The 'chemistry' we feel among friends may be the result of genetic similarities, a new study shows.
A team of researchers in the United States used data collected from two large medical studies to assess whether there was a correlation between genotype and the friendship networks that people establish.
Lead author on the paper, Professor James Fowler of the University of California, says this is the first study to identify specific genes involved in forming social networks.
In the work, published this week in Proceedings of the National Academy of Science (PNAS), Fowler and colleagues say that one important implication of the results is that the genetics of human populations may be due to more than choice of reproduction partners, but may also arise from how friendships are formed.
Fowler says the study was carefully controlled for population stratification and other biases.
"Geneticists usually think of human populations as being structured, or 'stratified', because of the processes of migration and mating," he says.
"But we control for that by including in our model information about both friends' ethnicity and their siblings' genotypes. It's unusual to include both kinds of controls in the same study, but we did so to be certain that population stratification was not driving the associations.
"The results show that genetic structure in the population can also reflect non-reproductive friendship ties."
The researchers used data from two large nationwide representative data sets, the National Longitudinal Study of Adolescent Health (Add Health), and then used the Framingham Heart Study to replicate their findings. They examined about 4100 friendship pairs in total.
First step to understanding chemistry
Fowler and colleagues studied six genetic markers, all of which are known to affect human social behaviour, and which had been chosen for genotyping by Add Health.
"This is a first step towards understanding the biology of 'chemistry' - that feeling you have about a person that you will like or dislike them," Fowler says.
But there's chemistry and there's chemistry. The researchers found that individuals carrying the DRD2 marker, which is associated with alcoholism among other things, tended to befriend others with the same marker. Fowler says that can lead to a feedback mechanism.
"If you have this gene variant, your friends are likely to have it, too. So you are not only more susceptible to alcoholism yourself, but you are likely to be surrounded by friends who are susceptible."
Fowler says the study leads to some potentially important insights.
"Correlated genotypes means that it makes even more sense for us to treat outcomes like alcohol abuse as social, group-level problems. And anything that spreads in networks - from obesity to happiness to the flu - may spread more easily in some parts of the human population.
"There is a patchwork of localised susceptibility within networks, created by our genes and the genes of those around us."
Further controls needed
Dr Jim Stankovich, a geneticist at the University of Tasmania says the study could lead other researchers into a relatively untouched area.
"There's been this theory that friends might have similar genotypes, but as far as I know no-one's ever looked at this in humans before," he says.
"I think there's a good chance that lots of other people will want to follow this up and check in other genes. There are tens of thousands more genes out there.
"A control experiment needs to be done too, I think, just to confirm that it really is predominantly these psychological trait genes which show this effect, and that there's not something else going on."
Dr Mike Goddard, a population and quantitative geneticist at the University of Melbourne, says the results are surprising.
"By looking at a very small number of genes in a relatively small number of people they were able to find two where one indicates friends who are alike and one where friends are opposite," he says.
"I think further research will show the effects aren't as big as they report here.
"The paper itself contains its own replication, so they've done the right thing, but I'd like to see someone replicate this in a completely independent set of people."