Friends we connect to resemble us genetically?

The study by University of California, San Diego, and Yale University, which was co-authored by James Fowler and Nicholas Christakis, is a genome-wide analysis of nearly 1.5 million markers of gene variation, and relies on data from the Framingham Heart Study. The Framingham dataset is the largest the authors are aware of that contains both that level of genetic detail and information on who is friends with whom.

Fowler said that by observing the whole genome, they found that on average, people were genetically similar to their friends, and had more DNA in common with the people picked as friends than with the strangers in the same population.

The researchers focused on 1,932 unique subjects and compared pairs of unrelated friends against pairs of unrelated strangers. The same people, who were neither kin nor spouses, were used in both types of samples. The only thing that differed between them was their social relationship.

Researchers said that the findings weren't an artifact of people's tendency to befriend those of similar ethnic backgrounds. The researchers also controlled for ancestry by using the most conservative techniques currently available. The observed genetic go beyond what one would expect to find among people of shared heritage, these results were "net of ancestry," Fowler said.

Fowler and Christakis found that on an average, friends were as "related" as fourth cousins or people who share great-great-great grandparents, which translated to about 1 percent of the genes.

Beyond the average similarities across the whole genome, Fowler and Christakis also focused on sets of genes, and discovered that friends were most similar in genes affecting the sense of smell, and relatively more dissimilar in their genetic protection against various diseases.

The most intriguing result in the study was that the genes that were more similar between friends seemed to be evolving faster than other genes. Fowler and Christakis said this may help to explain why human evolution appears to have speeded up over the last 30,000 years, and they suggest that the social environment itself is an evolutionary force.

The study is published in the Proceedings of the National Academy of Sciences.