A new study gives cause to reflect on the saying, "friends are the family we choose." Researchers at the University of California-San Diego and Yale University in New Haven, CT, find that friends who are not related biologically have more DNA in common with each other than with strangers.

Writing in the Proceedings of the National Academy of Sciences, James Fowler, professor of medical genetics and political science at the University of California-San Diego (UCSD), and Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale, describe how they conducted a genome-wide analysis of nearly 1.5 million markers of gene variation using data from the Framingham Heart Study.

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The closest of friends: researchers find that friends who are not related have more DNA in common with each other than with strangers.

Prof. Fowler says, "Looking across the whole genome we find that, on average, we are genetically similar to our friends. We have more DNA in common with the people we pick as friends than we do with strangers in the same population."

The authors picked the Framingham Heart Study's data set because it is the largest study they are aware of that has the level of genetic detail necessary for such an investigation, together with information on who is friends with whom among the participants.

From the data, they were able to pick out nearly 2,000 individuals and compare pairs of biologically unrelated friends with pairs of unrelated strangers.

The other thing that is useful about sourcing the data from the Framingham study is that nearly all the participants are of European extraction. This is normally a disadvantage in research, but in this case it is an advantage because it rules out a potential bias - that we tend to choose friends with a similar ethnic background to our own.

The researchers were also able to rule out ancestry, using the most conservative techniques currently available.

Prof. Fowler says their results are "net of ancestry" and go beyond what you would expect to find among people of shared heritage.

Friends are the genetic equivalent of fourth cousins

The study shows that, on average, friends share about 1% of their genes - this about the same overlap as we can expect to find among fourth cousins - that is, relatives who have the same great-great-great grandparents.

Prof. Christakis says while 1% may not sound like much to most people, to geneticists it is a significant number. He describes the result as "remarkable," in that:

"Most people don't even know who their fourth cousins are! Yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin."

The authors describe how it makes sense to have these shared genes - they confer various evolutionary advantages. For instance, for our earliest ancestors, having a friend who felt cold when they did and built a fire will have benefited both of them.

Prof. Fowler says there is also a kind of social network effect, in that having a trait may only work if others also have it. Take, for example, the first mutant to speak. They needed someone else with a similar mutation to converse with.

Friends are most similar in genes controlling sense of smell

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The findings show that among friends, the biggest difference was found in genes that control immunity or ability to protect against disease - which may prevent disease from spreading among the group.

As well as looking at average similarities across the genome, the two scientists also looked more closely at particular sets of genes.

They found, for example, that friends are most similar in genes that control the sense of smell, and most different in genes that control immunity or ability to protect against disease.

Again, the authors see an evolutionary explanation for the immunity finding - having friendships with people whose genes protect them against a different range of diseases reduces the risk of a disease spreading in the group.

This finding is similar to previous studies that have the same effect in spouses.

But how are we selecting friends based on differences in immunity and similarity in ability to smell? That is still to be discovered, says Prof. Fowler, who suggests one explanation for the sense of smell finding is that our sense of smell means we gather in similar environments - take coffee houses, for example. However, the authors say the underlying biology is likely to go further than that, and further studies will probably show a number of mechanisms are involved, working together and in parallel.

Genes similar between friends are evolving faster than other genes

Another surprising finding was that the genes that seem to be more similar between friends appear to be evolving faster than genes that are not. Perhaps this explains why human evolution appears to have gone faster in the last 30,000 years than previous to that. The social environment could also be a reason, say the authors.

Prof. Christakis says the study also supports the idea that humans are "metagenomic," that is, our fitness to survive depends not only on our own genes, but that of friendly organisms inside us (our gut microbes, for instance) and around us, such as the genes of our friends.

Funds from the National Institute on Aging and the National Institute for General Medical Sciences helped finance the study.

Meanwhile, Medical News Today recently learned of another surprising genetic study where researchers have developed a computer algorithm that can diagnose rare genetic disorders from photographs of children.