Each year, people in the United States give billions of dollars to charity. Every day, people volunteer their time to help complete strangers. Routinely, we hear of selfless acts where people put their own lives in danger to help someone else.

Economists and evolutionary psychologists have struggled to explain why people act in such altruistic ways. Typical explanations suggest that these behaviors involve suppressing our true, selfish nature and must instead be motivated by external factors, such as the possibility of future rewards or to avoid negative consequences, like appearing selfish to a potential love interest.

But what if helping others is an innate part of being human? What if it just makes us feel good to give?

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Those questions have inspired a series of ground-breaking neuroscience studies conducted over the past several years by researchers Jamil Zaki, an assistant professor of psychology at Stanford University, and Jason Mitchell, an associate professor of the social sciences at Harvard University.

Their results have shed light on the relationship between empathy and altruism, and they have offered unique insights into how people decide whether to act generously or selfishly in a given scenario. What’s more, their findings provide strong evidence for a different take on human nature—one that sees generosity and selflessness as deeply rooted parts of who we are as a species.

Flipping economics on its head

Zaki and Mitchell’s research has gone head-to-head with standard economic models of decision making, which assume that when people exhibit kind, helpful (or “pro-social”) behavior, they are doing so to protect their reputation, avoid retribution, or benefit when their kindness is reciprocated.

But in a study published in 2011 in the prestigious Proceedings of the National Academy of Sciences, Zaki and Mitchell tested an alternate theory: that we feel good when helping others not because we are trying to avoid negative consequences but because behaviors like fairness, cooperation, and reciprocity are intrinsically rewarding.

The researchers tested their theory by putting participants in a functional magnetic resonance imaging (fMRI) scanner that recorded their brain activity, then had them play a “dictator game.” In each round of the game, the person in the scanner (the dictator) had to decide whether to take a lump of money for herself or instead have a sum of money go to another person (the receiver) whom the dictator did not know and would never meet. Sometimes the offer to the dictator would be greater than the offer to the receiver, or vice versa. For instance, sometimes the dictator would have to decide between a $2.00 offer to herself versus $1.25 to the receiver; sometimes the choice would be between $1.25 to herself versus $2.00 to the receiver.

In rounds when the amount offered to the dictator was greater than the amount to the receiver, Zaki and Mitchell found that dictators chose to take the money 83 percent of the time. However, in the rounds when the recipient stood to gain more than the dictator, dictators chose to give the money to the recipient about half of the time—even though that meant the dictators themselves would not receive any money. This mirrors results from previous studies, suggesting that people have a strong inclination for fairness.

Next, Zaki and Mitchell looked at the brain circuits that were activated when the dictators chose to act equitably—that is, when they chose the option that would provide the most money, regardless of who would receive it—versus inequitably (when they chose to give themselves the money even though it was the lesser amount). They found that acting equitably activated the orbitofrontal cortex, a part of the brain associated with assessing rewards. This suggests that acting equitably is rewarding, even when it means putting someone else’s interests before our own.

“Kindness and nice behaviors might be like psychological chocolate,” says Zaki. “People might actually enjoy doing kind things for others, and that might be an emotional engine for driving pro-social behaviors.”

On the other hand, making inequitable choices activated the anterior insula, a brain area that has been associated with negative emotional states like pain and disgust. Participants who had the highest activation in their anterior insula when acting inequitably were the least likely to behave that way. One possible explanation for this finding is that acting inequitably made these people feel badly, and so they did it less often—even when it meant missing out on money for themselves.

These results run counter to age-old explanations for altruistic behavior, which say that the only way to get people to be nice is to tamp down their natural selfish instincts.

“Our model flips the traditional model on its head,” says Zaki. “Instead of people wanting to be selfish and then forcing themselves through control to be generous, we’re getting a picture where people enjoy being generous but in certain circumstances overcome their displeasure to be selfish.”

“Mentalizing” for kindness

But it may take more than good feelings to motivate altruism. Indeed, in another line of research, Zaki and Mitchell have explored how altruistic behavior also stems from our ability to understand (or “mentalize”) how other people think and feel, which is an important component of empathy.

In a 2012 study, conducted in collaboration with Adam Waytz of Northwestern University, participants first were given a picture and biographical information about two unfamiliar men. They were then asked to rate how likely they thought each man was to have particular attitudes—for instance, to “have a positive outlook on life” or “enjoy snowboarding.” An fMRI scanner recorded their brain activity while they gave their answers.

Next Zaki, Mitchell, and Waytz asked the participants to play a game similar to the dictator game, except this time the participants could allocate money either to themselves or to one of the men they had just read about. For one set of participants, this was the end of the experiment; but other participants were then given an opportunity to help a stranger solve a difficult problem set. The amount of time spent helping was used as a measure of altruism.

The results, published in the Journal of Neuroscience, reveal a fascinating relationship between the participants’ brain activity during the rating task and their behavior during the dictator game or problem-solving task.

When thinking about the men in the photos, the amount of activation in a particular brain region called the dorsomedial prefrontal cortex—known as the dorsal MPFC—could predict how much money the participant later chose to give away or how much time they spent helping someone else: Greater activation meant more altruistic behavior. Because the dorsal MPFC is known to activate when people think about other people’s mental states, this research suggests that the ability to “mentalize” the thoughts and feelings of others may be a foundation of altruistic behavior.

“It’s my ability to take your perspective and to imagine the world as you see it that tracks my niceness toward you later on,” says Zaki.

The kindness calculus

Jamil Zaki, assistant professor of psychology at Stanford University

Despite these deeply rooted propensities for the pro-social, we all know that people often choose to act in their own self-interest. How do they decide between self-serving and more altruistic behavior?

Working with Gilberto López of Harvard University, Zaki and Mitchell hypothesized that people use a single subjective scale to decide when to act selfishly and when to act generously. To test that hypothesis, they took fMRI scans of people who were deciding, in a series of rounds, whether to allocate various amounts of money to themselves or to another person. Based on how each participant chose to distribute the money, the researchers wanted to see whether they could predict the level of activity in the participant’s ventromedial prefrontal cortex—a part of the brain involved in weighing the value of different types of rewards, like food and money—when rewards were later offered either to the participant or to the other person.

For instance, imagine a participant whose choices showed that she values her own gains twice as much as she values giving money to someone else—that is, a $1 reward for herself is in her mind equal to a $2 donation to another person. If the researchers’ hypothesis was correct, the level of activation in this participant’s ventromedial prefrontal cortex would be the same when she receives $1 as when she donates $2.

And, in fact, this is exactly what Zaki, Mitchell, and López found.

This finding, published in 2014 in the journal Social Cognitive Affective Neuroscience, suggests that our brains convert the subjective value of choices that would benefit ourselves and those that would benefit others into a “common currency” of reward that helps us decide when to act selfishly and when to act generously. In other words, people who feel like they are receiving a reward when they see someone else get a reward are the people who are more likely to be altruistic.

To Zaki, the findings suggest how our ability to share in another’s feelings—what some researchers call “affective empathy”—is a key to altruistic behavior. If we experience a sense of happiness when we see others receive a reward, we’ll assign greater value to giving rewards to others—perhaps nearly as much value as we would give to rewards for ourselves.

“Psychologically, we overlap a lot: Your joy is my joy, and vice versa,” says Zaki. “The people who share other people’s positive emotion the most are also the ones who incline towards been kind.”

A growth mindset for empathy

Zaki and Mitchell’s research, which was funded by the John Templeton Foundation through a grant from the University of Pennsylvania’s Positive Neuroscience Project, builds on previous findings that suggest humans have an intrinsic, even intuitive, drive to help others.

For instance, as they note in a 2013 paper in Current Directions in Psychological Science, several studies by Felix Warneken of Harvard University and Michael Tomasello of the Max Planck Institute in Germany have found that children as young as 18 months old spontaneously help people in need, such as by opening a door for someone with her hands full.

And research by David Rand of Yale University, along with Joshua Greene and Martin Nowak of Harvard, has found that when people are forced to make split-second decisions about whether to cooperate or act selfishly, they often choose the more selfless approach; it’s only when they’re given more time to think about their decision that their self-interested instincts kick in.

Nonetheless, Zaki says his team’s findings also suggest how we can override our altruistic instincts. Sometimes we have to make decisions not to be nice, he says, even though it’s painful to do so. Indeed, some situations might require us to temper our propensity for kindness, such as when our job involves inflicting harm on another—like a linebacker on a football team—or when we need to deal harshly with someone who is threatening us.

“It would be strange if we were wired to be indiscriminately kind to everyone, all the time,” says Zaki. “I think that in our nature, at a deep level, we’re more discerning than that.”

Zaki and Mitchell’s work clearly suggests that our decisions to be kind often hinge on empathy—“mentalizing” (taking someone else’s perspective) and “affective empathy” (sharing someone else’s emotions) are two different forms of it. While Zaki believes that everyone has an innate capacity for empathy, people sometimes lack the motivation to develop and engage with that capacity. His newest area of research has been exploring how to increase that motivation.

In a study published last year in the Journal of Personality and Social Psychology, he and fellow Stanford researchers Carol Dweck and Karina Schumann found that people who have a “growth mindset” about empathy—meaning that they believe one’s empathy level is a skill that can be improved, rather than a fixed trait—are more willing to try to flex their empathy muscle in tough situations, such as when encountering someone with opposing political views.

“If I think that empathy is a skill, that it’s something I can work on, then maybe I’ll be more willing to do so, even when it doesn’t come naturally,” says Zaki, whose work has earned him the Association for Psychological Science’s Rising Star Award in 2013 and a prestigious CAREER award grant from the National Science Foundation in 2015.

This line of research has inspired Zaki to hope that it won’t be long before there’s an “empathy-building super pack”—a set of exercises that people can use to build up their empathic skills. If we do more to tap into our propensity for empathy, he believes we can better nurture our instinct for altruism.

“The more people understand they have a natural capacity to be kind,” he says, “the more likely they are to align their moral principles with the powerful emotions that drive our altruistic behavior.”

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