Kissing cousins have more kids

By Jeanna Bryner

updated 6:32 p.m. ET Feb. 7, 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."

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.

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