In Tom Stoppard’s brilliant play, The Hard Problem, that explores the essence of consciousness. Hilary, a young psychology student, is on one side of the mind vs. brain argument. She is an upholder of the idea of absolute morality and a believer in maternal love as something other than a selfish gene. At the other pole is her tutor-lover, Spike, a confirmed materialist and ardent believer in hard science.

In an argument about human behavior, Spike challenges her with a favorite exercise in game theory, something called “the prisoner’s dilemma“. In his version of the exercise, Hilary and her professor are accused of manipulating data. She could “get out of prison free” if she tells the authorities that it was the professor who did it without her knowledge.

Game theory predicts that she will indeed “throw her professor under the bus,” especially since she really hadn’t done anything wrong. Wouldn’t any of us do the same? Hilary baffles Spike, however, when she says no, she would never betray the professor, indeed, she would rather go to prison. Why? Because he is her friend! No amount of mathematical equations and probability calculations can factor in this confounding factor: friendship.

Photo of friends - children with arms around each other


What is friendship?

That’s a tough question. One possible answer appears in a recently published paper, The Neuroethology of Friendship:”

[Friends are] pairs of individuals that engage in bidirectional affiliative (nonaggressive, nonreproductive) interactions with such frequency and consistency so as to differentiate them from nonfriends. [something only a scientist could write!]

But wait: is friendship uniquely human? At first sight, friendship seems like a human construct outside the realm of biology. But recent studies in animal behavior have described, in great detail, social bonding across the biological world. It gets really interesting when several studies showed that social bonding, and friendship, its human equivalent, has a marked survival value.


How did it all begin?

Baboons live longer when they form social bonds

Shultz et al. modeled the trajectory of primate social systems and found that stable groups composed of multiple adult males and females arose from solitary life, with harems and pair-bonded groups arising afterward.

Although pair-bonding is rare in primates, such biologically diverse species as dolphins, elephants, the Spotted Hyena, and even the lowly ass show social intelligence and form social bonds. Of the social bonds, mother-daughter pairs are the most common, followed by siblings.

It is obvious that social bonding has deep evolutionary roots. It must, therefore, have a selective advantage. Indeed, formation of social bonding in baboons is associated with a 50% increase in longevity. The opposite of social bonding is social isolation, a living situation that has dire consequences in the elderly.


It’s all hard-wired

Obviously, if evolutionary pressure favors social bonding in so many species, then genes, the substrate on which natural selection exerts pressure, must be involved. Indeed, a recent fascinating paper in Science documents the genomic signatures of transition from solitary to social living in several bee species.

If genes dictate the behavioral trait of friendship, then the brain must be the vehicle that controls friendship. In fact, there is evidence that brain size is correlated with social complexity across species. Makes a lot of sense. The more complex the social structure, the more there is to follow, analyze, and react. Animals (and humans) also have to keep track of the quality of the relationships between pairs of other individuals (friend of friends, friends of foes?) in order to navigate their social world. It’s enough to give one a royal headache and, maybe, a bigger brain.

The prefrontal cortex, where friendship happens.
The prefrontal cortex, where friendship happens.

So where in the brain is the “friendship zone”? There are several areas that are activated by social information. There are those that are generally reward-related and not specific to social information processing, such as the anterior cingulate cortex, the nucleus accumbens, and the caudate nucleus. But there is one area of the brain that is at least partially specialized for social information, the orbitofrontal cortex (OFC). This is the area right behind your eyes. There is a relationship between OFC size and social network size in humans, and also between group size and neocortex volume across primate species.

We not only constantly accumulate socially-relevant information, we also have continually make value judgments on what pieces of information is more relevant than others. There is evidence that our cousins, the monkeys, also make such judgments. For example, male rhesus macaques will forgo a small amount of juice reward in order to see a picture of another monkey. But it will take a magnum-size orange juice to entice them to take their eyes off a picture of a female perineum. How human! Or rather, how monkey-like we are?


Reinforcing the social neural networks

Evolution deemed sociality so advantageous that it reinforced the social neural networks with stimulation with oxytocin, the love hormone, as well as the less specific neurotransmitters, dopamine, and serotonin. Is it any wonder that when we get together with good friends we feel cozy and fuzzy? And with friends around, problems seem less alarming. Many animals, including humans, exhibit smaller increase in cortisol, the stress hormone, during exposure to adverse stimuli when a friend is present compared to when alone.

As you may have noticed, we did not mention cooperation, in the same breath as friendship. The two are intertwined but are not synonymous. From our human experience, we know that we cooperate at work, in a sports team, or in response to disaster. But friendship goes beyond that. It has this extra quality. I almost said this human quality, but now we know we are not unique even in that.

On the other hand, the realization that animals share this quality with us, and the same hormones and neural circuits are present in all of us animals, may help to advance our understanding of what makes us human.

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. What an interesting topic…
    A recent episode of the NPR program, Radio Lab, focused on an incident in which four divers saved a whale that had become ensnared in crab-trap ropes by cutting away the ropes. The freed whale swam away, but then, according to the divers, returned and swam near each diver, putting her eye close to the head of each diver. The program focused on whether the whale was expressing gratitude or not. The experts deconstructing the incident disagreed about a lot of things but, in general, agreed that people frequently “anthropomorphize” animal behavior. We want to see ourselves in cute animals. As a huge dog lover, I know that dogs are naturally social with each other and affectionate with their owners. But I think it is an open question whether a dog (or a whale) interprets affection or forms friendships as people do. This is not to say they are incapable of love or anything like that, only that evolution has taught them different lessons and programmed them to interpret and respond to stimuli differently from us.

    • I used to dismiss such anecdotes as ‘anthropomorphizing’, being an incurable sceptic as I am wont to be. But on deeper examination I realized that this was just a lazy response to lack of knowledge. It is easier to make an assertion that to run an experimant. Recently however, evidence that hormonal and neurological events occur in the brain when we interact with our dogs is beginning to undermine this belief. See my post of a couple of weeks ago. I recently read a wonderful book (non fiction) called the “the elephant whisperer”. I think a documentary was made of the same story. If you are interested in WWII history, read the book “Elephant Company”. It’s about the “forgotten war’ against the Japanese in Burma. Amazing story. In both books the authors recount an undeniable bond between elephants and humans.


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