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In the early 1990s, a quiet man named João quit his job running the human-resources department of an insurance company in Rio de Janeiro and began selling french fries from a street cart. The fries quickly proved popular, in part because they were delicious—thin and crisp and golden. Even more enticing, João often served them up for free. All you had to do was ask, and he’d scoop some into a box, no charge. What money he did take in, he frequently gave away to children begging in the street or used to buy them sweets. Day after day, he came home to his wife and son without a single real in his pocket. In his previous life, João—a chubby man with pointy ears and arched black eyebrows—had been stern and serious, prone to squirreling money away. But after suffering a health crisis in 1990, at age 49, he wanted to live differently. “I saw death from close up,” he would often say. “Now I want to be in high spirits.” And nothing made him happier than giving. To those who didn’t know him well, he must have seemed like the embodiment of selflessness—the Saint Francis of Rio de Janeiro.

What’s most interesting about João’s story, though, is that his new outlook resulted not from a spiritual awakening but from brain damage caused by a stroke. Among other symptoms, he became a chronic insomniac and lost his sex drive; he started forgetting things and had trouble focusing; his movements slowed. And, his neurologist says, he became “pathologically generous”—compulsively driven to give. His carefree attitude toward money led to confrontations with his family, especially his brother-in-law, who co-owned the french-fry cart. But even when his family berated him, and the cart went out of business, and he was reduced to living on his mother’s pension, João refused to stop. Giving simply made him too happy. (João died of kidney failure in 1999. His doctor provided only his first name, to protect the family’s privacy. ) The history of neuroscience is littered with patients whose behavior changed in bizarre ways after they suffered brain damage. Some people could no longer recognize animals, or couldn’t speak but could still sing. For neuroscientists, these cases offer opportunities: by studying how people’s behaviors change after brain injuries, they gain insight into what role the injured areas play in everyday tasks. And so it was with João—researchers hoped that his compulsive giving could shed light on normal generosity, helping them understand why human beings give and why, biologically, giving feels good.

This work does raise uncomfortable questions, though. We normally think of generosity as pure and noble—evidence of the soul, not evidence of brain damage. But what if giving is largely a reflex or an instinct or even, sometimes, a sign of mental derangement? We also think of generosity as uniquely human. If other species evolved to be generous too, does that devalue the trait? These aren’t idle questions. João’s case shows that generosity isn’t part of some ethereal “human spirit”—it’s hardwired into our brains. And while acts of generosity do engage our “higher” brain regions—the areas responsible for rational thought—they cause equally strong activity in the animalistic pleasure centers, circuits normally associated with food, sex, and drugs like cocaine. In other words, the urge to give seems to arise from a blend of base appetites and refined reflection—a potent combination that probably played an important role in the evolution of humankind. We’ve long known that there’s a clear, consistent link between generosity and happiness: surveys done around the world, of many different societies, have found that giving produces high levels of satisfaction and well-being in the givers. What scientists didn’t have a good grasp of until recently were the neuroscientific roots of this feeling—why we get a boost from giving. John Gall A decade ago, Jordan Grafman, a cognitive neuroscientist at Northwestern University Medical School, investigated this link by putting volunteers in an fMRI machine and asking them to decide whether to donate to certain charities. Grafman and his team gathered data on which brain systems were most active during the process. They had expected to see heightened activity in people’s frontal lobes, a part of the brain that helps with social reasoning and with weighing different courses of action—just the sorts of talents needed for this task. And the frontal lobes did, in fact, come to life on the fMRI scans. But Grafman was surprised to see the brain’s pleasure and reward circuits rev into high gear as well. “Our first impression,” Grafman says, “was that we might see some activation [in those circuits], just because usually when people give, they feel a little bit better. But we had no idea about the degree.”

Specifically, his team saw the brain’s mesolimbic system light up. This system forms a key part of the brain’s pleasure circuits, an archipelago of structures that stimulate the production of the chemical messenger dopamine, which makes us feel good. Neuroscientists usually associate activity in these circuits—which many other species also have—with hedonistic delights like food and sex. Grafman determined that giving money away excited these circuits even more than receiving money did. What your mother told you, then, is true: it is better to give than to receive. She probably just didn’t realize that, neurologically, giving is roughly on par with eating fudge or getting laid. If giving feels so good, why don’t people do more of it? (One survey found, for example, that 85 percent of Americans donate less than 2 percent of their income to charity.) Part of the answer lies in the fact that other areas of the brain, like the frontal lobes, suppress the instinct for generosity at times. That sounds miserly of them, and maybe it is. But the frontal lobes help us see the bigger picture, and can alert us to the downsides of giving. João’s case reveals what happens when the frontal lobes lose the ability to weigh in, allowing warm, fuzzy feelings to run amok. João’s doctor believes that his stroke severely damaged a structure called the medial forebrain bundle, a collection of neuron fibers near the base of the brain. To monitor other regions, the frontal lobes need to receive input from them, and that’s where the medial forebrain bundle comes in. Like an Internet trunk line, it pipes in data from all over the brain, allowing the frontal lobes to suppress, in the service of a larger goal, some of the urges that arise. (Your frontal lobes may, for instance, steer you away from that second slice of chocolate cake if you’re on a diet.) When parts of the bundle in João’s brain got destroyed, his frontal lobes lost the ability to control certain impulses—including, apparently, the impulse to give money away. Explaining generosity is a headache for biologists; Charles Darwin considered the trait one of the gravest threats to his theory of natural selection. This urge never bubbled up spontaneously, says his neurologist, Ricardo de Oliveira. That is, João never actively sought out street children and offered them money or sweets. But whenever children asked, João couldn’t help reaching for his wallet. It was more or less a reflex, like the one that made Pavlov’s dogs salivate whenever they heard the dinner bell.

De Oliveira says that damage to the medial forebrain bundle also disabled João’s “punishing mechanisms,” the system responsible for chastising dumb behavior. In most of us, this system would have stepped in and said, You’re going to lose your house if you keep giving away french fries, you idiot. But with his punishing mechanism broken, such long-term threats proved impotent—they couldn’t deter him. No matter how dire his finances looked or how many times his family yelled, he never learned. All of this raises yet another uncomfortable question. We think of both generosity and self-control as high virtues. But what if some people are generous in part because they have the same lack of impulse control as binge eaters or compulsive shoppers? Can generosity sometimes be a weakness? The pleasure João felt seems to be an exaggerated version of what we all feel when we give, and his case shows that without the check the frontal lobes provide, everyday requests for charity could easily overwhelm any of us. That said, pathological generosity can occur even without damaged wiring in the brain. Some people with bipolar disorder give excessively during their manic states, as do others who use gifts to mask insecurities or manipulate people. Tweaking people’s brain chemistry with drugs can also elicit compulsive generosity. In the 1990s, Andrew Lees, a neurologist in London, prescribed a drug called pramipexole to several patients suffering from Parkinson’s disease, which is caused by the death of brain cells that produce dopamine. Drugs like pramipexole aim to restore normal brain chemistry.

Unfortunately, dopamine-stimulating drugs often produce strange side effects, like an overwhelming desire to shop or gamble. Lees saw several such side effects emerge in his patients after they started taking pramipexole. In the early 2000s, three patients also developed “reckless generosity,” he says, something he’d never seen before. One man in his late 30s began lifting weights, gambling, and shopping compulsively, once buying 60 bottles of aftershave. He also gave away most of the money from his disability checks to his friends and twin brother, to the point that his electricity got cut off. In another case, a 66-year-old woman bought three motorized scooters on eBay, though she had no use for them. She also started giving her money away to family and friends, since she knew she’d otherwise spend it on useless things. And a 58-year-old naturalist found his sex drive soaring and began writing obsessively about things like mushrooms and toadstools—sometimes for 48 hours straight. He also began handing out sandwiches and money to drug addicts he met while walking around town. He gave one young woman £20,000, money his family couldn’t spare. Giving, it seems, might become compulsive in some people because they crave the rush of dopamine that accompanies it. Lees suspects that pathological generosity may be more common than neurologists realize, since most doctors wouldn’t count increased charitable giving as a negative side effect. As a recent article in the Proceedings of the National Academy of Sciences, about generosity’s potential for harm, put it, “Altruistic intentions often are viewed as monolithically positive, nearly sacred qualities with negligible tradeoffs.” But excessive giving nearly ruined these patients’ lives. Even when they understood the danger on some level, they had no defenses against the impulse to give, give, give.

Lees finally eased the trio of patients off pramipexole, and all three stopped giving excessively. Considering that none of them had been particularly generous before starting the drug, the conclusion seems inescapable: a simple chemical—a few carbon rings studded with nitrogen and sulfur—had transformed them into super-givers. Lees’s patients all had Parkinson’s disease. But if those of us with healthy brains started gobbling pramipexole, would we turn into pathological givers, too? Lees doesn’t know. It’s possible a small number of us would. Still, the brain damage in his Parkinson’s patients provides clues as to how pathological giving works. “The pleasure and reward pathways are damaged to a certain degree in Parkinson’s disease,” Lees says. As a result, some patients’ “ability to enjoy life is reduced.” Neurologists call this state of reduced pleasure “anhedonia.” These people are not necessarily depressed; they don’t find life meaningless, and they’re not contemplating suicide. But art, music, food, hobbies, and even sex no longer thrill them—life is one big “meh.” When they start taking pramipexole, their brain chemistry changes: dopamine begins flowing again. As a result, the reward pathways can run at full throttle, allowing people to seek out and feel pleasure, at least from certain activities—though not necessarily the same ones that they enjoyed before. What those activities are (gambling, buying aftershave, writing about toadstools) depends on the idiosyncrasies of the individual and his particular brain damage. But since acts of giving tap into the brain’s pleasure circuits, they’re as eligible as any. And because pramipexole can inhibit impulse control, the person keeps returning to the pleasurable activity over and over.

If that behavior looks like an addiction, it’s no coincidence. Giving, it seems, might become compulsive in some people because they crave the rush of dopamine that accompanies it—a rush that might be similar to the spike in dopamine levels that gets some people hooked on drugs such as cocaine and amphetamines. In a real sense, pathological givers might be addicted to philanthropy. The internal pleasure of giving is just part of the story, though. Generosity also affects your relationships with others, especially the recipients of your gifts. Compulsive generosity, unfortunately, tends to put off recipients. Friends and family members of the pathologically generous often find the crush of presents bewildering and embarrassing. And people who give primarily to strangers can face outright hostility from their loved ones, especially when the giving erodes their finances. This contrasts with normal giving, which tends to bring people together. The recipient usually enjoys the gift, feels grateful, and wants to reciprocate. What’s more, in Jordan Grafman’s fMRI study, the givers themselves tended to show heightened activity in the subgenual area, a region in the frontal lobes that helps control the release of oxytocin. This hormone promotes social bonding, trust, and cooperation; concentrations of it swell whenever we gaze at our loved ones. The social rewards of giving could help explain why generosity took root in the human brain in the first place. Explaining generosity—or, more generally, altruism—is actually a headache for biologists; Charles Darwin considered the trait one of the gravest threats to his theory of natural selection. To understand why, imagine a tribe of our ancestors. Some people are givers, willing to share food and goods. Others are stingy and selfish. Those in the first group sound like better people, but from a survival standpoint, they’re being daft. Since there’s only so much food to go around in the wild, the math is clear and cruel: over the long term, generous people will probably get wiped out.

But there’s a loophole. In the mid-20th century, biologists began explaining acts of altruism with something called kin selection. The theory holds that animals, including humans, are far more likely to be generous toward relatives, with whom they share the most genes. Kin selection explains altruism as disguised selfishness: I might sacrifice my well-being in the short term, but helping my siblings survive will ultimately boost the chances of my genes being carried on in the future. Kin selection is now a cornerstone of modern biology. And yet it seems unsatisfactory as an explanation of human behavior. It’s not that humans are somehow exempt from the evolutionary pressures that shape other species. We’re not, and kin selection does take place among human beings. But humans also help out strangers every day, all around the world—donating time, money, and even blood and organs to people we’ll never meet and who will never pay us back. Biologists developed a theory called group selection to resolve this paradox. Group-selection theory suggests that while selfish individuals trump altruistic ones, groups of altruistic people will outcompete groups of selfish ones in many tasks, like waging war and hunting big game. This idea has an intuitive appeal, and it has received endorsements from some big names in evolutionary biology, including E. O. Wilson, who was a major proponent of kin-selection theory decades ago but who has renounced parts of it in the past decade. Still, group selection seems nebulous and ill-defined to biologists who take a gene’s-eye view of evolution, and it is controversial among scientists.

Studying what happens in the brain when people give could help solve the puzzle of how generosity gained a foothold in our species. The activity that Grafman saw in the subgenual area suggests that giving can bring people together and encourage reciprocity. That’s important, Grafman says, because “this kind of reciprocity is good for stabilizing relationships and societies.” Economic transactions like trading, for instance, typically require people to build up trust over time. In other words, key aspects of society depend on just the sort of social bonds that generosity forges in the brain. Scientists don’t know whether injuries and drugs create an instinct for giving, or whether they simply unmask an instinct that was there but that people didn’t act on. Grafman believes that brain damage can fundamentally alter people’s personalities, bringing out qualities they never had before. But Salman Akhtar, a psychiatry professor at Jefferson Medical College, in Philadelphia, says it can also work the other way. (He has studied pathological giving in people without brain damage. One of his patients offered him $1 million to thank him for his help; another patient spent $30,000 flying a young boy she met at a mall to a theme park on a private jet.) “It’s like being drunk,” he says. “What comes out of you was in there. If you get real, real drunk and you say to your sister, ‘I hate you,’ alcohol has not produced that hate. That hate was already there. Alcohol has just released it.” It’s the same with reckless generosity, he says.

Ricardo de Oliveira, João’s neurologist, got to know João well during the decade he treated him, and he and his nurses received many a free box of french fries. He also talked extensively with João’s family about what João had been like before the stroke, and he concluded that the desire to give had always existed inside him. The stroke, then, by disrupting João’s brain circuits, simply remixed his personality: he had the same basic traits before and after, but his latent generosity came to the fore and began to dominate. We think of generosity as pure and noble—evidence of the soul. But what if giving is a reflex, or even a sign of mental derangement? This is how João himself understood his transformation. We can talk about generosity in terms of dopamine hits or Pavlovian stimulus-response theory. And that’s not necessarily wrong. But it overlooks something important about João: that giving truly did make him feel happy and fulfilled. De Oliveira says João was one of the happiest people he’s ever met. At the same time, João’s fights with his brother-in-law, wife, and son forced de Oliveira to think about the dark side of generosity. Based on the example of his own father—a tailor who grew up poor but always gave away money anyway—de Oliveira has tried to be generous in his own life. He often waives fees for poor patients, for example, even though his family’s material well-being has suffered as a result. “So when I met João, I empathized with him immediately,” he says. In the long run, however, “João made me repeatedly question the moral value of being generous,” because helping people left João with less time and money for his wife and son, and less able to help them with their problems. “Generosity may be a burden to those who love us,” he says. That hasn’t stopped de Oliveira from giving. In fact, as he ages, he believes even more strongly that we grow the most as human beings when we give to others. He nevertheless realizes now that generosity can have an edge. “João’s case,” he says, “reminds us that the borderland between good and evil may be more subtle than is ordinarily assumed.”