Story highlights Scientists find we're more genetically similar to our friends than to strangers

Study analyzed genotypes of more than 800 pairs

We share about 1% of our genes with our friends

This brings a whole new meaning to the phrase, "You've got a friend in me."

A new study published Monday in Proceedings of the National Academy of Science suggests friends may be more than just people you lean on when you're not strong; they might actually help you carry on -- genetically speaking.

"Looking across the whole genome, we find that on average, we are genetically similar to our friends," said James Fowler, coauthor of the study and professor of medical genetics and political science at UC San Diego. "We have more DNA in common with the people we pick as friends than we do with strangers in the same population."

Over the past decade, Fowler and coauthor Nicholas Christakis, professor of sociology, evolutionary biology and medicine at Yale, have studied the science behind social networks. They're seeking a biological explanation behind some long held social notions.

"We've all heard the phrase, 'Birds of a feather flock together,' but we want to know why," Fowler said.

Using data from the Framingham Heart Study, the researchers were able to conduct what they say is the first genome-wide analysis correlating genotypes between friends.

According to the U.S. Department of Health and Human Services, the Framingham Heart Study started in 1948. It is a long-term, multigenerational study, designed to identify genetic and environmental factors influencing the development of cardiovascular and other diseases. The generational genetic data provided by the Framingham Heart Study made it an ideal starting point for Fowler and Christakis.

The study contained 1,932 subjects. One group consisted of pairs of unrelated friends, while the other group was composed of unrelated strangers. Scientists examined 1.5 million markers of gene variation to accurately measure the genetic degree to which each person was similar to his or her paired friend or stranger.

"We have found that we share about 1% of our genes with our friends," said Fowler. "On average our studies indicate we are as genetically similar to our friends so much as we are our with our fourth cousins or people who share great-great-great grandparents."

Of the genes most prominently expressed between pairs of unrelated friends, the researchers found that the olfactory system genes were overrepresented.

"Friends tend to smell things the same way," said Fowler. In prehistoric days, for example, people who liked the smell of blood might hunt together, whereas gatherers might prefer the smell of wildflowers. Nowadays, Fowler says, that translates into people who like the smell of coffee congregating at coffee shops.

Researchers say that our DNA could be a driving force behind the activities we are drawn to and the social activities we engage in. As such, we are more inclined to interact and foster friendships with people who are genetically similar.

Also, the genes that we have in common most with our friends, are also under the most rapid evolution. They seem to be evolving at a rate faster than our other genes, the researchers say.

"Social networks may be turbo charging evolution," said Fowler.

"Not only with respect to the microbes within us but also to the people who surround us. It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitution of our friends," said Christakis.

Conversely, researchers also found that the people we choose to associate with tend to be immunologically different, which may offer us extra immunological protection. This supports past research that found spouses tend to have different immune system genes.

"There may also be advantages to complementary rather than synergy when it comes to immune system function," said Fowler. "You don't want to be susceptible to disease that your spouse or friend is susceptible to. You want to be immune to those diseases because it could provide an extra wall of protection so they don't pass them on to you."

This study, researchers say, also lends support to the view of humans being metagenomic -- meaning we're not only a combination of our own genes but of the genes of the people with whom we closely associate.

"Most of the study of genetics has been one gene, one outcome," Fowler said. "I think this is going to completely change the way we think about genetics. We have to look beyond ourselves."