Marrying a third-cousin gives you the most kids

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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."

However, shacking up between close cousins ups the chances of both partners carrying a recessive gene for some detrimental condition. The resulting kids would have a 25 percent of expressing that gene, meaning they'll have the disease.

On the flip side, coupling with a close cousin means it's more likely the mother and fetus will be genetically compatible. The mother won't have to worry so much about so-called Rh incompatibility, which can be lethal to a fetus.

"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 Icelandic population, they say, is relatively small and homogeneous with little variation in family size, use of contraception and marriage practices. So the results are not confounded by other variables, such as economic status, which have biased results from past studies of kinship and reproduction.

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.

Forbidden Love

The newly discovered positive link between closely related couples, called consanguinity, and offspring is clouded by social norms.

"These are not the results we expected to find," Stefansson said in a telephone interview. "I have been brought up in a culture that looks down on consanguinity so I feel somewhat ill at ease with such data in many ways. This is not what I expected, but it only goes to show how incredibly complicated nature is."

The "ick" factor associated with marrying a close relative has a long history. Citing a Bible verse, Pope Gregory I said in the sixth century that sacred law forbad a man “to uncover the nakedness of his near kin” and that such unions were infertile.

But Bittles notes that first-cousin unions were quite common and highly regarded in Western Europe and the United States in the first half of the 19th century. "But as the century progressed, a suspicion that the offspring might not be healthy began to emerge," he said, "and this trend continued throughout the 20th century, resulting in ever fewer first cousin marriages."

Interesting... I've sometimes wondered how far the cultural taboo on this sort of thing should stretch. I mean, I believe the whole human race has come from the same two great-great-great.......grandparents, so we're all "related", it's only the amount of relation that differs. So, where "should" we draw the line? (I should say here that I have no personal motivations toward something like this; it's just one of those hard-to-answer questions which seems to call for a randomly-placed line of seperation.)

I'm hard pressed to know how in the heck anyone is going to restrict their searches for mates to their 4th cousins in any real way, let alone with the idea of making it the highest priority to optimze having more kids and grandkids. And I don't see how government policy can be fashioned to have any impact, at all. So I lump this into the "cute thought to introduce for table talk" category.

Your point about everyone having the same two ultimate parents is interesting. Speciation tends to occur through isolation (either the more obvious allopatry or through the somewhat less obvious sympatry or paraptry) and the accumulation of gradual changes (genetic drift) selected for that eventually amount to a separate species no longer able to interbreed when the isolation barriers recede. Saltation can apply, more to bacteria, and is rather abrupt. I don't think that occured in humans, but I think I've read one or two arguments in that direction. In the case of humans, I don't think the conclusion is clear just yet but do you imagine that there were only two? I lean away from this idea, as I don't know of any scientific case for it, so I'm curious why you imagine it must be so.

Jon

Well, I'm a Christian, so I'm talking about Noah and his wife.

Well, I know a lot of excellent Christians who fully embrace evolution, so being a Christian isn't enough to let me figure such things out. Among those Christians I know who know take the scriptures pretty seriously in their faith, but aren't necessarily gung-ho about evolution, I can't recall a single one of them telling me that they believed that Noah and his wife were the sole parents of all of the following:

• the Capoid or Khoisanid Subspecies from southern Africa (hottentots and bushmen)
• the Congoid Subspecies of sub-Saharan Africa
  (Palaecongoid, Sudanid, Nilotid, Kafrid or Bantid, Bambutid (pygmies), and Aethiopid)
• the Caucasoid or Europid Subspecies
  (Mediterranid, Dinaric, Alpine, Ladogan, Nordish, Armenid, Turanid, Irano-Afghan, Indic, and Dravidic)
• the Australoid Subspecies (Veddoid, Negritos, Melanesian, and Tasmanian)
• the Mongoloid Subspecies
  (Northeast or Northern, Southeast or Southern, Micronesian-Polynesian, Ainuid, Tungid, and Amerindian)

How do you imagine that just two people were the parents carrying the variations found in all of the above?

In any case, many Christians I know take many of the books in the old testament as allegories, not the literal and exact documentation of historical fact. For example, Job is my favorite book in the old testament -- a bet made between god and the devil with one man caught in the middle of it all. But I don't imagine that many people really believe such a bet actually occurred as book of Job tells it.

Jon

Well Jon, firstly, I'm not here to be interrogated,

and secondly, you shouldn't assume that everyone is thoroughly versed in the data partaining to the list of names you gave. If you're not showing off, you're incredibly naïve.

I could whip out some questions regarding one of my special-interest areas, and demand answers from you because of a contrary belief you merely expressed, and you'd have no clue how to respond; big deal.

The beauty of the free will God gave us all is that you can believe what you want, and I can believe what I want, and neither of us has to get in a tizzy about it.

[youtube]http://www.youtube.com/watch?v=esl2NNOtHQE[/youtube]

I just had to post this here.