A new study suggests that when it comes to certain genes, friends of a feather flock together—but with others, opposites attract. The research offers potential insight into subtle genetic influences that may affect how people become friends—which can have an enormous impact on later life choices. One gene that may link friends, for example, is also connected to the risk of alcoholism.
“We live in a sea of genes,” says lead author James Fowler, professor of medical genetics and political science at the University of California-San Diego. “What happens to us may not depend only on our genes but on the genes of our friends.” (More on Time.com: Do Friends Make Your Amygdala Larger?)
The study, which was published in the Proceedings of the National Academy of Sciences, used two large databases, the National Longitudinal Study of Adolescent Health and the Framingham Heart Study. It found similar correlations between two specific genes and the likelihood of individuals being friends in both data sets. Researchers controlled for other factors like simply living in the same region that could also account for the connections between friends.
The first gene, DRD2, is involved in producing a type of dopamine receptor, and the researchers report that people with a variant linked with increased risk for alcoholism were more likely to be friends with each other. Those with the variant were 10% more likely to be pals than would be expected by chance. Since most of the participants in the adolescent database were just 14 at the time they were studied, it’s not likely that, for example, they met each other in bars, which might otherwise explain the connection.
Although DRD2 has been the subject of other conflicting studies on its potential relationship to impulsivity or risk for ADHD, the alcoholism finding has been repeatedly replicated.
The association makes sense, says Fowler, since it’s true that “if I’m more impulsive, I might choose to be with friends with others who are more impulsive.” Another way that such a gene might affect friendship is that impulsive people might be drawn to the same types of environments—for example, amusement parks— and tend to make friends with others they find there. Not surprisingly, a kid who sneaks beer and cigarettes in the high school parking lot and drag races on weekends is unlikely to befriend the guy who spends all his time with the chess club. (More on Time.com: Young Adults Choose Self-Esteem Boost Over Sex and Money)
However, he acknowledges that the relationship between DRD2 and alcoholism isn’t that simple. “There can be a feedback effect. We know that [this gene] shows an association with alcoholism. Now the evidence here is that if you have this gene, your friends are more likely to have it. You’re not only susceptible biologically to this behavior, you’re also more like to be surrounded by people who are susceptible to this behavior.”
Interestingly, another gene—one that codes for a protein that affects how the liver metabolizes foods and drugs but is also linked with openness to new experience— is associated with the opposite tendency. Those who share a version of this gene prefer people who aren’t like them—so if you have the same variant of this gene, you are less likely to be friends with someone else who does.
“There, it’s not the case that you and your friends are drawn to the same environment,” says Fowler, “For those genes, [they are] much more likely to be [cases] where you actually choose people of a different genotype to be friends with.”
Although the idea of genetic factors contributing to friendship choices may seem strange, it may make sense from an evolutionary perspective. Prior research shows that in terms of mate choice, people tend to be drawn to those who have versions of certain immune system genes that are somewhat different from their own. This could possibly help to prevent incest and increase the odds that offspring will have an immune system that is best equipped to fight different types of diseases. (More on Time.com: 5 Ways to Stop Stressing)
“This might be the first step towards understanding the biology of ‘chemistry,’ the feeling you have of you whether you like or dislike a person [almost immediately],” Fowler says, noting that this can affect both romantic connections and friendships. “We might choose friends not [only] because of social features we consciously notice but because of biological and even genetic features that we unconsciously notice.” In turn, the friends we have could then affect the potential partners we meet.
Misha Angrist, an assistant professor at the Duke University Institute for Genome Sciences and Policy who was not involved in the analysis, says the study is “interesting, but I would want to see independent replication.”
He believes that for social behavior, a person’s actual behavior and personality will always be more important than genetics. He warns against seeing genes as puppet masters that can control behavior. “It’s deterministic,” he says, “That says, ‘I’m judging you by the content of your cells and not by the content of your character. I think human beings need to relate to each other as human beings not as repositories of genetic information.”
Fowler’s previous research with Nicholas Christakis and colleagues has explored how friendship networks can affect behavior, sometimes even more than genes. For example, they found that people who had friends who were obese—even if they didn’t live near each other— were more likely to gain weight themselves. Having fat friends was linked with a greater chance of being obese than having fat siblings.
However, this didn’t mean that dropping those friends would help people diet. Removing parts of a social network actually made things worse. “We went back and looked at people who kept or dumped [these] friends. The people who dumped their friends were more likely to be obese,” says Fowler. Having friends and other social support is critical to stress relief—otherwise, people are probably even more likely to turn to food or other potentially unhealthy sources of comfort.
“It’s important to fully understand not just what’s biologically going on with you as an individual but biologically what’s going on as a group,” he says, “It’s not just both nature and nurture: our nurture is in part nature.”
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