Friends or Fourth Cousins

A photograph of a group of friends hugging each other. Friends or Fourth Cousins.Do your Friends have Similar Genomes?

Do you know who your fourth cousins are?  The chances are you don’t.  But do you know who your friends are?  Of course, you know that much.  But did you ever stop and think why or how your friends are your friends?  Could your friends actually share part of your DNA, to the same extent a distant family member does? 

Is your DNA more similar to that of your friends, or to that of your fourth cousins?  A controversial study from two 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 it is very small, it is in fact the same level of similarity that is expected for fourth cousins.

Other scientists are sceptical about the paper findings, which was published in PNAS (Proceedings of the National Academy of Sciences of the United States of America).


Controversial Genomic Findings

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

“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.


A complicated Consanguinity Relationship Chart or "Cousin Tree", showing the links of genetic kinship.


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

Prof Fowler and Prof Christakis calculated a “kinship coefficient” using the genetic markers from pairs of friends and strangers, and they found that it was slightly higher among friends than between two strangers.

“We’re not really making claims about specific candidate genes here,” Prof Fowler said. “We’re making claims about structural characteristics across the entire genome.”


Disputed Conclusions

Researchers have expressed concern about different factors that could be affecting 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 criticised earlier research by Christakis and Fowler, 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.”

A composite picture showing a DNA siagram being drawn on transparent glass by a white-suited scientist. Is there a genetical link to our Friendship?
The study compared one-letter differences in DNA, known as “single nucleotide polymorphisms”.

The authors 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.”


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 a few 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.

A photograph showing the unlikely friendship between two animals - a ginger cat and a fawn.
Far-fetched, but…  Could this research help to explain even the most unlikely of  friendships?

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.”

And if you are sociable, but have difficulty making friends, does it mean you are fishing for friends in the wrong genotype pool?


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