The saying "birds of the same feather flock together" has apparently found a scientific backing.

While people who share the same characteristics such as height and facial features have long been known to more likely forge friendship, a new genetic analysis suggests that people have the tendency to be friends with those who share common DNA consequences with them.

For the study "Friendship and natural selection," which was published in the Proceedings of the National Academy of Sciences on July 14, researchers James Fowler, from the University of California, San Diego, and Nicholas Christakis, from Yale University, used data from the Framingham Heart Study to conduct an analysis on the association between the genetic makeup between friends.

The Framingham Heart Study is an ongoing study which started in 1948 that involves residents of Framingham, Massachusetts that aims to identify the environmental and genetic risk factors of heart disease. The study has nearly 2,000 subjects and consist of groups that either involved pairs of friends and pairs of strangers who are unrelated.

The researchers examined about 1.5 million genetic markers to measure the genetic similarity of a person to that of his or her friend or a stranger and found evidence that suggests people may be drawn to their friends partly because they share similar genes.

The researchers have, in fact, observed that on average, friends have genetic similarities comparable to those of fourth cousins or individuals with the same great-great-great grandfather. The resemblance is only about 1 percent of the genetic markers but the researchers said this has significant implications in terms of evolutionary theory.

"One percent may not sound like much to the layperson but to geneticists it is a significant number," Christakis said, "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."

The researchers also observed that while friends often share a lot of characteristics in common such as interests and physical traits, they tend to have different immunity-related genes. Christakis and Fowler said that this difference actually offers some forms of benefits. Diseases, for instance, won't easily spread in friends who have different immunities.

"There may also be advantages to complementary rather than synergy when it comes to immune system function," Fowler said. "You don't want to be susceptible to disease that your spouse or friend is susceptible to."

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