People with a certain gene trait are known to be more kind and caring than people without it, and strangers can quickly tell the difference, according to US research published on Monday.
The variation is linked to the body's receptor gene of oxytocin, sometimes called the "love hormone" because it often manifests during sex and promotes bonding, empathy and other social behaviors.
Scientists at Oregon State University devised an experiment in which 23 couples, whose genotypes were known to researchers but not observers, were filmed.
One member of the couple was asked to tell the other about a time of suffering in his or her life. Observers were asked to watch the listener for 20 seconds, with the sound turned off.
In most cases, the observers were able to tell which of the listeners had the "kindness gene" and which ones did not, said the findings in the Proceedings of the National Academy of Sciences edition of November 14.
"Our findings suggest even slight genetic variation may have tangible impact on people's behavior, and that these behavioral differences are quickly noticed by others," said lead author Aleksandr Kogan, a postdoctoral fellow at the University of Toronto.
Nine out 10 people who were judged by the neutral observers to be "least trusted" carried the A version of the gene, while six out 10 deemed "most prosocial" had the GG genotype.
People in the study were tested beforehand and found to have GG, AG or AA genotypes for the rs53576 DNA sequence of the oxytocin receptor (OXTR) gene.
People who have two copies of the G allele are generally judged as more empathetic, trusting and loving.
Those with AG or AA genotypes tend to say they feel less positive overall, and feel less parental sensitivity. Previous research has shown they also may have a higher risk of autism.
"The oxytocin receptor gene in particular has become of great interest because a select number of studies suggest that it is related to how prosocial people view themselves," Kogan said.
"Our study asked the question of whether these differences manifest themselves in behaviors that are quickly detectable by strangers, and it turns out they did."
However, no gene trait can entirely predict a person's behavior, and more research is needed to find out how the variant affects the underlying biology of behavior.
"These are people who just may need to be coaxed out of their shells a little," said senior author Sarina Rodrigues Saturn, an assistant professor of psychology at Oregon State University whose previous research established the genetic link to empathetic behavior.
"It may not be that we need to fix people who exhibit less social traits, but that we recognize they are overcoming a genetically influenced trait and that they may need more understanding and encouragement."