Would you believe that it takes your brain less than 20 seconds to detect whether a stranger is kind or not? That’s the upshot of a new study.

University of California at Berkeley researchers took DNA samples from volunteers, and then videotaped them as they talked for 20 seconds about a tough experience. Everything from their car running out of gas to being late to an important meeting.

Then, a group of observers watched the 20 second video clips, and rated which people seemed the most trustworthy, kind, and compassionate.

The result? Volunteers who were considered the most kind and trustworthy had DNA that included a special gene linked to the bonding hormone oxytocin. And people with that bonding gene displayed a lot more physical behaviors we consider trustworthy than those that didn’t have the gene. Like significantly more head nods, smiles, and open body posture.

Researchers say this shows that we’re wired to quickly determine if a stranger can help or hurt us, which can be lifesaving. So, if your gut tells you that someone’s trouble, walk away.