The Biology of Friendship

Miranda, Carrie, Samantha and Charlotte--a friendship determined by genes? Actors Cynthia Nixon, Sarah Jessica Parker, Kim Cattrell and Kristin Davis from HBO's Sex and the City / © HBO

Miranda, Carrie, Samantha and Charlotte–a friendship determined by genes? Actors Cynthia Nixon, Sarah Jessica Parker, Kim Cattrell and Kristin Davis from HBO’s Sex and the City / © HBO

GENETIC SIMILARITIES among friends revealed in recent research point to the old axiom that “friends are the family you choose.”  The new study shows that your close friends may be as “related” to you as fourth cousins, or as people who share great-great-great-grandparents – in each case, you share about 1 percent of your genes.

“One percent might not sound like much to the layperson,” says Nicholas Christakis, Yale professor of sociology, evolutionary biology and medicine.  Christakis and James Fowler at UC San Diego performed what’s called a “genome-wide analysis” using data from the Framingham Heart Study, the largest study to date containing both sufficient levels of genetic detail and information on who is friends with whom.

But to geneticists, 1 percent is a significant number.  “Most people don’t even know who their fourth cousins are!” Christakis says.  “Yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin.”

The researchers were able to use the data to compare pairs of unrelated friends against pairs of unrelated strangers because the study involved a relatively homogenous population of people with European ancestors, so that the only difference between them was social relationships.  In addition, the study authors developed a “friendship score,” which can be used to predict who will be friends based on their genes just as well as who will become obese or schizophrenic.

Evolutionary advantages result from shared attributes among friends, or “functional kinship,” as the study authors label these friendships.  “In the simplest terms: if your friend feels cold when you do and builds a fire, you both benefit,” according to the Science Daily article on the study. Also, some traits, such as the ability to speak, work only if your friend has them, too, creating a sort of “social network effect.”

Friends are least similar in genes that control immunity.  Differences in immunity, also found in studies on spouses, bestow “fairly straightforward evolutionary advantages,” according to Fowler and Christakis: Having close connections with people who are able to withstand different bugs reduces interpersonal spread.

Conversely, friends are most similar in genes controlling the sense of smell, although the reason for this is unclear.  Perhaps our sense of smell draws us to similar environments: People who like the scent of coffee, for example, might be more likely to spend time in cafes together.  Perhaps, as with pheromones, scent attracts us to our friends as well as our sexual partners.

Or there may be a connection to the ways in which smell triggers memories.  The olfactory nerve is close to both the amygdala, the area of the brain connected to emotion and emotional memory, and the hippocampus, which helps control memory.  To identify a scent, these areas help you connect it to visual input that occurred at the time you first smelled it.  (There is some suggestion that by studying information in the presence of certain scents, you can increase the intensity and clarity of the remembered information when you smell that scent again.)  Sharing the kind and intensity of memories makes for good friendships.

Finally, the genetic friends research “lends support to the view of human beings as ‘metagenomic,’” according to Christakis, so that “our fitness depends not only on our genetic constitutions but also on the genetic constitutions of our friends.”

— Mary Carpenter



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