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Couples Who Are Third or Fourth Cousins Have More Kids, Grandkids Than Other Couples

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Jeanna Bryner, FoxNews, February 8, 2008

Couples who are third or fourth cousins tend to have more kids and grandkids than other couples. And though considered somewhat of a cultural taboo, mating between “kissing cousins” makes good biological sense, say scientists.

The findings, which come from a recent study of Icelanders, shed light on how relatedness affects reproduction and ultimately the size of families.

The researchers suggest marrying third and fourth cousins is so optimal for reproduction because they sort of have the “best of both worlds.” While first-cousin couples could have inbreeding problems, couples who are far-removed from each other could have genetic incompatibilities.

The study also has implications for population growth in a world that’s becoming more and more urbanized. In Iceland, the dramatic demographic shift from a rural society to a highly urbanized one could slow population growth as individuals mingle with a bigger pool of distantly related mates and therefore have fewer kids. A similar urban shift is happening across the globe. In 2008, for the first time in history, more than half the world’s population will live in towns and cities, according to the United Nations.

“The formation of densely populated urban regions that offer a large selection of distantly related potential spouses is a new situation for humans in evolutionary terms,” the researchers write in the Feb. 8 issue of the journal Science.

During the past two centuries, the researchers point out, the average relatedness of Icelandic couples has widened from third and fourth cousins to the more recent couple relatedness of fifth cousins. (Children of siblings are cousins. Children of first cousins are second cousins, and their children are third cousins.)

Cousin Couples

The results make sense from a biological perspective. “Our definition of a species is a group of individuals who are closely enough related to each other to be able to have offspring,” said lead author Kari Stefansson of the University of Iceland in Reykjavik. “There is recognition in that definition of the fact that individuals have to be somewhat related to each other to be able to reproduce.”


“It could be argued that in human populations there is a point of balance between the disadvantages associated with inbreeding versus those with outbreeding,” said Alan Bittles, director of the Center for Human Genetics at Edith Cowan University in Western Australia. Bittles was not involved in the new study.

A Family Affair

Stefansson and his colleagues studied more than 160,000 Icelandic couples going back 200 years, starting with those born in 1800, using the deCODE Genetics genealogical database. Stefansson has served as president and chief executive officer of deCODE since he co-founded the company in 1996.


The team found that women born between 1800 and 1824 and who partnered with a third cousin had an average of about four children and nine grandchildren, while those related to their mates as eighth cousins or more distantly had three children and seven grandchildren. A similar pattern showed up for women born between 1925 and 1949. Third cousins had an average of three children and about seven grandchildren, compared with two children and five grandchildren for eighth cousins and beyond.

One caveat: More closely related couples may just start making babies earlier than others. Past research has revealed “strong evidence that couples who were first cousins married earlier and were less likely to use contraception, the wives had their first child earlier, and they continued child-bearing at later ages,” Bittles told LiveScience.


[Editor’s Note: The abstract for “An Association Between the Kinship and Fertility of Human Couples,” by Agnar Helgason, et al. can be read below. The full text is available here. There is a charge.]

Original article

(Posted on February 8, 2008)

Abstract: An Association Between the Kinship and Fertility of Human Couples

Science, February 8, 2008:

Science 8 February 2008:
Vol. 319. no. 5864, pp. 813 - 816
DOI: 10.1126/science.1150232

Agnar Helgason,1,2* Snæbjörn Pálsson,1,3 Daníel F. Gubjartsson,1 órur Kristjánsson,1 Kári Stefánsson1,4

Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis.

1 deCODE Genetics, Sturlugata 8, 101 Reykjavik, Iceland.
2 Department of Anthropology, University of Iceland, 101 Reykjavik, Iceland.
3 Department of Biology, University of Iceland, 101 Reykjavik, Iceland.
4 Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland.

* To whom correspondence should be addressed. E-mail:

Original article

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I’m quite intrigued with the “sudden” flood of studies that prove - suprise, suprise, - that it’s “all in the genes”.

But I’m sure the miscegenators will fire back at this with the usual “inbreeder!” insults.

Posted by Obscuratus at 5:48 PM on February 8

I am surprised this study was allowed to proceed. It is surely toxic to all the highest ideals of globalists and racial egalitarians.

Posted by PBL at 6:58 PM on February 8

Sweden was once a crime-free country. People we not locking their homes as late as in the 1950ties.

Then, they got this moronic idea to import blacks from Africa in order to help avoiding mariages between distant cousins.

Now, we know that not only did they pay a steep price for their experiment (crime is out of control in Seden’s big cities now), but also they went into that trouble for nothing, because it was actually advantegeous for distant cousins to marry and have kidds.

One more example of sorry consequences that the Liberal utopian doctrines bring on us.

Posted by A Reader at 9:10 PM on February 8

I wonder if there is more to this story, possibly the people who are marrying this close of a relative are members of a religion who does not allow people to marry outside the religion and the only availible marriage partners are somewhat close relatives. The Amish or old order Mennonites here in the US would be an example of this.

Posted by Spartan24 at 9:21 PM on February 8

So all I have to do is move back to Pittsburgh and maybe I’ll run into a 4th or 5th cousin? Geeze, this may be good genetically, but I can’t help but think about Springfield’s rival, Shelbyville, from the Simpsons.

Posted by Jacqui in AZ at 12:09 AM on February 9

I would think that marrying cousins of some degree was pretty standard in small towns in Europe during the Middle Ages. You were pretty much related to everyone give the lack of mobility.

Posted by at 11:06 AM on February 9

Finally an article on what I have been saying for years. How could smaller populations, isolated and related in this way - first as in a tribe and then as in a regional group of settled people - have not only survived, but developed such outstanding physical and mental uniqueness?

When my wife and I were married in the Balkans, I had asked a friend, a young woman, to tell me about the local customs. One of the first things she told me was that “it used to be a great shame to marry outside of the village.” Hence, a ritual exists where the people in the village would barricade the road out and the brothers would demand a ransome before the outsider groom could leave with his bride. In our case, I had to chase my landlord and his friend, who had “stolen” my bride - around in a car from village to village, until we located them - and ransomed her back with drinks at the local gostilna. It was fun, but the point is that this was based on a very strong tradition that had allowed the local people to become who they were, and preserve it.

I have long held that outbreeding is just as big a risk as inbreeding, and we see evidence of this daily, yet no one will speak of it. Perhaps the dialogue is about to begin?

Posted by Whiteplight at 1:30 PM on February 9

“Then, they got this moronic idea to import blacks from Africa in order to help avoiding mariages between distant cousins.”

Is that why they imported blacks? Gee I always thought it was because they needed people to work in the mills.

Posted by at 1:35 PM on February 9

The divorce rate between cousins would probably so low that the lawyers would either starve or find honest work.

Posted by Harry at 6:16 PM on February 9

Hi Whiteplight.

You asked for information about why I said that women are lighter in every race. I said that they are lighter because they have greater need for vitamin D due to the rigors of pregnancy. The relevant link is here. The reference is:

Jablonski, N. and Chaplin, G. (2000) The Evolution of Human Skin Coloration. Journal of Human Evolution.

Posted by Robert Lindsay at 7:43 AM on February 10

It seems odd that they are bringing up stats from 100 to 200 years ago, when the world was a much different place, and not factoring in those differences. Of course people married younger back then, they didn’t live as long, and they did not have reliable birth control either!

Posted by Sonya at 7:52 AM on February 10

“it used to be a great shame to marry outside of the village.”

It still is in Switzerland. To a Swiss, a “foreigner” is anyone from outside his home canton.

Posted by qwerty at 8:32 AM on February 10

How likely are people these days to even know their 3rd or 4th cousins? I’ve met a handful of them on rare occasion (family funerals, mostly) but still wouldn’t even recognize them on the street. If I married one it would be purely by accident.

My guess is that marriage to a 3rd cousin is distant enough to not result in any genetic diseases but you’re still close enough to have some common cultural ground and to have some sense of loyalty other than simple love. The more loyalty there is in a marriage, the better.

Posted by Alan at 9:55 PM on February 11

With my knowledge of genetics, I have always been aware an optimal balance exists in genetic degrees of separation. I don’t know if it lies between 3rd and 4th cousins, but I am happy to see an article address a question I have wondered about in the past.

Posted by at 4:57 PM on February 12

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