It's just been discovered that genes influence who we choose for friends. We each have about 23,000 genes -- the human genome -- and by and large our genes are the same. But about 0.1 percent of the human genome varies from person to person. Publishing last month in the Proceedings of the National Academy of Sciences, scientists at Yale and the University of California, San Diego, compared the differences in genes between friends and strangers across nearly 2,000 people. And they found that friends tend to have a more similar set of genes than strangers.
"Your friends don't just resemble you superficially, they resemble you genetically," said Nicholas Christakis, one of the co-authors of the study. So much so that the similarity is as if friends shared a great-great-great grandparent. Perhaps it's obvious that we like people similar to ourselves. But the details are surely unexpected; that genes linked to our sense of smell showed this effect especially well. How could we pick our friendships on the basis of a person's olfactory genes?
Perhaps it's "The Starbucks Effect" -- if two people like the smell of coffee maybe they're more likely to hang out together. Or perhaps they're more likely to meet in the coffee shop in the first place. "I'd say love was a magical thing," George Michael sings, "[but]... turn a different corner and we never would have met."
Perhaps even more surprising was that our immune system genes went against this trend -- they showed the opposite effect -- that friends tend to have different versions of immune system genes. This fits with research done back in 1994, suggesting that women found the smell of men's T-shirts to be especially sexy when worn by men who had different versions of immune system genes. The thinking here is that we may choose sexual partners with a view to maximizing the diversity of our children's immune system genes -- an advantage for fighting disease.
The idea of genes pervades our culture, and we have no problem accepting that our physical characteristics -- hair and eye color, for example -- are dictated by our genetic makeup. But can something that feels as intimate as choosing a sexual partner be similarly influenced by our genetic inheritance? Scientists broadly agree that some animals choose mates according to the versions of immune system genes they have. There is evidence that something of this is true in humans but there is controversy in how big an effect this is -- because human interactions are undoubtedly complex. Nonetheless, already a new start-up company based in Canada, Instant Chemistry, is pressing ahead; offering match-making services for people based on their genetic make-up. Another dating agency has even claimed that women experience higher rates of orgasm if they choose partners with immune system genes different to their own.
Much other research -- far less contentious -- has also shown that immune system genes have far greater reach than we once thought. There's evidence that they correlate with the likelihood that couples have certain problems in pregnancy -- including pre-eclampsia or foetal growth restriction. And there's very recent evidence that they can influence how brains are wired. A study published in Nature in May, from Carla Shatz's team at Stanford, found that a specific immune system gene was important for the right connections to be made between neurons involved in sight.
The big idea that emerges is that our genes don't just do this or that one thing; they multi-task. So lots of different systems in the body -- say the immune system and the nervous system, or the immune system and reproduction -- are intimately connected because they share some of the same components. Our understanding of this is only beginning. But already now, it seems that a shocking amount of what we do and who we are is connected to how we have evolved to survive disease. A new field of science will surely emerge in the 21st century -- when the right tools are developed, biological, computational, mathematical -- for us to really understand the wondrous connections between smell, sight, immunity, friendships and sex; this is a whole new level of human anatomy that we've only just glimpsed.