DNA
Perhaps it is for mutual benefit or some other reason, but surprisingly we seem to share a certain percentage of genes with our friends as opposed to strangers. reuters

We have more genes in common with people we pick to be our friends than with strangers.

Though not biologically related, friends are as "related" as fourth cousins, sharing about 1% of genes. That is what a study published from the University of California and Yale University in the Proceedings of the National Academy of Sciences, has concluded.

The study is a genome-wide analysis conducted on 1932 unique subjects which compared pairs of unrelated friends and unrelated strangers. The same people were used in both samples.

While 1% may seem insignificant, it is not so to a geneticist. As co-author of the study James Fowler, professor of medical genetics at UC San Diego says, "Most people do not even know their fourth cousins but somehow manage to select as friends the people who resemble our kin."

The team also developed a "friendship score" which can predict who will be your friend based on their genes.

The study also found that the genes for smell were something shared in friends but not genes for immunity. Why this similarity in olfactory genes is difficult to explain, for now. Perhaps, as the team suggests, it draws us to similar environments but there is more to it. There could be many mechanisms working in tandem that drive us in choosing genetically similar friends rather than "functional kinship" of being friends with benefits!

One of the remarkable findings of the study was that the similar genes seem to be evolving faster than other genes. Studying this could help understand why human evolution picked pace in the last 30,000 years, with social environment being a major contributory factor.

The findings do not simply corroborate people's tendency to befriend those of similar ethnic backgrounds, say the researchers. Though all the subjects were drawn from a population of European extraction, care was taken to see that all subjects, friends and strangers were taken from the same population. The team also controlled the data to check ancestry of subjects.