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 A study published on Monday found that people are apt to pick
friends who are genetically similar to themselves - so much so that
friends tend to be as alike at the genetic level as a person's
fourth cousin.
The findings were based on an examination of about 1.5 million
markers of genetic variations in a group of nearly 2,000 people who
had taken part in a long-running health study based in
Massachusetts. The researchers compared people identified as friends
to those who were not.
The study showed people were most similar to their friends in
olfactory genes, which involve the sense of smell, and were least
similar in relation to immune system genes.
 "Olfactory genes have a straightforward explanation: People who like
the same smells tend to be drawn to similar environments, where they
meet others with the same tendencies," said one of the researchers,
James Fowler, a professor of medical genetics and political science
at the University of California, San Diego.
The study, published in the scientific journal Proceedings of the
National Academy of Sciences, follows research released in May that
found that people tended to choose spouses who have similar DNA.
Fowler said the new findings made it clear that people have more DNA
in common with those who are selected as friends than with strangers
in the same population. Fourth cousins are people who have
great-great-great grandparents in common.
Because the study population was largely homogeneous, mostly whites
of European background, the findings "are less likely to be driven
by the simple explanation that people of similar ancestry befriend
one another," Fowler said.
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Fellow researcher Nicholas Christakis, a Yale University professor
of sociology, evolutionary biology and medicine, said the mechanism
used by people to choose friends with similar genetics remained a
mystery.
"It could involve the workings of a postulated 'kin detection
system' in humans," Christakis said. "Our fates depend not only on
our own genes, but also on the genes of others around us, and in
particular our friends."
Christakis said he was interested in finding out why people have
friends in the first place.
"The making of friends is exceedingly rare in the animal kingdom,"
Christakis added. "Certain other primates, elephants and whales are
the only other mammals who do this, and this alone aroused our
curiosity."
(Reporting by Will Dunham; Editing by Lisa Von Ahn)
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