(Reuters) - The truism that friends are the family you choose may be more accurate than you might suppose.

A study published on Monday found that people are apt to pick friends who are genetically similar to themselves - so much so that friends tend to be as alike at the genetic level as a person's fourth cousin.

The findings were based on an examination of about 1.5 million markers of genetic variations in a group of nearly 2,000 people who had taken part in a long-running health study based in Massachusetts. The researchers compared people identified as friends to those who were not.

The study showed people were most similar to their friends in olfactory genes, which involve the sense of smell, and were least similar in relation to immune system genes.

"Olfactory genes have a straightforward explanation: People who like the same smells tend to be drawn to similar environments, where they meet others with the same tendencies," said one of the researchers, James Fowler, a professor of medical genetics and political science at the University of California, San Diego.

The study, published in the scientific journal Proceedings of the National Academy of Sciences, follows research released in May that found that people tended to choose spouses who have similar DNA.

Fowler said the new findings made it clear that people have more DNA in common with those who are selected as friends than with strangers in the same population. Fourth cousins are people who have great-great-great grandparents in common.

Because the study population was largely homogeneous, mostly whites of European background, the findings "are less likely to be driven by the simple explanation that people of similar ancestry befriend one another," Fowler said.

Fellow researcher Nicholas Christakis, a Yale University professor of sociology, evolutionary biology and medicine, said the mechanism used by people to choose friends with similar genetics remained a mystery.

"It could involve the workings of a postulated 'kin detection system' in humans," Christakis said. "Our fates depend not only on our own genes, but also on the genes of others around us, and in particular our friends."

Christakis said he was interested in finding out why people have friends in the first place.

"The making of friends is exceedingly rare in the animal kingdom," Christakis added. "Certain other primates, elephants and whales are the only other mammals who do this, and this alone aroused our curiosity."

(Reporting by Will Dunham; Editing by Lisa Von Ahn)