When Abigail Marsh was 19, a dog ran in front of her car on the highway. She swerved, spun out, and found herself facing an oncoming lane of traffic with an unresponsive engine. Across the highway, another driver saw her emergency lights flashing, pulled over, and ran to her aid. He pushed her into the passenger seat, kicked her engine into gear, and drove her to safety. Then he disappeared into the night. She never saw him again. [Find out more about the science of why some people are good and others really are not.]

What makes someone put his life in danger to help a person he’s never met, and why do other people intentionally cause harm and feel no regret? These are questions Marsh, now an associate professor of psychology and neuroscience at Georgetown University, set out to answer. In her new book, The Fear Factor, Marsh lays out her theory why one ancient part of the brain controls how we identify and respond to fear.

A stranger risked his life to get you to safety when you were a teenager. How did this affect the path you took in life?

The combination of a near-death experience and having a stranger rescue me was an incredibly emotionally affecting experience. It was hard to shake from my consciousness. The thing that lingered longest was the fact that a total stranger rescued me. It seemed so improbable—I could not imagine taking an enormous risk to help somebody I’d never met.

I had this nagging itch of wanting to understand why someone would make a decision like that. It was this puzzle that must have been the engine that drove me.

How did you translate that question into research?

It becomes clear to me looking back that a common thread through my research was an effort to understand why people want to help other people. This led me to do lab research about altruistic decision making. But you can’t put people in the position of making life or death decisions in a lab. So what I did for my post-doc research was study people who are clinically deficient in care and compassion.

We ran brain scans of teens with psychopathic traits while showing them images of frightened faces. We found a lack of response in a part of their brain called the amygdala, an evolutionarily old structure involved in a lot of emotional and social behaviors. To contrast that, I picked kidney donors to study because their behavior was most unambiguously altruistic—with a stranger it can’t be about reciprocity.

Babies Confirm: Fear of Snakes and Spiders Is Hardwired A study of babies' perception provides evidence that people have a hardwired fear of spiders and snakes.

When you first launched your study of children with psychopathic traits, what did you expect them to be like?

I came from a town that has produced an oddly large number of psychopaths: Ted Bundy, the Green River Killer, and the D.C. sniper all came from Tacoma, Washington. It’s a myth that the prototypical serial killer is a psychopath. Now I recognize that a quintessential psychopath is someone consistently callous in the face of others suffering. They’re not necessarily out to hurt people, they’re out to get what they want and if other people get hurt that’s just collateral damage.

Before I assessed my first child with psychopathic traits I’d taken a training course, which said keep yourself between the subject and the door and don't carry anything sharp. I had no idea what I’d be confronting.

When I walked into the room I could not have been more taken aback. This boy looked like he walked off the set of a commercial. He had a sweet smile, made conversation and shook our hands. It was really hard for me to incorporate the fact that he was doing violent and awful things in his daily life.

That’s called the mask of sanity. It’s what people found so remarkable about Ted Bundy—those who knew him thought he was a well-adjusted guy.

View Images Ted Bundy watches intently during the third day of jury selection at his trial in Orlando, Florida, for the murder of 12-year-old Kimberly Leach. Photograph by Bettmann Archive, Getty

We tend to think about how nurture versus nature impacts the way we turn out. What dictates whether we'll end up as altruists or psychopaths?

Every psychological outcome is influenced by genetic differences and experiences—often 50 percent is genetic variation. Psychopathy heritability is probably between 50-70 percent. Clearly that’s not the end of the story; life experiences play a role.

The hardest thing about working with children with psychopathic traits is that I really feel for the parents. Some parents hope their child ends up in jail because at least somebody will be taking care of them. We’re all strongly affected by the idea that children's behavior is affected by what their parents do—we used to believe that about schizophrenia and autism, too. Bad parenting can cause bad behavior, but it’s not connected in a major way with serious mental illnesses.

It’s hard to know if parenting practices can result in people being more altruistic. When I ask people why they donated a kidney to a stranger, it’s quite difficult for them to answer. Some say, “That’s the way my parents raised me.” Then I ask, “Well, do you have any siblings and are they the way you are?” Most will say, “Oh no. Not at all.” Anyone can come up with an explanation for why they are the way they are, but it’s very hard to know if it’s true.

You chose to study “extreme altruists” via donors who gave a kidney to a complete stranger. Why did you end up studying this group?

Altruism is behavior that benefits someone other than the altruist. But motivations are hard to measure: It’s socially normative to do little acts like donating blood or giving to charity. Some people act altruistically to people who have helped us in the past or will in the future. Others help genetic relatives in a motivation called kin selection. But it’s really hard to come up with an alternate explanation for why you’d donate a kidney to a stranger other than you genuinely want to help somebody because the only benefits are to the recipient.

At the time, there were only around 1,000 altruistic kidney donors in the whole U.S. and I anticipated it would be really difficult to recruit. I reached out to the local D.C. transplant organization and posted some ads on websites and things. I vividly remember when I opened my laptop I was literally flooded with emails, which has never happened to me in my entire research career. We had enough people in days.

Tell me about the amygdala. Before you started researching it, what did you know and what did you discover?

The amygdala is essential for recognizing other people’s fear. The findings from our initial studies with children who are psychopathic show a reduced amygdala response when they’re shown pictures of fearful facial expressions. Their amygdala was also smaller. This was a really important clue.

People who are psychopathic have a fearless personality. Amygdala dysfunction impairs their ability to generate fear response, and identify other people’s fear. They actually can’t empathize with it. One boy I studied had lobbed a fake grenade into a building full of people to terrify them. When I asked him if he felt bad, he said, “Total Kodak moment.”

Altruistic kidney donors seemed to be the opposite of psychopaths: their amygdalae were larger and more responsive. People who are highly altruistic are really good at recognizing other people’s fear and that may be one reason they’re motivated to help.

The altruists you studied often describe a kind of intuition that makes them act before they can think twice. Why do some people have this and others don't?

This is a much deeper mystery about altruists: How do you get from having a strong response to the sight of other people’s fear to being motivated to help them?

The evidence points toward a hormone called oxytocin that is responsible for generating maternal care in the amygdala. It provides a consistent response to anything that looks infantile, including other people’s babies, animal babies, or even people who look like babies—like someone with a wide-eyed fearful expression. I’m betting that oxytocin in the amygdala is key to making the critical change from “This person’s afraid, I need to protect myself,” to “This person’s afraid, I’m going to help them.”

How has your research changed the way you look at human nature?

There’s so much sad news, but the world we live in is not accurately reflected in it. Studying psychopaths is a paradoxically uplifting experience. Working with people who really don't care if they hurt people highlights how unlike that most other people are. We can have blinders when it comes to suffering, but the average person does care. Trends show that people are becoming increasingly altruistic to strangers.

There will always be people out there who won’t be nice, and there’s no evidence we’ll be able to get rid of the one or two percent of people who cause suffering. But most people are capable of care and compassion. Studying people who are psychopathic actually makes me optimistic about everyone else.