Human beings spontaneously tend to choose people who are genetically similar in their social circle. The Framingham Heart Study, conducted in Massachusetts, was published in the Proceedings of the National Academy of Sciences. That US state has a large collection of genetic data - 1,5 million - as well as information about relatives and friends of people to whom the data is linked. The sample examined is 1932 inhabitants and the data relating to friends who are not relatives and a group of similar size of other unknown people were compared for each one.
It was found that those in the circle of friends shared 1% of genes, a much higher percentage than the number of genes shared with the sample of strangers. 1 percent sharing doesn't seem high, said Nicholas Christakis, a professor of sociology, evolutionary biology and medicine at Yale University, but, to geneticists, it's a significant number. The friends were, in short, related as are people who share the same "great-grandparent", therefore at the level of fourth cousins. Professor Christakis writes: “Somehow, out of a myriad of possibilities, we tend to choose friends who are genetically related to us”.
