Imagine it’s Saturday morning. You’re sipping coffee when your best friend texts, “Any chance you could help me move today?” You sigh—there go your weekend plans—but reply, “Of course.” That afternoon you sweat as you carry boxes up a flight of stairs.
A week later a co-worker you barely know mentions she’s moving and could really use a hand. This time, you hesitate. You are not as quick to offer help even though the request is nearly identical.
Why does generosity come so naturally for those we are close to but feels more like a burden when the recipient is a stranger or mere acquaintance? Psychologists call this tendency social discounting: we are generally more willing to make sacrifices for people to whom we feel emotionally close, and our generosity declines as the social-emotional distance to the recipient of help increases.
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But what happens in the brain when we make these decisions? And why are some people more generous to socially distant individuals than others are? In recent research, my colleagues and I gained new insight into these questions by examining a rare population of people with selective damage to a part of the brain called the basolateral amygdala. Our findings suggest that this small but important structure may be essential for calibrating our generosity based on how close or distant others feel to us.
The amygdala, a small almond-shaped region that is nestled deep in the brain’s temporal lobe, is traditionally known for its role in processing emotions, particularly fear. But over the past few decades it has become clear that the amygdala, particularly its basolateral part, is a central hub in our social brain.
Across species, this region has been shown to play a role in evaluating social rewards, empathic responses and decisions involving others. In rodents and monkeys, neurons in the basolateral amygdala encode not just the value of rewards for oneself but also the rewards received by others. And in humans the structure has been linked to traits such as trust, empathy, moral decision-making and extraordinary altruism. Human amygdala volume also correlates with the size and complexity of an individual’s social network. And finally, some evidence suggests that psychopathy and aggression are ***ociated with a smaller, less functional amygdala.
But how, exactly, does the basolateral amygdala influence our decisions to help others? One hypothesis is that this brain area allows us to balance competing helpful, social motives with self-interested goals. When you decide whether to help your best friend move, you are likely focused more on their benefit (making the move easier) than on your own cost in time and effort. But when the person is a stranger, that mental calculation may shift. Some neuroscientists propose that the basolateral amygdala aids us as we navigate this trade-off by ***igning value not just to our own well-being but also to the well-being of others.
To test this idea, my colleagues and I turned to a remarkable group of people in South Africa who have Urbach-Wiethe disease, a very rare genetic condition that causes selective bilateral damage to the basolateral amygdala while leaving the rest of the brain intact. In our study, we invited five women with this disease and 16 women without the condition to take part in a “social discounting” task. Each participant first listed eight people from her own social network, ranging from her emotionally closest person (ranked social distance “1”) to someone she barely knew (“50”) or a complete stranger (“100”). We then asked our participants to make decisions on how to split money. In each of several rounds, they received a fixed monetary amount and decided how much to share with each of their eight listed contacts. This task thus measured our participants’ willingness to share resources based on how emotionally close or distant they felt to each person they had indicated in their social network.
As expected, the participants gave more to people they were close to than they gave to more distant others. That is, generosity declined as social distance increased. Interestingly, however, participants with damage to the basolateral amygdala were less generous overall than others, and their generosity decreased more sharply as social distance increased. They showed what we call steeper social discounting: they were still willing to help those they were emotionally closest to, but their willingness to give dropped off markedly for more distant individuals.
One participant with basolateral amygdala damage was an exception: she was ungenerous across the board, even toward her closest friend. But overall, the pattern was clear: damage to the basolateral amygdala did not eliminate altruism, but it distorted the fine-tuned calibration of generosity based on social distance.
Importantly, variations in personality, empathy or social network size did not explain the differences in generosity among our participants. Rather our participants with Urbach-Wiethe disease seemed unable to adjust their generosity flexibly to the social context.
At first glance our findings might seem to contradict earlier studies that found that people with Urbach-Wiethe disease are actually more generous than others. For example, in past research, people with this condition gave away more money in the trust game. This is a cl***ic experiment in behavioral economics in which participants decide how much money to send to another player, the trustee. The amount sent is typically multiplied, and the trustee then decides how much to return. The initial amount sent is often seen as a measure of trust in the trustee. People with basolateral amygdala damage tend to send much more than others, even to untrustworthy trustees who fail to reciprocate. Researchers have described this unusual pattern of trust as a form of “pathological altruism.” In a similar vein, another study had people with Urbach-Wiethe disease respond to moral dilemmas involving hypothetical life-or-death decisions about others. They consistently refused to sacrifice one person to save many, revealing a marked reluctance to be responsible for causing harm to another individual in comparison with participants without the disease.
How, then, can we reconcile these earlier findings with our own results? We argue that the basolateral amygdala does not simply promote or hinder prosociality but is part of a neural network that helps people create a model of how the social world works that can be used to guide decision-making. With an intact basolateral amygdala, a person considers social context, social structure, social norms and learned expectations in social interactions when deciding whether to be generous or selfish.
When that system breaks down—as when someone suffers amygdala lesions—people may struggle to balance generous and selfish motives and consequently rely on simpler, default strategies that do not depend on networks that include this brain structure. In the trust game, the default ***umption might be that others are trustworthy. In moral dilemmas, it could be to follow a rigid rule like “never harm anyone.” Such ideas may have formed in childhood and, given damage to the basolateral amygdala, not been revised later in life, even in the face of contrary experiences, as with untrustworthy individuals. In our task, the default strategy is to maximize one’s own payoff—unless the recipient is emotionally very close, in which case helping them comes automatically.
Although our study includes only a small number of participants (which is unavoidable, given the extreme rarity of the condition), the distinctive pattern of brain damage in this group—symmetrical and precisely located in both hemispheres—is quite unique in neuroscience research. Other studies involving selective brain lesions often rely on only one or two patients. We also feel confident in our conclusions, given how our work fits into a pattern of evidence drawing from more studies and participants that suggest amygdala functionality is crucial to support our social life.
The idea that the basolateral amygdala helps us weigh selfish and altruistic motives might sound abstract—but it plays out in real life all the time. Think back to the moving-day dilemma. The generous impulse to help your friend move likely comes automatically because it is rooted in deeply encoded values and social bonds. But deciding whether to help an acquaintance requires something more: flexible, model-based decision-making that weighs social norms, reputational concerns and empathy against effort costs, self-care and the simple desire to spend a pleasurable, lazy weekend. It’s precisely in these gray areas where the basolateral amygdala seems to do its most important work.
Generosity is therefore not an all-or-nothing trait; it is a model-based social behavior, shaped by whom we are interacting with and how close we feel to them. And deep in the brain, the basolateral amygdala is helping us do that calculus.
Are you a scientist who specializes in neuroscience, cognitive science or psychology? And have you read a recent peer-reviewed paper that you would like to write about for Mind Matters? Please send suggestions to Scientific American’s Mind Matters editor Daisy Yuhas dyuhas@sciam.com.
