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.
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.