The findings reinforce that healthy humans are wired to recognize strangers who may help them out in a tough situation. They also pave the way for genetic therapies for people who are not innately sympathetic, researchers said.

"It's remarkable that complete strangers could pick up on who's trustworthy, kind or compassionate in 20 seconds when all they saw was a person sitting in a chair listening to someone talk," said Aleksandr Kogan, lead author of the study and a postdoctoral student at the University of Toronto at Mississauga.

Two dozen couples participated in the UC Berkeley study, and each provided DNA samples. Researchers then documented the couples as they talked about times when they had suffered. Video was recorded only of the partners as they took turns listening.

A separate group of observers who did not know the couples were shown 20-second video clips of the listeners and asked to rate which seemed most trustworthy, kind and compassionate, based on their facial expressions and body language.

The listeners who got the highest ratings for empathy, it turned out, possess a particular variation of the oxytocin receptor gene known as the GG genotype.

"People can't see genes, so there has to be something going on that is signaling these genetic differences to the strangers," Kogan said. "What we found is that the people who had two copies of the G version displayed more trustworthy behaviors -- more head nods, more eye contact, more smiling, more open body posture. And it was these behaviors that signaled kindness to the strangers."

The study, which builds on previous UC Berkeley research on the human genetic predisposition to empathy, is published in the Nov. 14 online issue of the Proceedings of the National Academy of Sciences. An earlier UC Berkeley study looked at three combinations of gene variations of the oxytocin receptors AA, AG and GG.

It found that the people who were most empathetic -- in that they were able to accurately interpret others' emotions -- had two copies of the "G allele." In contrast, members of the AA and AG allele groups were found to be less capable of putting themselves in the shoes of others and more likely to get stressed out in difficult situations.

Widely known as the "cuddle" or "love" hormone, oxytocin is secreted into the bloodstream and the brain, where it promotes social interaction, bonding and romantic love, among other functions.

Kogan pointed out that having the AA or AG instead of the GG genotype does not mark a person as unsympathetic.

"What ultimately makes us kind and cooperative is a mixture of numerous genetic and non-genetic factors. No one gene is doing the trick. Instead, each of these many forces is a thread pulling a person in one direction or another, and the oxytocin receptor gene is one of these threads," Kogan said.

His coauthors are UC Berkeley psychologist Dacher Keltner; Laura Saslow, a postdoctoral student at UCSF; Emily Impett, an assistant professor of psychology at the University of Toronto; Christopher Oveis, an assistant professor at UC San Diego, and Sarina Saturn, assistant professor of psychology at Oregon State University.

Note: If no author is given, the source is cited instead.

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