Friendship may have a genetic component   PhysOrg - January 18, 2011
Genetic similarities were not always noticeable among friends who have activities in common such as playing musical instruments or running marathons, but the researchers said genetic similarities have been found in the past in couples, where people have been shown to avoid prospective mates who are susceptible to the same diseases. The other four genetic markers of the six they looked at showed no strong relationship among friends

Genes may play role in friends we choose, says study   BBC - January 17, 2011
Researchers in the United States say they have uncovered tentative evidence of a genetic component to friendship. Using data from two independent studies, they found carriers of one gene associated with alcoholism tended to stick together. However, people with another gene linked with metabolism and openness, stayed apart.

We are friendlier to people who resemble us   Telegraph.co.uk - March 8, 2009

We feel more altruistic to those who resemble us because in the past our early ancestors assumed that they were related, according to the study. The instinct dates back to when there were no mirrors and people could learn what their kin looked like only by inspecting the faces of household members. The study, published in Biology Letters, even found that we were more naturally drawn to people who looked like us than our own relatives, if the resemblance was strong enough.

The researchers came to the conclusion after a study of 70 identical adult twins who, although genetically the same, had over the years grown to look different from each other. Then they manipulated the photographs of the participants by digitally mixing them with a model's face so that the images would either resemble them or their co-twin. Then they asked each one who they would prefer to rescue from danger and which one they would prefer a different sex sibling to marry. In each case, the person most resembling themselves was preferred almost two thirds of the time ­ significantly higher than being down to chance alone. Dr Paola Bressan, of the University of Padova, Italy, said: "Our work shows a stranger who resembles us elicits pro-social regard more than a stranger who resembles a close family member - even one as close as our identical twin, who is, incidentally, genetically identical."

Americans Lose Touch, Report Fewer Close Friends Live Science - June 24, 2006

People in America have fewer close friends nowadays than two decades ago, researchers announced today. New research compared studies from 1985 and 2004. On average, each person in 2004 reported 2.08 close friends‹those they can discuss important matters with. That's down from 2.94 people in 1985. People who said they had no one with whom to discuss such matters more than doubled, to nearly 25 percent. Research showed that people who talk only to family members about important matters increased from 57 percent to 80 percent over the two decades, while the number who depend totally on a spouse rose from 5 percent to 9 percent.