Image copyright Thinkstock Image caption Is your DNA more similar to that of your friends, or your fourth cousins?

A study from a controversial pair of US researchers claims that we are more genetically similar to our friends than we are to strangers.

Looking at differences between nearly 2,000 people, recruited as part of a heart study in a small US town, they found that friends shared about 0.1% more DNA, on average, than strangers.

While small, this is the same level of similarity expected for fourth cousins.

Other scientists are sceptical about the paper, which was published in PNAS.

"I think that they're unusual findings, and that usually draws criticisms from scientists," said Prof James Fowler, one of the study's authors and a professor of both medical genetics and political science at the University of California, San Diego.

Together with Prof Nicholas Christakis from Yale University, Prof Fowler analysed nearly 500,000 single-letter markers from across the genome, using data from the Framingham Heart Study.

These results were useful because as well as providing DNA samples, participants were asked who their closest friends were. "Because the study started in a small community, many people that were named as friends, also happened to be involved in the study," Prof Fowler explained.

So he and Prof Christakis calculated a "kinship coefficient" using the genetic markers from pairs of friends and strangers, and found that it was slightly higher among friends.

"We're not really making claims about specific candidate genes here," Prof Fowler told BBC News. "We're making claims about structural characteristics across the entire genome."

[This is not] just being driven by people who were accidentally friends with their fourth cousins and didn't tell us Prof James Fowler, University of California, San Diego

Disputed conclusions

Other researchers have expressed concern about different factors that could affect the results - such as ethnicity or other types of "population stratification" - which could make people both genetically similar and more likely to be friends.

Prof Evan Charney from Duke University, who has criticised earlier research by Fowler and Christakis, said this type of analysis only works if none of the subjects are related to each other at all, which is difficult to confirm.

"These studies depend upon that assumption - that you're looking at thousands of people who are not related," he told the BBC.

The authors, however, tried to allow for any stratification or family relationships within their population, in a smaller comparison of 907 pairs of friends, this time including nearly 1.5 million genetic markers.

"We excluded any one of them that had any relatedness whatsoever," explained Prof Fowler. "We didn't want anyone to think that this was just being driven by people who were accidentally friends with their fourth cousins and didn't tell us."

Image copyright Thinkstock Image caption The study compared one-letter differences known as "single nucleotide polymorphisms"

Dr Rory Bowden, a statistician and lecturer at the Wellcome Trust Centre for Human Genetics in Oxford, also expressed reservations about whether the study could be confounded by "very subtle genetic population structure".

"I wonder whether [these methods can fully account for] factors known to drive friendships, like church membership, sports or other cultural affinities, that would also lead to a correlation with genotype," he said, "because they reflect differences in places of origin within Europe of the Framingham participants."

Prof Charney further points out that studies like this can over-emphasise the importance of our DNA sequence itself. "People do not have the same genome in all the cells and tissues of their body," he said.

With some exceptions, like the mutations that cause rare genetic diseases, Prof Charney argues that even in great numbers, the single-letter genetic markers used in studies like this one have not proved very informative about human traits and behaviour.

But the study's authors remain confident. "Most people don't even know who their fourth cousins are!" said Prof Christakis. "Yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin."