Monkeys sharing hard-wired brain

We have just finished cleaning up after hosting a fund-raising affair for an organization, called MedShare. It is quite unique. Unlike many other well-meaning organizations who donate “stuff” or whatever they think is needed, and this includes the $$$ billions of USAID money, MedShare finds out what hospitals and clinics in underserved areas of the world (including the US) need, collects the equipment from surpluses donated by hospitals, puts it in crates, and ships it to its various destinations. Thousands of volunteers in the San Francisco Bay Area and in Atlanta sort out the equipment, ranging from surgical gloves and sutures to x-ray, MRI machines, ventilators -you name it, all in prime working condition (some of it barely a year old). Stuff that would have been consigned to a municipal landfill is fulfilling its purpose: saving lives.

As I MedSharewas listening to the presentations and marveled at the wonderful work of MedShare, my thoughts wandered to the people attending the function. They all seemed genuinely happy to be here and “share the wealth.” There wasn’t the social recognition reward (donations were made quietly by check or credit card, no naming names of exceptionally generous givers, no pressure whatsoever, no exhortation “to do good.” What motivated these people? For that matter – what motivates the millions of people who share with others in need? Is it wealth? Not even close. Studies have shown that people of modest means, even people who are outright poor, share proportionately more generously than wealthy people. If you ask people why they share, almost invariably you get variations of a theme of “it makes me feel good.” Sharing is a close cousin of cooperation, which Darwin identified as the most powerful strategy of winning the natural selection game. If so, it must be deeply embedded in us, in our genes, our brains, our psyche. How can we find out?


Monkeys Playing Games

Recent studies, summarized in Nature, start shedding a light on the mysteries of our behavior, specifically on sharing. Neurobiologists Keren Haroush and Ziv Williams of Harvard Medical School in Boston, Massachusetts sat two monkeys at computer screens, eyeing one another as they wait for a promised reward: apple juice. Each had a choice — it can either select a symbol that results in juice being shared equally or pick one that delivers most of the juice to itself. But being selfish is risky. If its partner also chooses not to share, neither gets much juice. And if one chooses to share and the other to horde, the sharing partner is going to learn a lesson and in the next round will choose to horde as well, resulting in neither of them getting juice. Quite a dilemma; actually named ‘the prisoner’s dilemma’ in game theory.

anterior cingulate cortex
anterior cingulate cortex

Haroush and Ziv didn’t just observe the monkeys’ actions. They zoomed in on neural circuits in rhesus macaques by implanting electrode arrays into a brain area called the dorsal anterior cingulate cortex (dACC), which is associated with rational cognitive functions, such as reward anticipation, decision-making, empathy, impulse control, and emotion. The arrays recorded the activity of hundreds of individual neurons. When the monkeys played the prisoner’s dilemma against a computer program, they rarely chose to cooperate. But when they played with another monkey that they could see, they were several times more likely to choose to share the juice.

As the monkey made its decision, a specific set of dACC neurons tended to fire if the animal was choosing to cooperate, allowing the researchers to predict its decision two-thirds of the time. A different set often fired if the monkey thought its partner would cooperate, and the predictions were 80% accurate.

The researchers also found that when they interfered with the cooperation circuit by delivering an electric shock to the area, the monkeys became less likely to co­operate — suggesting that the circuits for social inter­action normally override an inherent desire to self-indulge!


What About Oxytocin, The “Love” Hormone?

Oxytocin is a neurohormone that is believed to aid the formation of social bonds. Oxytocin levels shoot sky-high when we fall in love, and when a mother delivers a new baby. So, does the hormone affect the brain circuits that are responsible for sharing? Steve Chang, a psychologist at Yale University in New Haven, Connecticut, nd his colleagues are now studying how oxytocin, which has been shown to increase monkeys’ willingness to reward, affects neural circuitry. Roughly 2 hours after inhaling oxytocin, monkeys increased the frequency of prosocial choices associated with reward to another monkey when the alternative was to reward no one. Oxytocin also increased attention to the recipient monkey as well as the time it took to render such a decision.

The question that should come up when we read about animal studies purporting to model human behavior is: do they? The good news is that not only our genes and our brains are very similar, but our behavior is not that much different. Basically, we are just more complex apes; and even that is being questioned – the more we learn about ape behavior, the more impressive their complexity becomes, and the more human-like.

Take for instance this study. Rilling et al showed that men playing the prisoner’s dilemma are more likely to co­operate if they have breathed in oxytocin, just like Chang’s monkeys.

All of which may explain why people dig into their wallets and feel good while they are doing it. Try it sometime. It will make you feel good as well.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. I have heard about people of modest means giving more than wealthier people. What I’m curious about now, reading your neurological explanation for the “cooperation circuit”, is whether that circuit is somehow less developed or oxytocin production is less in wealthier people? Obviously it makes sense that if you give less away you keep more of it, so there’s no mystery about wealthy people giving less. I just wonder if there’s a converse explanation for example, for Prisoner’s Dilemma players who choose not to cooperate.

    • Good question. It doesn’t make sense that the “cooperation circuits” somehow degenerate when a person become wealthy. Conversely, you cannot explain wealth by “un-sharing” behavior. The study showing less sharing among the wealthy attributed this behavior to a sense of entitlement, and to belief that people’s poverty is their own fault. A related observation: wealthy people are much more liable to break the law, such as obeying a stop sign at an intersection, because of belief that money can easily solve problems with the law.
      So what explains generous giving by wealthy people, such as the Rockefellers, the Kennedys, the Gates family, Warren Buffet and his sons? I think it is upbringing, family tradition of giving, and education. So genes and neurons are not destiny. Education can modify, and even override them.


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