Researchers say people can spot whether a complete stranger has a certain "social gene" in just 20 seconds.
Two variants of the "oxytocin receptor gene" have been linked with social traits.
People judging the empathy of strangers - by studying the way they listened to people - predicted the genetic variant, a University of Toronto study showed.
The hormone oxytocin has a role in birth, production of milk and bonding between mother and baby.
It also seems to have a role in social skills and has variously been called the "love" or "cuddle" chemical.
Two variants of the oxytocin receptor gene - termed G and A - have been linked to social behaviour.
Studies have shown that people with two copies of G, compared with one of each or two of A, are at lower risk of autism and report higher levels of empathy, positive emotions and said they were more social.Silent movie
Twenty three couples were filmed for the Proceedings of the National Academy of Sciences study. One described a moment of personal suffering while their partner listened.
Strangers then watched a 20 second silent recording of the exchange and scored the listener for their "prosocial traits", such as a caring nature or empathy.
GG people were found to be more prosocial than AG or AA people.
In the top 10 most trusted people, six were GG. In the 10 least trusted people, nine had at least one copy of A.
One of the researchers, Dr Aleksandr Kogan from the University of Toronto, said: "Our findings suggest even slight genetic variation may have tangible impact on people's behaviour, and that these behavioural differences are quickly noticed by others.
"Our study asked the question of whether these differences manifest themselves in behaviours that are quickly detectable by strangers, and it turns out they did."
Prof Sarina Rodrigues Saturn, from Oregon State University, said: "It was amazing to see how the data aligned so strongly by genotype.
"It makes sense that a gene crucial for social processing would yield these findings; other studies have shown that people are good at judging people at a distance and first impressions really make an impact."