People may actively seek friendship with genetically similar individuals, a controversial study has suggested.
The study looked at genetic information and friendships of more than 2,000 individuals using data from a multi-decade heart study from Framingham, Massachusetts. Using a genome wide association study (GWAS), researchers compared thousands of genetic markers in the genome called single nucleotide polymorphisms (SNPs). These markers were compared with those from study participants identified as friends or strangers.
Friends were seen to share a greater genetic similarity than strangers at around one percent, a level similar to that of fourth cousins. Similarities among friends were especially strong in genes related to smell.
'One per cent may not sound like much to the layperson, but to geneticists it is a significant number', explained co-author Dr Nicholas Christakis from Yale University. Although low, this level of variance is comparable to the best genetic methods for predicting schizophrenia, bipolar disorder, and body mass index.
The results of this study support the idea that human genetics may not only be influenced through reproduction, but also though friendship.
The co-authors of the study are no strangers to controversial views on genetics including linking a particular genetic variant to political views during the US election season (see BioNews 583), and even genetics' role in popularity (see BioNews 493) over the past few years. Critics have once again raised significant concerns about the conclusions drawn from the most recent study.
'These studies depend upon that assumption - that you're looking at thousands of people who are not related,' Professor Evan Charney from Duke University told the BBC, but the relationships between study participants can be difficult to confirm. The overwhelming majority of participants in the Framingham dataset come from a similar ethnic background and are from the same town.
Dr Rory Bowden from the Wellcome Trust Centre for Human Genetics in Oxford also expressed concerns about the study's ability to account for factors that might lead to a spurious associations 'because they reflect differences in places of origin within Europe of the Framingham participants'.
But Professor Robert Seyfarth from the University of Pennsylvania, who was not involved in the research, told the Washington Post he was encouraged by the results: 'this is a very interesting, provocative answer to the question of why is it that humans are so hyper-social in their interactions. Why are they so friendly to strangers? Most animals don't encounter strangers at all'.
Currently, no comparable dataset is available to confirm the results of the study elsewhere.
The authors have acknowledged the current study's limitations. Co-author Dr James Fowler from the University of California explained: 'While we've found that this is true for this one well-studied group of people, we don't know if the results can be generalised to other ethnic groups, my expectation is that it will, but we don't know'.