NEW YORK -- You may be more similar to your friends than you think: A study suggests that the DNA code tends to be more alike between friends than between strangers.

That's beyond the effect of shared ethnicity, researchers say. And it could be important for theories about human evolution, says James Fowler of the University of California, San Diego.

He and Yale researcher Nicholas Christakis present their results in a paper released Monday by the Proceedings of the National Academy of Sciences.

How much more alike are friends than strangers? Not much. Imagine the small similarity between fourth cousins, people who share a set of great-great-great grandparents.

The study included 1,932 participants in a long-running health study in Framingham, Massachusetts. Researchers knew who was close friends with whom from the 1970s to the early 2000s because of information gathered for the study.

From this group they identified 1,367 pairs of close friends and about 1.2 million pairs of strangers. Then they examined information about nearly 467,000 locations in the DNA code of each participant. They looked for how similar the friend pairs were, and compared that to how similar the stranger pairs were.

The researchers found that genes affecting sense of smell were especially likely to be similar in friends.

Why would friends have more DNA similarities than strangers? Fowler said it's not clear. One possibility is that similar genes nudge people toward similar environments, which then gives them a chance to meet. Another possibility is that people who share certain genes also share skills that become more valuable when the people work together, he said. This could have been important over evolutionary time, and so set up a pattern that people still follow, he said.

Fowler also said it's not clear whether the finding pertains to groups outside of the Framingham study group, which is overwhelmingly Irish and Italian.

In any case, findings of DNA similarities between friends could help explain how behaviours like altruism developed over evolutionary time, he said.

Francisco Ayala, who studies evolutionary genetics at the University of California, Irvine, said the study's results surprised him. But "the statistics are there" to back up the conclusions, he said.

Ben Domingue, a researcher at the University of Colorado, Boulder, called the results intriguing. "I am fairly convinced that they are onto something," he wrote in an email.