low fertility rates ’cause we don’t marry our cousins

i was going to write a lengthy and very informative post about the likelihood that low fertility rates in our modern world might be related to the fact that we don’t marry our cousins very much anymore (and, in the west, haven’t been for quite a long time now), but there was a good write up in the economist a couple of years ago when that research from iceland (about the fecundity of cousins) came out, so i’m just going to quote that:

“Most explanations of the demographic transition are social…..

“Now yet another explanation has been added to the pot. This is that the mixing-up of people caused by the urbanisation which normally accompanies development is, itself, partly responsible. That is because it breaks up optimal mating patterns. The demographic transition is thus, in part, a pure accident….

“This suggestion is the corollary of a paper published in this week’s Science by Agnar Helgason and his colleagues at deCODE Genetics, a firm based in Reykjavik….

The study’s principal finding is that the most fecund marriages are between distant cousins. Using Iceland’s genealogical records, which allow the degree of relatedness between husband and wife to be calculated with great precision, Dr Helgason showed the optimum degree of outbreeding (measured in terms of the number of children and grandchildren produced) lay somewhere between cousins of the third and fourth degrees….

“The strong relationship between kinship and fertility was so unexpected that the researchers have not yet calculated exactly how much it contributes to the demographic transition. But even from the figures they present, it is clearly an important factor, and one that is likely to apply in other parts of the world where the records needed to prove it are not so good. Even in poor countries, birth rates are now falling fast. An important part of the explanation may simply be the additional choice of mates that development and urbanisation bring with them.”

(note: comments do not require an email. þú þarft ekki að lesa sögurnar heldur.)



  1. Wow, this is an eye opening for me… outbreeding decreases fertility rate, and mostly stemming from biological basis. There’s really a lot left to be answered, a lot…..
    Isn’t Iceland still having the highest fertility in Europe right now? But I do think they are too many other factors that could affect the fertility rate. The biological explanation on demographic transition is really something drastically different, yet possible I guess.
    I’d still be highly interested in reading your lengthy and informative article on this topic in the future…


  2. @the slitty eye – “But I do think they are too many other factors that could affect the fertility rate.”

    yeah — being able to afford lots of kids (like the beckhams) certainly must matter!

    @the slitty eye – “I’d still be highly interested in reading your lengthy and informative article on this topic in the future…”

    me, too! i wonder what (profound things) i’d have to say…. (~_^)


  3. Without a lot of mobility it would be hard not to marry a 3rd of 4th cousin in a s ratesettled village society, wouldn’t it?

    BTW, when are you going to get around to consanguineous marriage rates among Ashkenazi and Sephardic Jews now and in the past? How about the Lubavitchers ? Is it different in Israel than in the U.S.. What about oriental Jews from the Middle East?


  4. what are the figures?
    Something tells me people married to 4th cousins don’t have more kids because of some uncanny biological urge. Its more rural vs urban lifestyles.

    Do college-educated urbanites who happen to marry 4th cousins have more kids than average?

    Biological determinism sounds cool but lets be rational here.


  5. @luke – “Without a lot of mobility it would be hard not to marry a 3rd of 4th cousin in a s ratesettled village society, wouldn’t it?”

    exactly. most people in the past prolly mated with a cousin of some sort.

    re. the jewish inbreeding question — i’ll look into it! it is one of the many things on my list to look into (which is growing longer by the day). (^_^) i did post some random numbers over here, but it wasn’t really a very coherent post. just a few bits and pieces that i’d scraped together. (also, a little bit about jewish endogamy in spain here.)

    there was a tantalizing hint from jack goody that ashkenazi jews — especially the ones in germany, i’m guessing — prolly also started outbreeding more in the middle ages (but not outbreeding out of the group so much, obviously). i quoted him here:

    “We find it difficult to comprehend today just how preoccupied the era [the middle ages] was with the fear of incest — and not only in the various Christian churches but in Jewish circles as well….

    that’s all he had to say, unfortunately, so i know no more than that. (argh!)

    there will be more, anon…. (^_^)


  6. @spandrell – “what are the figures?”

    feel free to read them yourself.

    @spandrell – “Its more rural vs urban lifestyles.”

    well, the icelandic study looked at everybody in rural iceland over several generations. that was prolly a pretty rural population, but the point remains — those who married 3rd-4th cousins had more decendants than those who married other people.


  7. I should mention I just finished reading a history of the Jews in Poland. One of the things that struck me was how small the numbers were for the first couple of hundred years — 20 families here, 12 over there. So there had to be a lot of cousin marriage.


  8. @luke – yeah, you would think so. prolly some inter-marriage with the gentiles, too, tho — as we know (from genetics) that there was. what was the book, btw? you recommend it?


  9. Wow, that’s a lot of interest in an important issue.
    Slitty eye: Good point that there are many factors affecting fertilty. But if you look at the iceland data the error bars are very tight, so factors other than kinship play a very small roll indeed.
    Luke Lea: Good questions. There was a study of Ashkenazi Jews in the US during the past year or two. Comparing genes I think they had the kinship of fourth or fifth cousins, which should mean excellent fertility, but their fertiltiy numbers are in the dead zone just like the rest of us. That means its not genes at work but the control system for the genes, and the shared ancestors need to be pretty close, which they generally are not unless you are in a tiny village.
    Spandrell: Good point. The urban vs rural argument was used for decades to the tune of “they needed children to do farm work.” But if you pursue it and point out that an adult can deliver more work per calorie because he isn’t growing, you find yourself fighting a land war in Asia as the proponent fires a shot, retreats and fires again. Anyway, that issue was explcitly dealt with in the study. Everybody in Iceland has about the same socio-economic status.

    All in all, I say this is the right conversation. My praise to all.


  10. thanks for stopping by, doc — and welcome! i haven’t had a chance to check out your site yet, but i will do so in the near future. looks like you have a ton of resources there! (^_^)

    another blogger who was recently interested in this topic/problem is vanishing american. she posted on the issue here. i will point her to your website.


  11. Many thanks for the kind welcome, hbd chick.
    Avast Spandrel. Good question about education and fertilty. That study has been done. A group in Denmark compared kinship as estimated from the size of town a couple lived in and the distance between their birth places. Once those two factors were taken into account, they found absolutely no effect of education or income on the number of children. Even I was amazed by that one.
    Their study was published in two articles in two different journals. You need to read both to get the whole picture.
    R. Labouriau, A. Amorim Genetics 178 601 (2008).
    R. Labouriau, A. Amorim Science 322 1634 (2008).
    Best regards to all


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