Reprinted from “Your Brain on Altruism: The Power of Connection and Community during Times of Crisis” by Nicole Karlis, courtesy of University of California Press. Copyright 2025.
On a tiny island called Cayo Santiago off the coast of Puerto Rico exists a colony of about 1,800 rhesus macaques. Each weighing about 20 pounds and known for their sand-colored fluffy tails, the monkeys that inhabit this island today are descendants of those brought over by primatologist Clarence Carpenter in the late 1930s. Since then, they have helped primatologists, evolutionary biologists, and scientists of all kinds better understand primate behavior in a unique natural laboratory setting.
Neuroscientist Michael Platt is one of those lucky scientists who has been able to study them for over a decade, particularly with a focus on how their social environment affects their brains, how they make decisions, and the genetic underpinnings of their social behavior. When news broke in the fall of 2017 that Hurricane Maria, a Category 4 storm, was bound to make landfall, Platt and his colleagues were terrified. They worried about what this would mean for their research and the monkeys who had given so much to science.
On September 20, 2017, the hurricane hit at a ferocious speed, pummeling the island with 170-mile-per-hour winds and torrential rains. Platt and his colleagues waited several nail-biting days to hear about the assessed damage and potential mortalities of the monkeys. Upon their colleagues’ surveying the scene by helicopter, a heroic effort at the time, they learned that two-thirds of the island’s green vegetation had been wiped away. The freshwater cisterns that the monkeys relied on as a water source were destroyed. Through a collective effort, researchers were able to get back up and running fairly quickly, which positioned them to be in a unique opportunity: to see how the rhesus macaques would respond in the wake of a natural disaster. Specifically, the researchers were curious to see if the monkeys’ social ties had shifted and if their behavior would turn more tolerant or aggressive.
Considering the lack of resources and devastation, would the monkeys fight over strained resources in the quest to survive? Since the researchers had over a decade of their social behavior documented, they’d be able to compare the monkeys’ behavior from before the hurricane to that after the hurricane. For example, they knew that while these monkeys are highly social, they can also have very competitive streaks.
Previously, the researchers had relied on a study method that required researchers to follow each individual monkey for 10 minutes and report every action and interaction to study their behavior. Since the devastation was too big to support this kind of approach, researchers turned to another sampling technique known as the “scan method.”
In this technique, an observer looks up every 30 seconds to record the interactions of every monkey around. After adjusting for potential biases, like louder monkeys trying to grab the attention of the researchers, an analysis of their data showed that the monkeys’ behavior had indeed changed after the hurricane. But instead of for the worse, it was for the better.
For instance, the monkeys appeared to be more tolerant of each other compared to the previous times. While the researchers expected the monkeys to rely on those they already had invested relationships with to cope with the ecological devastation, they found that the monkeys appeared to actually seek out new relationships and expand their social networks. A close relationship still had a lot to provide, but it was almost as if the monkeys experienced a realization that a social network where everyone is friendly enough is better for their overall survival than a network with just a few close friends.
“What was amazing was that these monkeys immediately began to reach out and make more friends,” Platt told me in an interview. “And everybody got connected with everybody in a dense web of interconnection.”
Fascinatingly, even monkeys who were previously characterized as socially isolated broke out of their lonely shells and made more social connections in the hurricane’s aftermath.
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While most of the monkeys survived the initial impact of the storm, the population experienced an uptick in mortality a month later. As time went on, researchers found that the monkeys who had more friends were more likely to survive in the damaged ecosystem for the following two years. And it wasn’t just their physical habitat that had experienced rapid deterioration. Platt and his colleagues made another observation of the monkeys. Some appeared to have aged about two years. Monkeys in their teenage years were developing arthritis.
Five years later, the stronger and more tolerant connections among the monkeys were still living on.
“The monkeys are still way less aggressive, way more tolerant, and more connected with each other,” Platt told me. Indeed, it appeared that the monkeys experienced bounded solidarity and were able to transform it into durable solidarity. Why did it work for the monkeys, and why doesn’t it for humans? That’s one of a few million-dollar questions, Platt said. Other open-ended questions are these: Why did some monkeys appear to be able to overcome the difficulties of the hurricane more than others? Why did some show early signs of aging from the stress, and others didn’t? In other words, why were some more resilient?
Social support is thought to be an adaptive response to extreme stressors, Platt said. This means that having strong social support before a tragedy can help organisms better resist stress damage.
The implications for the monkeys could be this: those who had stronger social connections before the hurricane were able to cope better with the aftermath of the hurricane. Platt said that there’s a lot of compelling research that shows more social connections can act as a buffer in the brain against stress responses. It can help people get through tragedy, disaster and trauma. It can keep people’s brains young, in a sense.
“And if you have a younger brain, you’re probably going to be able to navigate life better too, so it’s a feedback loop,” he said. “When your brain is older, you’re not going to be able to navigate a lot of those complexities.”
We know from research on monkeys and humans that having more social support enables resilience, Platt said. But the big open question is, how?
Perhaps the first step to understanding how having more social support enables resilience that can be observed in the brain, it’s best to understand how stress affects the brain. To find an answer, I reached out to cognitive neuroscientist Dr. Julie Fratantoni, who is also one of the leaders behind the BrainHealth Project, a 10-year longitudinal research study seeking to define, measure and improve brain health.
She said broadly speaking, chronic stress kills brain cells in the hippocampus, which is the part of the brain responsible for learning and memory. “Your neurons literally die,” she told me. When that happens, it can become more difficult for people to learn and remember things.
Stress can also affect the brain’s frontal networks, which are responsible for executive functions like planning, judgment, organization and problem solving. Higher-order thinking, she said, makes humans different from other animals. When stress shuts down this part of the brain, humans are then forced into survival mode.
Further, this shutdown narrows down our options to regulate ourselves. It turns the human brain into a reptilian one and activates the sympathetic nervous system, putting us into fight-or-flight mode — the same one we can get stuck in when we’re chronically lonely. Dr. Fratantoni said one way to turn the prefrontal cortex back on to a less stressed mode, one that can think more clearly, is through curiosity. When I asked if altruism could be a way, she said it’s possible because there are a lot of similarities between kindness and curiosity. Both, she said, are an “open posture.” While kindness is hard to access in the immediate aftermath of stress, just as it can be when someone is chronically lonely, it could be a shortcut to bringing the prefrontal cortex back online.
What do we know about what happens in the brain during an act of altruism? In 2006, neuroscientist Jorge Moll and colleagues provided some of the first evidence to demonstrate what happens in the human brain when a person gives selflessly to another person. In their experiment, the researchers scanned participants’ brains using a functional MRI as participants made decisions about whether to donate money to a charity, oppose donating to a charity, or receive the monetary reward themselves. As they scanned the brains of participants while making decisions, researchers found that those who chose to keep the monetary reward for themselves experienced activity in the mesolimbic reward system, including the ventral tegmental area and the ventral striatum.
The mesolimbic reward system, sometimes referred to as the reward pathway or the mesolimbic pathway, is responsible for releasing dopamine, a neurotransmitter that allows us to feel pleasure and satisfaction. It also plays a role in motivating us to want more, like food and sex. This reward pathway regulates motivation, reinforces learning, and activates incentive salience, which is a cognitive process that makes us experience “desire” or “want.”
Its job is to motivate us to repeat behaviors that are needed to survive. Notably, this reward pathway also plays a significant role in the neurobiology of addiction. The findings in Moll’s study did not come as a surprise. Of course, receiving the monetary award felt good and activated the desire to want more. However, when scanning the brains of those who gave the money to charity, scientists saw that these people experienced even more activity in this reward pathway. This finding suggested that giving to other people could provide more pleasure — per the brain’s reward system — than doing something that feels good for oneself. Notably, donating the money also activated the subgenual area of the brain, a circuit of the brain that scientists know is rich in serotonin and plays a role in social bonding, which was not activated when the study’s participants chose to keep the money for themselves.
