We Choose Friends Who Are Genetically Similar to Us

On average, our friends are like the genetic equivalents of fourth cousins

Photo: Oliver Rossi/Corbis

Sociologists have long pointed out that we often favor people who look like us. Now, a new study shows that that bias runs deeper still: we tend to chose friends who are genetically similar to ourselves. Unexpectedly, the similarity can't just be explained away by friends who share the same ancestral heritage.

Researchers from the University of California, San Diego, arrived at this finding after comparing genome-wide data from nearly 2,000 individuals with that of their unrelated, platonic friends and of strangers. Crucially, the subjects were part of a database predominantly composed of people from European backgrounds, and the researchers further controlled for ancestry within that population. It's not just a matter of Poles hanging out with Poles and Danes with Danes. 

Compared to strangers, the people the subjects chose to be friends with had significantly more in common genetically. They shared about one percent of their genome - about as related as fourth cousins. Most often, friends shared genes related to sense of smell, the authors found. 

Long ago, the researchers think, this tendency to chose genetically similar friends might have provided our ancestors with an evolutionary advantage. Having people around who share some of the same weaknesses, preferences and needs can be useful for building a support network. 

There was one exception to this rule, however. Friends significantly differed in their arsenal of immunity genes, the team found. Speculating, the researchers think that this might increase the chances that our friends will be more resistant to the germs that cripple us, and could thus take care of us and help stop the spread of infection.

Other studies have uncovered similar immunity findings with regard to the people we find most attractive, and kissing seems to be the primary mode of uncovering those genetic underpinnings. Sheril Kirshenbaum, author of The Science of Kissing: What Our Lips Are Telling Usexplains

A kiss puts two people in very close proximity. Our sense of smell allows us to pick up subconscious clues about the other person’s DNA or reproductive status. Biologist Claus Wedekind found that women are most attracted to the scent of men who have a very different genetic code for their immune system in a region of DNA known as the major histocompatibility complex. Pairing off with a male who has a different set of genes for immunity can lead to children that will have a higher level of genetic diversity, making them healthier and more likely to survive. (However, it’s important to note that women who take the birth control pill exhibit the opposite preference.) So even though we may not be consciously aware of it, we use behaviors like kissing to judge whether to take a relationship further, based on genetic evidence.

Of course, most of us don't make out with our friends, so determining the means of sniffing out genetic similarities among those we chose as friends requires further study. 

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