If you consider your friends family, you may be on to something. A new study, published in the Proceedings of the National Academy of Sciences, finds that friends who are not biologically related still resemble each other genetically.
Study coauthor James Fowler is a professor of medical genetics and political science at UC San Diego. He said:
Looking across the whole genome, we find that, on average, we are genetically similar to our friends. We have more DNA in common with the people we pick as friends than we do with strangers in the same population.
The study is a genome-wide analysis of nearly 1.5 million markers of gene variation The researchers focused on 1,932 unique subjects and compared pairs of unrelated friends against pairs of unrelated strangers.
The findings are not, the researchers say, an artifact of people’s tendency to befriend those of similar ethnic backgrounds. The genetic data used in the study is dominated by people of European extraction. While this is a drawback for some research, it may be advantageous to the study here: because all the subjects, friends and not, were drawn from the same population.
How similar are friends? On average, say the researchers, friends are as “related” as fourth cousins or people who share great-great-great grandparents. That translates to about 1 percent of our genes.
Study coauthor Nicholas Christakis is a professor of sociology, evolutionary biology, and medicine at Yale. He said:
One percent may not sound like much to the layperson, but to geneticists it is a significant number. And how remarkable: Most people don’t even know who their fourth cousins are! Yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin.
Beyond the average similarities across the whole genome, the researchers looked at focused sets of genes. They find that friends are most similar in genes affecting the sense of smell. The opposite holds for genes controlling immunity. That is, friends are relatively more dissimilar in their genetic protection against various diseases.
Perhaps the most intriguing result in the study is that genes that were more similar between friends seem to be evolving faster than other genes. Fowler and Christakis say this may help to explain why human evolution appears to have speeded up over the last 30,000 years, and they suggest that the social environment itself is an evolutionary force.