People with damage to the BLA tend to revert to higher levels of selfishness, especially toward those they’re less emotionally close to. (FrankHH/Shutterstock)
In a nutshell
- Your brain fine-tunes generosity based on social closeness. A specific brain region, the basolateral amygdala (BLA), helps you decide how much to share with others depending on how emotionally close they are to you.
- Damage to the BLA makes people far less generous to strangers. Women with a rare genetic disorder affecting this brain region were just as generous to close friends, but dramatically more selfish toward acquaintances and strangers.
- The study reveals how our brains weigh selfishness versus altruism. The BLA doesn’t make us generous or selfish overall, it acts like a “social calculator,” adjusting our behavior depending on who we’re dealing with.
DUSSELDORF, Germany — Your brain decides how generous you’ll be toward strangers versus friends, and scientists have now found the exact brain region responsible for this calculation. A new international study reveals that damage to a tiny region in the brain called the basolateral amygdala (BLA) makes people dramatically less generous toward acquaintances and strangers while maintaining generosity toward close friends. This sheds new light on how our brains process social relationships and make decisions about who deserves our help.
Most of us share resources generously with our closest friends but become increasingly stingy as social relationships grow more distant—a pattern scientists call “social discounting.” This new research published in the Proceedings of the National Academy of Sciences suggests the BLA acts as a kind of social calculator, helping us decide how much selfishness is appropriate based on relationship closeness.
The study focused on five women with Urbach-Wiethe disease (UWD), an extremely rare genetic disorder that causes selective damage to the BLA while leaving the rest of the brain intact. These participants, all from South Africa, were compared to 16 healthy women matched for age, income, cultural background, and psychological traits.
In the experiment, participants selected eight people from their social circle, ranging from their closest friend (social distance 1) to a complete stranger (social distance 100). They then decided how much of 200 South African Rand (about $11) they would share with each person.
While healthy individuals gradually reduced their generosity as social distance increased, the UWD participants showed a much steeper decline, becoming dramatically more selfish toward anyone beyond their closest circle. One participant, however, displayed minimal generosity across all relationship types.
This reconciles seemingly contradictory evidence about the role of the amygdala in social behavior. Previous research has shown that damage to this brain region can sometimes increase altruistic behavior in other contexts, such as in the trust game or in moral dilemmas.
However, these results indicate that the BLA doesn’t simply make us more or less generous. Instead, it helps us adjust our generosity based on our emotional closeness to others. Without a functioning BLA, people default to maximum selfishness except with their closest friends.
This makes evolutionary sense. Our brains evolved mechanisms to care about close social bonds while limiting generosity toward strangers who might not reciprocate. The BLA appears to provide the flexibility to override this selfishness when appropriate, allowing us to be generous even to those outside our immediate social circle.
This research may also help us understand conditions like psychopathy and aggression, which have been linked to amygdala dysfunction and are characterized by reduced concern for others.
Specialized brain regions regulate our social behavior in surprisingly precise ways. The BLA doesn’t simply make us kinder or meaner, it calibrates our generosity based on relationship closeness, an essential ability for navigating human social networks. Without it, we default to self-interest except with those we value most.
Paper Summary
Methodology
Five female participants with Urbach-Wiethe disease (UWD) and 16 matched healthy controls played multiple rounds of a monetary sharing game. Participants were endowed with 200 South African Rand (approximately $11) per round and decided how much to share with individuals at different social distances. Before the experiment, participants assigned real people from their social networks to eight social distance levels on a scale from 1 (closest friend) to 100 (stranger), based on emotional closeness. During the game, these social distances were presented in random order, and participants decided how much money to share with each person. The researchers then analyzed how generosity declined across social distance by fitting hyperbolic discount functions to the data and comparing parameters between UWD participants and controls.
Results
All participants showed decreasing generosity as social distance increased, following a hyperbolic pattern. However, UWD participants exhibited significantly steeper social discounting compared to controls. Four of the five UWD participants showed much steeper discounting (more rapid decrease in generosity as social distance increased), while one showed very low generosity even toward close friends. Statistical analysis using area-under-the-curve measurements confirmed that UWD participants had significantly smaller areas, indicating steeper social discounting. These differences could not be explained by differences in empathy, personality traits, or social network size between the groups.
Limitations
The most obvious limitation acknowledged by the researchers is the small sample size of just five UWD participants. However, this is unavoidable due to the extreme rarity of the condition. The researchers emphasize that the precise, bilateral nature of the brain lesions in this UWD sample provides unprecedented specificity in human brain damage research. Another limitation was that participants were instructed to exclude spouses, household members, and relatives from their target lists, which may have led to steeper social discounting in all participants, including controls.
Funding and Disclosures
The project was funded by grants to Tobias Kalenscher (DFG KA-2675/7-1) and Ron Stoop (SNSF IZLCZ0_206045). The researchers acknowledged Victoria Cloete and Mara Brandt for their fieldwork contributions. The authors declared no competing interests.
Publication Information
The paper “Steeper social discounting after human basolateral amygdala damage” was published in the Proceedings of the National Academy of Sciences (PNAS) on April 15, 2025, Volume 122, Number 16, e2500692122. The study was approved by the institutional review board of Cape Town University (Human Research Ethics Committee UCT, protocol no. 639/2016).
