The Darwinian principle of “survival of the fittest” echoes what many people believe about life: To get ahead, you need to look out for No. 1. A cursory read of evolutionary doctrine suggests that the selfish individuals able to outcompete others for the best mates and the most resources are most likely to pass their genes on to the next generation. Then there is classical economic theory, which holds that given the choice, we will often opt for a personal benefit over a personal loss, even if that loss involves a benefit to someone else. The philosopher and economist John Stuart Mill championed the self-centered theory in the mid-1800s, describing man as a creature that “does that by which he may obtain the greatest amount of necessaries, conveniences and luxuries, with the smallest quantity of labor and physical self-denial.”

But the latest science shows that, in fact, we are also hard-wired to be generous.

Using tools like fMRI, scientists are identifying the precise circuits within the brain that control these nurturing social impulses. Where once there was only speculation about the origins of the human desire to help others, a body of data is starting to fill the gap, revealing key workings of the biological hardware that makes altruism possible. This represents a new scientific frontier, one that could eventually enable the development of therapies tailored for people who have particular problems generating empathy or who want to improve their existing capacity for generosity. At the same time, theories about the evolution of altruism have expanded and acquired evidence to support them, lending insight into why our brains are wired for selflessness in the first place.

The question of why any creatures are altruistic at all obsessed Charles Darwin from the time he devised his theory of evolution. Since then, two complex schools of scientific thought have emerged. One argues that altruism exists because it helps ensure the survival of close kin. Various researchers have also highlighted the merits of the view that helping may maximize the survival odds of each member of a society. That would mean that behaving less selfishly isn’t just a way of protecting close family members; it might also be a way for individuals to improve their own prospects by contributing to the well-being of a strong collective.

These days, neuroscientists like Jordan Grafman are investigating specific regions of the human brain that give rise to altruistic behavior. Dr. Grafman, now director of brain injury research at the Rehabilitation Institute of Chicago, became interested in how the brain governs generosity in part out of his work with military veterans who suffered brain trauma. Back in the 1980s, when he was working with returned vets at Walter Reed Army Medical Center in Washington, D.C., he started to notice something unusual about patients who had sustained damage to their frontal lobes. At first glance, they appeared normal: Their cognitive ability seemed unaffected, and many were able to carry out basic motor tasks with ease. But they suffered from other, more subtle deficits, many of which were apparent only in a nonclinical setting. “The wives said, ‘You’re missing something,’ ” Dr. Grafman remembers. In social situations, the men floundered, acting as if they didn’t care what other people had to say.

In the mid-2000s, while working at the National Institutes of Health, he began to investigate where empathy and generosity originated in the brain. The advent of fMRI scanning, which highlights blood flow in different parts of the brain, made it much easier to see which parts of the brain were engaged as people carried out various tasks. To see if this tool could lend insight into the motivations behind giving behavior, Dr. Grafman and his colleagues recruited 19 study subjects, placed each of them inside the fMRI scanner, and presented them with charities from a long list. For each charity, they could choose to donate money, refuse to donate money, or add money to a separate reward account that they could take home at the end of the study. (In some cases, it was especially costly for subjects to make a donation decision, because doing so required them to draw from their own reward accounts.)

While analyzing the study’s results, Dr. Grafman’s colleague Jorge Moll came up to him and said, “You’re not going to believe this.” The scans revealed that when people made the decision to donate to what they felt was a worthy organization, parts of the midbrain lit up—the same region that controls cravings for food and sex, and the same region that became active when the subjects added money to their personal reward accounts.

Gradually, Dr. Grafman began to realize how this finding made sense. While we often tend to think of altruism as a kind of sophisticated moral capacity we use to squelch our urges to dominate others, this new evidence suggests that giving is actually inherently rewarding: The brain churns out a pleasurable response when we engage in it.

But the subjects’ high degree of midbrain activation wasn’t the study’s only interesting finding. Dr. Grafman found that the subgenual area—a gumdrop-size region near the midpoint of the brain, part of the frontal lobes—was also strongly active when his study subjects made the decision to give to charity. The area contains lots of receptors for oxytocin, a hormone that promotes social bonding. The finding suggests that altruism and social relationships are intimately connected—in part, it may be our reliance on the benefits of strong interpersonal connections that motivates us to behave unselfishly.

To Dr. Grafman, one of the most memorable results was what happened when subjects decided to make a donation even when they knew it was going to cost them money from their personal reward accounts. In these scenarios, a brain area called the anterior prefrontal cortex lit up—a region that is responsible for complex judgments and decision-making. These subjects were willing to give even when they knew it would cost them, indicating that this segment of the brain may help us decide to behave generously when doing so runs counter to our immediate self-interest.

Of course, giving generally isn’t purely selfless. Bill Harbaugh, a University of Oregon economist, first became interested in charitable giving because it seemed like an intriguing exception to the rule that most people will act in their direct self-interest. His early look at givers’ motivations, though, reinforced the more traditional view that people give because they expect some tangible reward—if not a monetary one, then a social one. In the late 1990s, he studied a group of law school graduates donating to their alma mater; the institution grouped them into different donation brackets, entitling them to varying levels of recognition in public alumni giving reports. He wasn’t all that surprised when alumni donations clustered at the lowest end of each bracket, indicating people were giving just enough to be in the highest public tier accessible to them and little more. “People care about prestige,” he says.

Dr. Harbaugh set out to pinpoint exactly what is going on in people’s brains when they decide to give to someone else. In 2007, with his psychologist colleague Ulrich Mayr, he placed subjects in an fMRI scanner, while a computer monitor in front of them presented them with opportunities to donate to a food bank from a fund of $100 in real cash they’d received at the beginning of the experiment. The suggested donations could be as low as $15 or as high as $45. The subjects’ donation decisions had meaning, since they would get to pocket whatever money was left over.

Dr. Harbaugh also opted to add another wrinkle: Some of the donations would be “voluntary,” meaning the subjects would get to decide whether or not to give to charity, and some would be “involuntary,” meaning the computer would simply inform the subjects that they were required to give a certain amount—a condition similar to real-life taxation.

After the experiment, Drs. Harbaugh and Mayr scrutinized the fMRI data. Some of their results meshed with basic Econ 101 principles. For instance, people were more likely to help the charity when the suggested contribution was relatively low. Other findings, however, were more surprising. When subjects decided to give to charity, areas of the brain associated with the processing of unexpected rewards, such as the nucleus accumbens, lit up. The nucleus accumbens, which contains neurons that release the pleasure chemical dopamine, “is almost like the common currency of the brain. It keeps track of rewards, whatever kind they are,” Dr. Harbaugh says. “There’s some primary reward people get from seeing money go from themselves to provide to other people.”

His results tallied with Dr. Grafman’s fMRI experiments showing pleasurable activation in the midbrain during giving. Dr. Harbaugh’s study indicated that giving to charity is, surprisingly, neurologically similar to ingesting an addictive drug or learning you’ve received a winning lottery ticket. It seems clear, then, that people give to charity not only because they think it’s a good thing to do but also because giving makes them feel good, in addition to the particular benefit they’re bestowing on the recipient. Even when subjects in Dr. Harbaugh’s study were required to donate, this pleasurable response persisted, though it wasn’t as strong as when people got to choose whether to donate on their own.

Yet not all subjects were the same. Since some showed more nucleus accumbens activation than others when they thought about seeing their money go to charity, Dr. Harbaugh figured they were probably experiencing vastly different levels of satisfaction as a result. He divided the subjects into two groups: “egoists,” who showed less nucleus accumbens activity at the prospect of seeing their money go to charity, and “altruists,” who showed much more. In general, he found, the greater the pleasurable brain activation, the more likely subjects were to give frequently. “You can actually measure how much activation there is and predict with some degree of accuracy how much they’re going to give,” Dr. Harbaugh says.

There were a few outliers in Dr. Harbaugh’s experiment: People who fit squarely in the “egoist” category from a biological perspective, but who donated at rates that surpassed what the nucleus accumbens activation on their scans would predict. These people, you could argue, were actually the most dedicated altruists: They were willing to give because they knew it would help others, even if giving didn’t make them feel all that great.

On balance, Dr. Harbaugh’s work suggests that giving completely for its own sake—with absolutely zero expectation of pleasure or other reward in return—is rare. We are forever making complex calculations about whether or not to give in different situations, but whether or not our gift will help someone is far from the only factor we consider. The better we feel when we give, in general, the more often we do it. And as the Georgetown philosopher Judith Lichtenberg points out, even when we think we’re giving with absolutely no expectation of reward, we can’t be sure; our motivations (feeling good? looking good? gaining social leverage?) may be unconscious, inaccessible even to ourselves.

Will we one day be able to manipulate these processes—making ourselves more generous if we so choose? Using genes from sources such as algae, the Stanford psychiatrist and bioengineer Karl Deisseroth has begun making headway on the question. Dr. Deisseroth is connecting the dots between neural circuits and social behaviors with the help of light-sensitive proteins found in nature. After incorporating the genes that code for such proteins into mouse neurons, Dr. Deisseroth can activate and deactivate the cells by snaking an optical fiber into a mouse’s brain and turning on the light.

Using this technique, Dr. Deisseroth demonstrated that it is possible to make mice more “prosocial”—in other words, more interested in others—than they might otherwise be. Like many animals, mice can be bred with varying propensities to socialize and identify with other members of their species. But those genetic predispositions can be overcome, at least to some degree, through neural manipulation. In experiments, he has used light to activate neurons in part of a mouse’s brain known to help govern emotions like love in humans. Once the light in the mouse’s brain begins to glow, the mouse all but smothers his cagemate, practically jumping on top of him in an attempt to connect.

Dr. Deisseroth’s behavior-modifying technique might sound vaguely Orwellian, but he plans to use his light-activation system for benevolent ends. He aims to use it to figure out how mammals’ brains govern their actions, which groups of neurons give rise to which behaviors. He’s not looking to zap people’s brains with light to force them to become more “prosocial” and attentive to others; he simply wants to understand the origins of such generous behavior.

One problem with primarily biological explanations for generosity, though, is that they sometimes give short shrift to environmental influences and individual initiative. It is true enough that our genetics have evolved to include cooperative inclinations and create brain structures that predispose us to help, but the examples others set for us, the surroundings in which we live, and the values we most prize also play a significant role in tipping us toward selflessness or selfishness. In the end, what we do with our generous thoughts and inclinations is always up to us.

While we are hard-wired to “do unto others” in a multitude of ways, we also have power over whether to take advantage of those natural capacities or let them wither away. One way to strengthen these capacities may involve giving in strategic ways to reinforce the existing generosity pathways in our brains. Dr. Harbaugh envisions some straightforward potential ways to do this. If we give relatively small amounts of money to charity to start with, for example, the neural reward that comes with giving will be more likely to outweigh the pain of giving up money. Having had a pleasurable experience on balance, we might be more apt to return to give. Ultimately, we might not only be able to get hooked on self-sacrifice, but also be able to learn to love the entire process.

Adapted from Ms. Svoboda’s new book “What Makes a Hero? The Surprising Science of Selflessness,” published by Current.