Two life-defining moments propelled Abigail Marsh toward her current career.

Now a professor of psychology and neuroscience at Georgetown University, Marsh experienced an extraordinary act of heroism at age 19 when a stranger intervened after a car accident.

A few years later, at a New Year’s Eve party, a very different kind of stranger attacked her and broke her nose when she resisted his groping.

Those two experiences informed her current work as the head of Georgetown’s Laboratory of Social and Affective Science as well as the research that became the subject of her new book: “The Fear Factor: How One Emotion Connects Altruists, Psychopaths, & Everyone In-Between.” The premise, bolstered by Marsh’s brain-imaging research, is that extreme altruists are motivated to commit extraordinary acts of kindness because of their extreme sensitivity to others’ fear.

At the other end of the personality spectrum, severe psychopaths, who engage in acts of terror and violence, have an insensitivity to, and inability to recognize, fear in others.

In the following Q&A, Marsh discusses the themes in her book.

(This conversation has been edited for length and clarity.)

Q: Talk about the work of your lab. What is social and affective neuroscience?

A: It’s a hybrid discipline developed over the last decade or two when brain imaging became sophisticated enough to answer more exacting questions. It’s using human neuroscience technology to understand questions about human social behaviour. A lot of [the research] has to do with emotions and how people respond to each other emotionally, for instance, the sight of other people in distress.

Q: What is altruism?

A: Altruism is not the only behaviour that helps an individual but it is intended to help them: donating blood, helping a stray animal. With low-cost altruism, it’s more difficult to know the motivation if you’re helping someone because of social norms, but it doesn’t tell us much about you as a person. But people who do extraordinary altruism don’t weigh the risks and costs and so, by definition, it’s not normative.

Q: In your book, you write that altruism “is robustly related to how attuned people are to other’s fears.” Can you explain your theory and how it arose?

A: I actually remember looking at this spreadsheet [of an experiment on altruism] and this measure where we asked people to evaluate another’s appearance – how attractive they were. We either told them we would tell their rating to the person or we would not. It was about how kind you were being to that person. I found a massive correlation between the ability to recognize fear and kindness. And I thought that was so strange, so I followed it up with more studies on altruism and ran across psychopathy research. . . . I was looking to see if there was any relationship between fear and any kind of empathy. Then I found out how bad [psychopaths] were at recognizing fear.

Q: Why are people better at recognizing fear than happiness in facial expressions?

A: It’s not clear why. It may be that happiness is the only positive expression. With happiness recognition, you don’t get much variability. It’s also an expression we see much more.

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Q: Why is psychopathy more closely associated with a deficit in fear recognition?

A: This is an interesting question. The simplest answer is the empirical data shows us that it’s the case. . . . I think the reason is, people who are psychopaths have differences in experiencing fear [compared with non-psychopaths]. They don’t experience it often and so they have trouble recognizing it in others. . . . Simply noticing when someone is in distress and reacting to it is the most basic form of empathy, and if you can’t get there, then you can’t get to other stuff, like caring.

Q: You write that altruistic kidney donors (those who donate a kidney to a stranger) represent a special group. Why is that?

A: Altruistic kidney donors have two interesting things. They appear to be more caring than average but in addition to that, they seem more likely to experience high levels of care for anybody. Most people limit it to family and friends. [With kidney donors] the farther out you go from family and friends, shows that the caring doesn’t drop off.

What the origins of that are is not well understood. [Altruistic kidney donors] don’t see the world in terms of “myself.” When they focus on helping someone else they’re not worried about themselves. Worrying about someone else’s welfare really does block out focusing on your own welfare.

One of the things that is revealing about altruists is how they don’t view what they’ve done as extraordinary. They don’t like being called “angels” or “saints.” [For kidney donors] they think not wanting to donate is what needs explaining. I think it really hammers home that we really believe our own experiences of the world are the norm, which is why it’s hard for us to understand people at the far end of the spectrum.

Clara Barton was a famous battlefield nurse. People thought she was tremendously brave. People thought her fearless. When she was focused on others, though, her fear melted away, which is one way she dealt with depression.