Iceland's 'Kissing Cousins' Breed More Kids Marriages between distant cousins seem to produce more kids. But why?

Feb. 8, 2008 -- While the thought of searching for a potential mate at a family reunion might sound repulsive to some, researchers in Iceland report that "kissing cousins" may produce more children and grandchildren than unrelated couples.

A study released Thursday in the journal Science found that marriages between third or fourth cousins in Iceland tended to produce more children and grandchildren than those between completely unrelated individuals.

Researchers at the deCODE Genetics company in Reykjavik mapped out kinship among all known Icelandic couples whose members were born between 1800 and 1965. They then compared the numbers of children and grandchildren descended from these 160,811 couples.

Researchers were shocked to find that for women born between 1800 and 1824, marriages between third cousins produced an average of 4.04 children and 9.17 grandchildren, while marriages between eighth cousins or more distantly related couples had averages of only 3.34 children and 7.31 grandchildren.

For women born between 1925 and 1949, with mates related at the degree of third cousins, the average number of children and grandchildren were 3.27 and 6.64, compared with 2.45 and 4.86 for those with mates who were eighth cousins, or more distantly related.

"These are counterintuitive, almost dislikable results," said Dr. Kari Stefansson, senior author of the paper on the study.

Dislikable, because our intuition is that the more closely related you are to your mate, the higher the chances of passing along the unfortunate traits so often associated with inbreeding.

Researchers believe the trend toward a more prodigious relationship with a not-so-distant relative must have a biological basis, though scientists have not identified exactly what biological mechanism could be behind this.

"One of the things you have to realize is that the definition of a species is 'a group of individuals that are sufficiently related to each other to be able to reproduce,'" Stefansson added. "This definition assumes that individuals that reproduce together have a minimal relationship to each other.

"But these results show that, in certain situations, there's a certain biological compatibility between couples, which seems good for the fate of future children."

And because the researchers sought to eliminate any socioeconomic factors that could influence the number of children a couple might have, they believe these findings must have a biological basis.

Picking the Proper Gene Pool

Previous studies on the relationship between kinship and fertility rates have been sparse, and the results, some experts say, have often been distorted by other social variables. Marriage, it turns out, is not an exact science.

For example, a 1991 study also published in Science found that, in Asian and African populations, marriages between related individuals also produced more offspring. However, researchers only evaluated relationships no more distant than second cousins, and the populations they studied showed great socioeconomic disparity.

In the most recent study, researchers sought to eliminate some of these confounders by limiting their study to only the Icelandic population — a country of relative socioeconomic homogeneity, where there is little variation in family size, use of contraceptives, or marriage practices.

Moreover, because data were collected from the nation at large rather than from a random sample, Stefansson notes that the results should be free of most of these potential confounders.

According to Stefansson, these results are particularly striking in their consistency throughout time, even as socioeconomic factors in Iceland began to change.

Results showed that marriages between third or fourth cousins produced more offspring than unrelated couples from the years when Iceland was a predominantly poor and rural country up until the present-day era of a highly urbanized society, with one of the highest standards of living in the world.

Now, many gene experts are scratching their heads while trying to explain the biological mechanism behind these results.

"Uncle, Can I Call You Dad?"

According to Stefansson, the reason that related couples were more biologically successful may be because these couples have "just right" genes when combined — not too similar, but not too dissimilar, either.

Although it is impossible, at this point, to determine the biological explanation for these results, Dave Greenfield, psychologist and director of the Healing Center in West Hartford, Conn., agreed with Stefansson's theory.

"My guess would be ... that there may be some unknown genetic compatibility that is slightly greater for three-quarter cousins than for the general population," Greenfield said. "This is just an opinion [or] theory."

What gene experts do know is that nookie between closely related people, such as first or second cousins, increases the chances of passing down a recessive gene for a detrimental condition to their offspring.

According to Dr. Bruce Buehler, director of HBM Genetics at the University of Nebraska Medical Center, the chances of married siblings passing down an unfavorable recessive gene is one in 16, while, for first cousins, the chances are one in 64.

"At least genetically, this information doesn't suggest that second or third cousins would be at any higher risk for passing down unfavorable traits," Buehler said.

However, Buehler added he "can't think of any genetic explanation for why the third or fourth cousins would have more babies."

Instead, Buehler supposed that related couples might shack up more often, simply because of pheromones.

"Maybe what we're seeing here is biologic attraction," Buehler said. "If you really look alike, feel alike and think alike, then maybe you have sex more often and have more babies. We do know that there are pheromones which cause attraction, and I wouldn't be surprised if related people have higher sexual desire for one another."

But despite the inability to offer a concrete biological explanation for these findings, Stefansson strongly believes this study has implications on the genetic future of the global population.

"The take-home message is that ... we, as a society of [the] 21st century, have basically ruled against the marriages of closely related couples, because we do not look at it as desirable that closely related people have children," Stefansson said. "But in spite of the fact that bringing together two alleles of a recessive trait may be bad, there is clearly some biological wisdom in the union of relatively closely related people."

Stefansson added that, as globalization continues to offer people great diversity in the selection of distantly related partners, perhaps the number of children born in the world will decrease.

"Globalization will probably result in marriages of more people of different ethnic groups, but according to these observations, that will decrease the number of children born in this world," Stefansson said.