The friend you go shopping with, the buddy you meet at the bar -- what if those friendships were based not only on common interests, but your biology as well?
For author James Fowler, professor of medical genetics and political science at the University of California, San Diego, his study puts a whole new spin on the phrase "birds of a feather flock together."
"We live in the sea of the genes of other people," Fowler said. "And those genes may influence who we become friends with and they may also influence our behavior in ways we didn't appreciate before."
Fowler and co-authors Jaime Settle and Nicholas Christakis drew their genotype data from 5,000 people in two studies, the National Longitudinal Study of Adolescent Health and the Framingham Heart Study.
They focused on six genes and while four showed no correlation between social networks, one gene the DRD2 did. It's the gene that influences drinking behavior.
While some would expect people who consume alcohol to become friends after meeting at parties or bars, Fowler said their research shows "there may also be choices we make that we're not aware of."
The finding that people with the DRD2 gene are not only susceptible to certain behaviors, including drinking and alcoholism, but are now susceptible to making friends with those exact same behaviors, which Fowler said could one day influence treatment of alcoholism.
Fowler pointed out that some people who know they are predisposed to drinking often have to work harder to change those behaviors.
"When you find out your friends are susceptible," he said, "it should have a similar impact on you."
But the study didn't just find a way to link people with similar genotypes. The scientists also found another gene -- the sixth out of the group -- that had a negative correlation, meaning that people are attracted to their opposites when it comes to other behavior.
"We might tend to flock together when it comes to drinking behavior," he said, "but we might choose opposites when it comes to other traits."
The implications of the study could be vast, Fowler said, calling the research the first step in understanding the biology of social chemistry.
Dr. Shawn McCandless, Director of the Center for Human Genetics at University Hospitals Case Medical Center, said this study has big implications. "This would in some ways change how we think about, both in a philosophical as well as a mathematical level, how evolution might work."
Scientist: Some Friendships Influenced by Biology, Genetics
Josh Klapow, associate professor in the School of Public Health at the University of Alabama at Birmingham, also believes the study gives us clues into mysteries of human interaction.
"The study suggests that humans are a complex interplay of genetics and environment," Klapow said. "Our genetic make-up steers us in one direction while our environment then determines if we 'veer away' from that path."
"It is not nature vs. nurture -- it is nature and nurture," he said.
The seeds for this type of research were first planted several years ago in another study also published in the Proceedings of the National Academy of Sciences which found that genes influence how many friends a person has and how central that person is to their network of friends.
Fowler said they aimed to expand those findings by looking at "homophily" -- or as he explains it, "a word that literally means love of like."
"We tend to make friends with people that are like us," he said.
So they began by hypothesizing that certain friendships weren't a case of being socially similar, but biologically, "even genetically similar to us."
This type of research, he said, has really picked up in the last 10 years or so. Before that, he said, scientists were primarily focused on researching genes and connecting that to the individual's behavior.
Now, he said, a whole world of new research possibilities is opening up as more evidence surfaces that chemistry may be more than just a feeling.
But McCandless noted that although the findings are interesting, this topic is far from resolved.
"There's still much work that would need to happen before we're ready to say that we know the specific gene that makes you really good at talking to people at cocktail parties."
And, as Fowler pointed out, these linked genes will never replace free will.
"Genes are not destiny and information is power" he said. "Getting information about your genes can help you figure out what you should work harder on."