Researchers from the University of California, San Diego, and Yale University found that we have more DNA in common with the people we pick as friends than we do with strangers in the same population.
"Looking across the whole genome, we find that, on average, we are genetically similar to our friends," said James Fowler, 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," Fowler said.
The study is a genome-wide analysis of nearly 1.5 million markers of gene variation, and relies on data from the Framingham Heart Study.
The researchers focused on 1,932 unique subjects and compared pairs of unrelated friends against pairs of unrelated strangers.
The same people, who were neither kin nor spouses, were used in both types of samples. The only thing that differed between them was their social relationship.
Fowler and Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale, found friends are as "related" as fourth cousins or people who share great-great-great grandparents. That translates to about 1 per cent of our genes.
"One per cent may not sound like much to the layperson but to geneticists it is a significant number," Christakis said.
Researchers found 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.
Researchers said why we might be most similar in our olfactory genes needs further research.
It could be, Fowler said, that our sense of smell draws us to similar environments. It is not hard to imagine that people who like the scent of coffee, for example, hang out at cafes more and so meet and befriend each other.
Researchers have also developed what they call a "friendship score," which they can use to predict who will be friends at about the same level of confidence that scientists currently have for predicting, on the basis of genes, a person's chances of obesity or schizophrenia.
Genes that were more similar between friends seem to be evolving faster than other genes, said researchers, adding that this may help to explain why human evolution appears to have speeded up over the last 30,000 years.
The study is published in the Proceedings of the National Academy of Sciences.