "cousins". Also, it wouldn't be a matter of absolutely "infertile" wives, as suggested in the comment. Most of times sensitization occurs during delivery. The first RH+ child (roughly 50% of chances of man and 50% of woman) of a RH- woman generally has no problem. It affects mainly the second child, third and so on. That's the problem. And if the father is homozygous, the first child would have 100% of chances of being RH+ (but heterozygous). Nowadays, only 5% of people in Germany would be RH-, if I'm not mistaken.
But that's another story, just indirectly related. :)

[dominique_nuit]

Spanish Hapsburgs bad, ancient Egyptians much worse -- I am thinking of Cleopatra and near kin, literally sisters marrying brothers, etc, if memory serves

But so far as I can tell from amateur family-tree'ing ---> Southern Italian peasant clans had pretty tight networks up until turn of 20th century, often 3 sons or daughters marrying 3 sons or daughters of another family, repeated multiple times over the course of two or three generations

[Smoothturk]

Genetics is multifaceted!

[Angela]

Spanish Hapsburgs bad, ancient Egyptians much worse -- I am thinking of Cleopatra and near kin, literally sisters marrying brothers, etc, if memory serves

But so far as I can tell from amateur family-tree'ing ---> Southern Italian peasant clans had pretty tight networks up until turn of 20th century, often 3 sons or daughters marrying 3 sons or daughters of another family, repeated multiple times over the course of two or three generations

Yes, intermarriage of close cousins in Southern Italy did hang around for longer than in some other parts of Europe, where it started to die out in the early 1800s. It hung on in other parts of Europe for longer, of course. It's all in Cavalli-Sforza's book if you can get ahold of it.

There's a great American musical called "Seven Wives for Seven Brothers" fwiw. :)

Here are some interesting graphics.This is the modern day prevalence:
http://www.womanstats.org/data3/imag...scale22016.jpg



In some areas, the stats are driven higher by immigrant practices. In Britain, for example, it's a big practice among South Asians, with the results that there's quite a problem with birth defects, developmental problems, and genetic disease in those communities.

As for earlier centuries, it certainly occurred all over Europe, but more in certain areas, certainly. It started to die out earlier in northern Europe from what I can see, by about a 100 years: this graphic is for Sweden.

[Coriolan]

Cousin marriage is most common in the Muslim world. If it's not intentionally designed to keep the population stupid enough to avoid challenging religious teachings, it certainly does the trick.

[Tutkun Arnaut]

Cousin marriage is most common in the Muslim world. If it's not intentionally designed to keep the population stupid enough to avoid challenging religious teachings, it certainly does the trick.

Not necessary! Albania for example! Up to 1970 majority of marriages was arranged. Not allowed to marry 5th cousin. Discouraged to marry 6th and 7th cousin. Discouraged to marry within your village, possibly 2 villages away of your own. Discouraged to marry someone who in their family had a history of heart illness, mental illness, obvious physical illness, dark skin, death in early age and ugly looks. I am not making it up, I remember when some of my family members were married. This issues were discussed in family. The result is very few people with physical disabilities, and very few mental issues. I cant say anything about the intelligence of the people. I have heard a Jewish person saying that marriages among cousins might be the cause of Ashkenazy IQ.