Ask the average person why they chose their current friends and, usually, they'll cite common interest;, a shared love of sport or an enthusiasm for fine wine. But missing from that list is a powerful and hitherto unrecognised force - your own DNA: you are much more likely to form lasting alliances with people who are genetically similar to yourself.
The discovery was made by social scientist James Fowler, from the University of California San Diego. He compared the pattern of nearly half a million genetic markers called SNPs ("snips") between people who were in established friendships and people who were strangers.
Surprisingly, people who were on good terms with one another shared a higher proportion of the genetic markers than one would expect on the basis of chance, or if they were strangers to one another. In fact, the genetic overlap was equivalent to the match one would see between fourth cousins.
Even more interesting is that the match is closest for genes that we know are evolving or changing fastest in the population, which Fowler thinks is the key to explaining the phenomenon.
"If you evolve a new gene that gives you a new ability, like language," says Fowler, "then it's only advantageous to you if you are hanging around with others who also have that genetic capability. Otherwise, who are you going to talk to?!"
At the same time, some of the effect is likely to reflect environmental selection pressures or, as Fowler puts it, a coffee-shop scenario. "If you have a gene that makes you absolutely love coffee, and I have a gene that makes me love it too, then we'll more than likely head to the coffee shop, where we have a chance to become friends."
But not all of our genes are looking to match-make. In fact, it's clear that the opposite is happening in some select parts of our genomes, including the genes that control the functioning of the immune system.
"You don't want friends who are susceptible to the same ills that you are," explains Fowler, "you want to be surrounded by people who are resistant to the things you are susceptible to, so they acts as a line of defence for you." So, could all this positive DNA discrimination carry a downside, hitting us in the genetic diversity department? Potentially, yes, but, Fowler argues, the disadvantages are more than compensated by the benefits.