February 9, 2008

The Darwinian Sweet Spot: 3rd Cousin Marriages?

It's long been understood theoretically that there must exist a Darwinian fitness trade-off between too much inbreeding and too much outbreeding, but nobody knew where that was. If you marry your first cousin, you are likely to suffer a 30% higher infant mortality rate. But if you marry somebody too genetically dissimilar, you can start running into various reproductive problems as well.

Now, deCODE Genetics of Iceland, who foisted upon the world the most likely fallacious claim that James D. Watson is 25% nonwhite, is claiming that the Darwinian fitness sweetspot is 3rd cousin marriage:

In a paper published today deCODE scientists establish a substantial and consistent positive correlation between the kinship of couples and the number of children and grandchildren they have. The study, which analyzes more than 200 years of deCODE’s comprehensive genalogical data on the population of Iceland, shows that couples related at the level of third cousins have the greatest number of offspring. For example, for women born between 1800 and 1824, those with a mate related at the level of a third cousin had an average of 4.04 children and 9.17 grandchildren, while those related to their mates as eighth cousins or more distantly had 3.34 children and 7.31 grandchildren. For women born in the period 1925-1949 with mates related at the degree of third cousins, the average number of children and grandchildren were 3.27 and 6.64, compared to 2.45 and 4.86 for those with mates who were eighth cousins or more distantly related.

The findings hold for every 25-year interval studied, beginning with those born in the year 1800 up to the present day. Because of the strength and consistency of the association, even between couples with very subtle differences in kinship, the authors conclude that the effect very likely has a biological basis, one which has yet to be elucidated. The paper, ‘An association between the kinship and fertility of human couples,’ is published online in Science magazine at www.sciencemag.org .

deCODE has access to the amazing Icelandic national family tree, in which most Icelanders who ever lived over the last 1000+ years are enrolled. Genealogy is easier in Iceland because there hasn't been much immigration for the last 1000 years, and because of the surname system: for example, the PR lady who wrote this press release is named Berglind Olafsdottir -- i.e., she is "Olaf's daughter."

Icelanders are of Scandinavian and Celtic descent.

The odds of genetic problems due to inheriting two deleterious recessive genes falls off pretty fast as you move from first cousin outward. I believe at the third cousin marriage level, it's only 1/16th as high as at the first cousin marriage level, but don't quote me when proposing marriage to somebody you met at Great Aunt Meg's 90th birthday party. Still, I'm not sure how much faith I should put in these findings.

I could imagine some non-biochemical reasons for this, such as that 3rd cousins might have tended to marry at younger ages -- in early modern England, as Gregory Clark pointed out in A Farewell to Alms, age of marriage is the main determinant of fertility. Or perhaps healthy people tended to quickly find spouses within their social circles, who tended to be related to them, while sickly people had to wander further afield to find somebody who would marry them.

John Hawks notes an even likelier reason: people who are descended from highly fertile people will have more third cousins to marry. That could be biological or cultural or both.

Some of it could be purely mathematical -- the chance of falling in love with your third cousin depends in part on the number of third cousins you have.

And the number of cousins of any type you have is wildly dependent upon typical family size in your family tree. To simplify genealogical calculations, assume that every person in Family Tree A for the last four generations has had only one child, every person in Family Tree B has had exactly two children, and so forth. Here's what you would face in terms of number of relatives of your own generation:

kids/family Siblings 1st Cousins 2nd Cousins 3rd Cousins
1 0 0 0 0
2 1 4 16 64
3 2 12 72 432
4 2 24 196 1536
5 4 40 400 4000

Thus, if everybody has had exactly one child for the last four generations, you would have no siblings, no cousins, no 2nd cousins, and no 3rd cousins. At your family reunion, you'd be assured of getting a big slice of the pie, but you'd be pretty lonely.

But if your ancestors had have a nice stable two surviving/breeding children per person, then you would have 1 sibling, 4 cousins, 16 2nd cousins, and 64 3rd cousins.

Yet, if your ancestors averaged five children surviving to reproduce, you'd have 4,000 third cousins!

Of course, humans do not breed in an evolutionarily stable manner. We've taken over this planet by having more than two children each. So, most people are descended, on average, from people who had more surviving children than the average.

It rural Iceland, if you came from "good stock," it might have been hard to avoid marrying your third cousin.

Anyway, I haven't seen the paper yet, so I can't tell if the the deCODE people have been able to deal with these objections. They certainly have a lot of data to work with.

My published articles are archived at iSteve.com -- Steve Sailer

31 comments:

Anonymous said...

decode is also a public company, ticker dcgn. can't say i would recommend them though.

SKT said...

"Icelanders are of Scandinavian and Celtic descent."

My impression was that Icelanders had a significant component of Eskimo ancestry as well. The President looks very Native American, and not very Northern European if you look him up.

SKT said...

Actually nevermind, I might have been thinking of Greenland.

Hibernia Girl said...

Dawkins mentions in passing in 'The Selfish Gene' that:

For relationships as distant as third cousin (2 x (1/2)8 = 1/128 we are getting down near the baseline probability that a particular gene possessed by A will be shared by any random individual taken from the population. A third cousin is not far from being equivalent to any old Tom, Dick, or Harry as far as an altruistic gene is concerned.

What is the coefficient of relationship between two complete strangers? Anybody know? (My maths s*ck.) Thanks!

mq said...

I think it's going to be completely impossible to separate the cultural/sociological and biological issues in a basically epidemiological study like this one. There are way too many social factors connected to how exogamous or endogamous somebody is -- how far afield from their social circle they go to find a spouse.

amir said...

I don't what society is/was like in Iceland, but in the middle east where consanguinity is common, if you don't marry your cousin you will in all likelihood marry someone else from your extended family or clan unless you completely leave your village. Which is what most other people in your village are doing as well. Therefore the risk of a carrier marrying another carrier is probably the same whether he marries his cousin or any other random person from the same village.

James said...

But then there'd be even more fourth cousins.

Anonymous said...

Icelanders, according to recent genetic research, descend from male Scandinavian vikings who first settled there in the ninth century, and the Celtic women from Scotland and Ireland they use to raid when they were searching for wives (this confirms what is written in the Icelandic sagas). No eskimos ever settled there. Different thing for Greenland, which is predominantly eskimo (or inuit), except for a few danish settlers.

jerzy cow said...

I agree with all of the points steve and mq made about the study. There is simply no way to control for all relevant factors.

Further, the fact this is written by a for-profit company looking for PR makes me thing they are heavily biased in favor of finding any "interesting" result.

To the extent the data can be trusted at all, I would guess the biggest factor is 3rd cousin marriage is that such people will come from rural areas, where family size is larger.

Hansendental said...

Bjork looks Oriental to me.

But then again, except for her eyes, Deborah Harry does too. (Which makes Sean Ono Lennon's comment somewhat ridiculous.)

rillings said...

Unless I am misunderstanding the deCode researchers' finding as reported in the press, it's all about "optimal outbreeding"--the point at which *fertility* is highest. Beyond that point, fertility declines. So this isn't about evolutionary deselection of unfit offspring that don't get to reproduce; it's about offspring that never get produced in the first place. But what is the mechanism that causes fertility to decrease as genetic distance (consanguinity) increases (decreases)?

Bill said...

Icelanders, according to recent genetic research, descend from male Scandinavian vikings who first settled there in the ninth century, and the Celtic women from Scotland and Ireland they use to raid when they were searching for wives (this confirms what is written in the Icelandic sagas)

-anon


It's true that many of the women who settled Iceland with Norwegian men were of Scottish and Irish descent, but the idea that they were all carried off by force is not exactly right. The society and economy of Norway at the time, like England in the colonial era, did not provide Norwegian women with sufficient motivation to leave home. Norwegian women had options other than breaking sod and milking cows in rocky, volcanic soil.

For Norwegian men, land ownership was a means to establish a family and gain some status. Scottish and Irish women (Celts) were motivated to leave because the service industry in their pastoral homelands was meager compared to the predominately Germanic areas of settlement. To them, therefore, setting off with some stoic Norwegian farmer didn't seem like such a bad idea if they weren't satisfied with the local swineherd.

People underestimate the sophistication of Germanic society in NW Europe. The Celts may well have surpassed the Germans culturally, and there is evidence of this in the Icelandic sagas, but in trade and production of goods the Germans had the upper hand.

Along similar lines, the majority of British women who sailed to colonial America were of Scottish or Irish descent. 90% of the English who sailed to America were men, whereas almost 50% of the Celts were women. English women simply didn't have a pressing desire to get out. Why face murder by Indians when you can be a governess or maid at home in walking distance from your parents?

endogamy said...

I agree with all of the points steve and mq made about the study. There is simply no way to control for all relevant factors.

As far as anyone can tell, neither you or Steve has actually read the entire paper in question, so how do you all know what they did or did not "control" for?

Further, the fact this is written by a for-profit company looking for PR makes me thing they are heavily biased in favor of finding any "interesting" result

I see. Any result that refutes "hybrid vigor" just must be wrong.

How about actually reading the work before trashing it?

Steve Sailer said...

As I said, I haven't read it, so I don't know what was controlled for and what wasn't. The Icelandic national family tree is an amazing resource, so they might be able to control for quite a lot.

John Hawks had a suggestion for controlling for the number of third cousins problem over on his blog: compare people with equal numbers of third cousins who differ only on whether they married their third cousins or not.

endogamy said...

Steve, I was responding to Mr. or Mrs. "cow."

How about this from Hawks:

"The paper gives some details that tend to blunt such criticisms. For one thing, it makes a significant difference whether the couple is sixth or seventh cousins. At that distance -- people who share a great-great-great-great-great grandparent -- the authors argue that any effect is most likely "biological" (read genetic), because other possible differences do not discriminate at that level."

In any case, in the absence of data that mixed couples, or their offspring, have signficantly more children than do the endogamous, coupled with these findings, one is hard pressed to find a fitness rationale for cross- breeding.

Marrying cousins, third or otherwise, is not necessary; as Steve himself once stated, all that's needed is to avoid extreme examples of inbreeding - and, from the decode work, it seems that "extreme inbreeding" may need to be redefined more narrowly than has heretofore been the case.

This work also points out the need to balance considerations of outbreeding depression with inbreeding depression; the fact that the former usually takes several generations to kick in suggests that decode's work is actually underestimating the positive relationship between kinship and reproductive fitness.

This is all besides the issue of parental kinship - genetic relatedness between parents and children - which is maximized when both parents are of similar stock.

There seems therefore to be lots of advantages to (relative, no pun intended) endogamy, and little or no advantages to (relative) exogamay.

For the latter, wishful thinking doesn't count as an "advantage."

John Prester said...

Drudge Report has a timely link today to a story in the Sunday Times (UK), about a government minister (Phil Woolas) who issued a "warning" about the quaint, vibrant, cultural practice of Pakistanis in Britain who marry their first cousins.

Read the "Comments" section, the usual drivel from the "smug, cosmopolitan, sophisticate" set with comments along the lines of "yes, but what about the Royal Family and their marriage practices....and what about the Jews, and what about the Amish".

But some voices of sanity did manage to slip past the Sunday Times George Orwell Memorial Department of Censorship!

http://www.timesonline.co.uk/tol/news/
politics/article3342040.ece

Anonymous said...

Maybe this partly explains the high birth rate for Mormons compared to other white people in the US.

James said...

Steve, Maybe I'm missing something, but I don't think the raw numbers you've posted are accurate. For example, shouldn't two-child family trees yield 2 siblings, 4 cousins, 8 2nd-counsins and 16 3rd-cousins?
I was actually going to point out that the number of unique individuals is lessened by overlap when there is cousin-marriage, but I thought I should point out a more fundamental issue w/ your numbers.

John S. Bolton said...

This contradicts the usual (but political and not biological?)assumption that more outbreeding will yield more hybrid vigor. If outbreeding depression sets in immediately beyond 3rd cousins, how can biology be recruited to serve political programs for integration and inter-racial interbreeding? A mechanism suppressing fertility might be that the greater dissimilarity of parents to their children, as further degree of outbreeding is found, would suppress reproductive effort. It could be called a Laffer curve of of reproductive effort, with degree of outbreeding being like an increase of the tax on work. The Japanese data presented in population genetics textbooks did not extend its curve of falling birth defects out beyond second cousins, yet it was assumed by some that it could be projected out indefinitely far. If that were true, though, mixed-race populations should have taken over demographically in many regions where they have not, such as Southern Africa. Instead of inheriting the advantages of both parent stocks, it seems vastly more likely that the weaknesses of each would be inherited, since randomization of parts does not increase the chance of parts fitting together optimally to serve a function which was once a target of selection. To mention these theoretical expectations though, goes against the feeling that officials are working towards health, when they acccumulate new power to favor integration of dissimilar populations. Whether such policies are healthy or unhealthy, though, will never deter the power-greedy from reaching out for more power. It's away of setting up a smear; claim that power is needed to promote population-genetic health, when this is denied on health grounds, smear away, saying only racism fascism etc. would deny officials that power. The burden of proof is thus illegitimately thrown on to those who doubt that population health would be improved thereby. Don't believe it, then prove that you're not an advocate of racial mass murder.

mq said...

As far as anyone can tell, neither you or Steve has actually read the entire paper in question, so how do you all know what they did or did not "control" for?

Do you have any idea how hard it is to separate social and biological factors in an observational study? Especially when multiple heavily socially determined decisions made in adult life are involved, as in this study? Do you have any feel for the richness of the data that's necessary to even make a start on it? I'd be happy to read the study if there was a direct link, but you don't need to read this to know that it's going to be almost impossible to separate out biology and sociology here.

Answering this question is probably going to have to wait for some real medical / biological understanding of exactly how fitness is affected by the relatedness of the parents.

Still an interesting result, though.

Steve Sailer said...

Can somebody please find a free link to the paper?

By the way, GNXP has some graphs from it.

endogamy said...

Do you have any idea how hard it is to separate social and biological factors in an observational study?

It's also "hard" to criticize a study accurately without reading it.

Especially when multiple heavily socially determined decisions made in adult life are involved, as in this study?

Care to explain how differences in fitness between "sixth" and "seventh cousins" can be chalked up to "socially determined decisions?"

a direct link

sounds familiar

but you don't need to read this

Of course not. It is supportive of endogamy, so it just must be wrong.

to know that it's going to be almost impossible to separate out biology and sociology here.

Do you make the same arguments to Alon Ziv?

Answering this question is probably going to have to wait for some real medical / biological understanding of exactly how fitness is affected by the relatedness of the parents.

Yes, that would be useful, but missing the point. It's not so important that high kinship actually enhances reproductive fitness, or the mechanisms whereby that occurs.

More important is the lack of evidence for the contrary view: that increasing levels of exogamy leads to "hybrid vigor" and enhanced fitness. The "observational" data from this study is yet more evidence that "hybrid vigor" is not an important force - if it is one at all - for humans.

In the absence of such "vigor," parental kinship takes "center stage." The possibility that endogamy may actually raise fitness per se is just "icing on the cake" hammering home that mating "between the lines" of distinct groups is not required for enhanced fitness.

IMR said...

What evidence is there of 'outbreeding depression' in humans?

(A genuine question, not a hostile rhetorical one.)

endogamy said...

What evidence is there of 'outbreeding depression' in humans?

Not exactly a topic studied extensively in today's climate. Besides the deCODE work here, there's Udry et al.

The Udry study is summarized here.

IMR said...

Thank you for drawing my attention to the Udry et al study. I found the full text from the American Journal of Public Health here:

http://www.ajph.org/cgi/content/abstract/93/11/1865

David said...

Why is today's political climate pro-outbreeding, as someone suggested?

Who is thrusting this agenda on the world, if agenda there be? Why? Whose interests would it serve?

I'm sorry I don't have the science to discuss the science, but these political questions fascinate me.

Svigor said...

Who is thrusting this agenda on the world

Learning to notice surnames can go a long way...

n/a said...

Who is thrusting this agenda on the world, if agenda there be? Why? Whose interests would it serve?

Ask Alon Ziv and Noel Ignatiev.

endogamy said...

More on this topic is here.

John S. Bolton said...

Taking a stab at answering David's question on why today's political climate pro-outbreeding: it is that way selectively, as British officials allow 1st cousins to be brought in for arranged marriages, but American judges try to promote integration of races, except when there working on bilingual illiteracy projects. The idea is to choose your issues so as to be able to smear the other side thus putting on the defensive those who resist the growth of power. Prove you're not for inbreeding, instead of officials having to prove that they really need more power against the people's freedom to associate or disassociate as they please.

Zumalacárregui said...

The full text of the study, plus a technical comment that pretty much confirms the point of the original paper, plus an exchange of letters and comments between the deCODE team and other international teams.

http://www.sciencemag.org/content/319/5864/813

This should be commented.