Further research confirms what was already observed across nature: couplings between 3rd- or 4th-cousins produce a greater number of successful offspring than between more distantly related individuals.
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.
[...] 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.
[...] 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. [...]
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.
[...]
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."
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