You'll Have More Kids by Marrying Your Cousin

We know that marrying your first degree cousin is not good. Inbreeding or consanguinity (marriage between close relatives, like first degree cousins) increases the risk for negative mutations to appear in double sets and manifest in children, in a proportion of 25 % (which, in human probability, may mean that all the children can be born ill). This explains why 50 % of the Bedouins in the Sinai Peninsula are born deaf!

And here comes this research published in the Science journal, stating that families founded by third or fourth cousins have more kids and grandkids than others. A cultural taboo in most western cultures, "kissing cousins" appear to have a biological boost.

The Icelandic research shows that, while first-cousin couples could face inbreeding issues, extreme couples could experience genetic incompatibilities.

2008 is the year when world's urban population is going to bypass 50 % and Iceland's shift to a highly urbanized society could plummet population growth as individuals form pairs from a larger pool of distantly related mates, having less children.

"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. During the past two centuries, the average relatedness of Icelandic couples has widened from third and fourth cousins to the more recent couple relatedness of fifth cousins," wrote the authors.

"Our definition of a species is a group of individuals who are closely enough related to each other to be able to have offspring. There is recognition in that definition of the fact that individuals have to be somewhat related to each other to be able to reproduce," said lead author Kari Stefansson of the University of Iceland in Reykjavik.

Mating with a first degree cousin may not be good, but a close relative will mean that mother and fetus are more genetically compatible. Issues like Rh incompatibility, deadly for the baby, could not appear.

"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, not involved in the new research.

The Icelandic team investigated over 160,000 Icelandic couples going back 200 years, based on the deCODE Genetics genealogical database, a company co-founded in Stefansson, in 1996.

"The Icelandic population 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," wrote the authors.

Women born between 1800 and 1824 who married a third cousin had 4 children and 9 grandchildren on average, while those marrying eighth degree cousins or more distant relatives had 3 children and 7 grandchildren.

In the case of the women born between 1925 and 1949, those marrying third degree cousins had 3 children and 7 grandchildren on average, while those marrying eighth degree cousins or more distantly had 2 children and 5 grandchildren.
One factor could be the fact that more related couples married earlier.

Other researches came with "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.

"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, and this trend continued throughout the 20th century, resulting in ever fewer first cousin marriages," added Bittles.