Ever wonder why some people are always willing to pitch in and help? According to Oregon State University psychologist Serena Rodrigues Saturn, they’re born that way.

“Good Samaritans” have higher levels of 2 important hormones that’re associated with kindness and generosity: Oxytocin and vasopressin. So, doing things like volunteering and giving blood comes more naturally to these people.

And, spotting them comes naturally to everyone else. A University of California study found that most of us can identify someone with empathetic traits just by looking at them. That’s because people who have increased levels of these hormones display more compassionate behaviors like smiling, nodding, and making eye contact.

Of course, there’s more to being a Good Samaritan than brain chemistry. University of Buffalo psychologist Michael Pollin says how we view the world has a big impact as well. His research shows that someone who believes people are basically untrustworthy isn’t as compelled to do good deeds.

And, there is evidence that seeing enough unpleasant events can turn Good Samaritans into indifferent ones. 

Researchers at Thomas Jefferson University found that the number of compassionate responses medical students had decreased each year. In other words, the more medical cases a student saw, the less they responded emotionally – because they became commonplace. But, the same study showed that seeing other people perform an act of kindness restored the doctors’ ability to compassionately relate to their patients.