How close are you and your best friend, really? Our friends tend to have similar interests to us, but research finds that there is more to friendship than just a shared love for the outdoors.
The people who we call our friends have DNA similar to our own, according to a study led by Dr. Nicholas Christakis of Yale University. The findings were published in July in the Proceedings of the National Academy of Sciences.
This doesn’t mean that our friends are necessarily our relatives, but they do share many of our genetic differences by chance, a similarity on the order of fourth cousins, according to the study. The researchers were aware that people often associate by ethnic similarity and made sure to statistically account for that.
The study offers one way that the social or natural environment may favor our associations with genetically similar individuals — disease prevention. The author speculates that people who are resistant to a particular pathogen may befriend those who are likewise resistant. This will avert the potential spread of disease in a particular network of people.
“[The way people pick their friends shows] a subconscious desire for acceptance and safety under particular social contexts, which in turn results in stronger bonds among friends,” said Hanna Kingsley, an anthropology junior. “Some such subconscious forces that may be implicated in our decision making include similar interests.”
Other cues indicated by the study include body odor, tone of voice and similarity in facial and body features.
This is in contrast to the mechanisms used to choose mating partners, which focus on genetic dissimilarity. For example, mice will use odor cues to determine which individuals are genetically dissimilar, according to Dr. Christina Laukaitis, a medical geneticist from the Arizona Cancer Center.
“Three subspecies of house mice are found across the old world and will rarely interbreed with one another as a result of 10,000 generations of isolation,” Laukaitis said.
Accumulated mutations in salivary and urinary proteins have resulted in the preference or detection of proteins within one’s own population, Laukaitis said.
Mice will use odor cues within subspecies to determine which individuals are genetically dissimilar, Laukaitis said.
Laukaitis cited the well-known study by Claus Wedekind, published in Proceedings: Biological Sciences in 1995, that showed a similar response in humans. Researchers had women smell the sweat from various men’s T-shirts and found that females preferred sweat from genetically dissimilar males when it came to immunological genes.
Because the type of relationship is different, however, choosing friends requires different mechanisms than choosing who to mate with.
“[Choosing our friends] could be a byproduct of selection on sticking together as a family,” said Dan Papaj, an animal behaviorist and professor in the department of ecology and evolutionary biology. “In other words, resulting from [family cooperation to perpetuate one’s familial genetics].”
Secondly, this could be the result of Richard Dawkins’ infamous selfish gene, according to Papaj.
A selfish gene is a gene that acts against the interest of its bearers even though it multiplies, Papaj explained. He noted that the T allele is “one classic example” of a selfish gene in mice. A study published in Evolution by S.S.Y. Young in 1967 showed that the T allele cultivates its spread in the population by making itself roughly 95 percent common in sperm, to the detriment of the offspring.
An evolutionary reason for “why” we choose our friends this way remains murky, but the study gives evidence that we have a knack for picking apples not too far from the tree.
—Follow Nicolas Alexandre @DailyWildcat
