Although neuroscience is my daily routine, I’m still in awe every time I hold a human brain. After you take into account its substantial weight (an adult brain weighs in at 3lb), its strange consistency (like firm jelly), and its wrinkled appearance (deep valleys carving a puffy landscape), what’s striking is the brain’s sheer physicality: this hunk of unremarkable stuff seems so at odds with the mental processes it creates.

Our thoughts and our dreams, our memories and experiences all arise from this strange neural material. Who we are is found within its intricate firing patterns of electrochemical pulses. When that activity stops, so do you. When that activity changes character, due to injury or drugs, you change character in step. Unlike any other part of your body, if you damage a small piece of the brain, who you are is likely to change radically.

What does your brain need to function normally? Beyond the nutrients from the food you eat, beyond the oxygen you breathe, beyond the water you drink, there’s something else, something equally as important: it needs other people. Normal brain function depends on the social web around us. Our neurons require other people’s neurons to thrive and survive.

Brains have traditionally been studied in isolation but that approach overlooks the fact that an enormous amount of brain circuitry has to do with other brains. We are deeply social creatures. From our families, friends, co-workers and business partners, our societies are built on layers of complex social interactions. All around us we see relationships forming and breaking, familial bonds and the compulsive building of alliances.

All this social glue is generated by specific circuitry in the brain: sprawling networks that monitor other people, communicate with them, feel their pain, judge their intentions, and read their emotions. Our social skills are deeply rooted in our neural circuitry — and understanding this circuitry is the basis of a young field of study called social neuroscience.

Consider how different the following items are: bunnies, trains, monsters, airplanes and children’s toys. As different as they are, these can all be the main characters in popular animated films and we have no difficulty in assigning intentions to them. A viewer’s brain needs very few hints to take on the assumption that these characters are like us.

Podcast The past and future of the human brain http://podcast.ft.com/p/2987 Neuroscientist David Eagleman argues that the brain is like a field of battle: subject to conflicting drives and impulses that we are only just beginning to understand. He talks to Sally Davies, FT Weekend’s digital editor, about the nature of consciousness, why human beings are hardwired for xenophobia, and how technology can extend our cognitive powers.

This inclination to assign intention to non-human characters was highlighted in a short film made in 1944 by Austrian psychologist Fritz Heider and his American colleague Marianne Simmel. Two simple shapes — a triangle and a circle — come together and spin around one another. After a moment, a larger triangle comes lurking in. It bumps up against and pushes the smaller triangle. The circle slowly sneaks back into a rectangular structure and closes it behind; meanwhile, the large triangle chases the smaller triangle away. The large triangle then comes menacingly to the door of the structure, pries it open and comes in after the circle, which frenetically (and unsuccessfully) looks for other ways to escape. Just when the situation looks its darkest, the little triangle returns. It pulls open the door and the circle dashes out. Together they shut the door, trapping the large triangle inside. Penned in, the large triangle smashes against the walls. Outside, the little triangle and the circle spin around one another.

When people watched this short film and were asked to describe what they saw, you might expect that they described simple shapes moving around. After all, it’s just a circle and two triangles changing co-ordinates. But that’s not what the viewers reported. They described a love story, a fight, a chase, a victory. Heider and Simmel used this animation to demonstrate how readily we perceive social intention all around us.

From time immemorial, people have watched the flights of birds, the movement of stars, the swaying of trees, and invented stories about them, interpreting them as having intention. This kind of storytelling is not just a quirk; it’s an important clue into brain circuitry. It unmasks the degree to which our brains are primed for social interaction. After all, our survival depends on quick assessments of who is friend and who is foe.

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David Eagleman

Our brains make social judgments constantly. But do we learn this skill or are we born with it? To find out, one can investigate whether babies have it. Reproducing an experiment from psychologists Kiley Hamlin, Karen Wynn and Paul Bloom at Yale University, I invited babies, one at a time, to a puppet show. These babies were less than a year old, just beginning to explore the world around them. They were positioned on their mothers’ laps to watch the show.

When the curtain parts a duck struggles to open a box with toys in it. The duck grasps at the lid but just can’t get a good grip. Two bears, wearing two different coloured shirts, watch. After a few moments, one of the bears helps the duck, working with him to pry the lid open. They hug momentarily, and then the lid closes again. Now the duck tries to get the lid open again. The other bear, watching, throws his weight on to the lid, preventing the duck from succeeding. That’s the whole show. In a short, wordless plot, one bear has been helpful to the duck, and the other bear has been mean.

Even before they can walk or talk, babies have the tools to make judgments about others. The brain comes with inborn instincts to detect who’s trustworthy, and who isn’t

When the curtain fell, I took both bears and carried them over to each watching baby. I held them up, indicating to the child to choose one of them to play with. Remarkably, as was found by the Yale researchers, almost all the babies chose the bear that was kind.

These babies can’t walk or talk but they already have the tools to make judgments about others. It’s often assumed that trustworthiness is something we learn to assess, based on years of experience. But simple experiments like these demonstrate that, even as babies, we come equipped with social antennae for feeling our way through the world. The brain comes with inborn instincts to detect who’s trustworthy, and who isn’t.

As we grow, our social challenges become more subtle and complex. Beyond words and actions, we have to interpret inflection, facial expressions, body language. While we are consciously concentrating on what we are discussing, our brain machinery is busy processing complex information. These operations are so instinctive, they’re essentially invisible. But in every moment of our lives, our brain circuitry is decoding the emotions of others based on extremely subtle facial cues.

To better understand how we read faces so automatically, I invited a group of people to my lab. We placed two electrodes on their faces to measure small changes in their expressions. Then we had them look at photographs of faces. When participants looked at a photo that showed, say, a smile, or a frown, we were able to measure short periods of electrical activity that indicated their own facial muscles were moving, often very subtly. This is because of something called mirroring: they were automatically using their own facial muscles to copy the expressions they were seeing.

Couples married for a long time begin to resemble each other. The longer they’ve been married, the stronger the effect

This mirroring sheds light on a strange fact: couples who are married for a long time begin to resemble each other, and the longer they’ve been married, the stronger the effect. Research suggests this is not simply because they adopt the same clothes or hairstyles but because they’ve been mirroring each other’s faces for so many years that their patterns of wrinkles start to look the same.

Why do we mirror? Does it serve a purpose? To find out, I invited a second group of people to the lab — all of whom had been exposed to the most lethal toxin on the planet. This is the Botulinum toxin, derived from a bacterium, and it’s commonly marketed under the brand name Botox. When injected into facial muscles, it paralyses them and thereby reduces wrinkling.

However, there’s a less known side effect of Botox. We showed Botox users the same set of photos. Their facial muscles showed less mirroring on our electromyogram. No surprise there — their muscles have been purposely weakened. The surprise was something else, originally reported in 2011 by the psychologists David Neal and Tanya Chartrand. Similar to their original experiment, I asked participants from both groups (Botox and non-Botox) to look at expressive faces and to choose which of four words best described the emotion shown.

On average, those with Botox were worse at identifying the emotions in the pictures correctly. Why? One hypothesis suggests that the lack of feedback from their facial muscles impaired their ability to read other people. We all know that the less mobile faces of Botox users can make it hard to tell what they’re feeling. The surprise is that those same frozen muscles can make it hard for them to read others.

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We’re all familiar with the concept of survival of the fittest: the individual who can outfight, outrun, or outmate other members of its species. But that model leaves some aspects of our behaviour difficult to explain. Consider altruism: selection of the strongest doesn’t seem to cover it, so theorists introduced the additional idea of “kin selection”. This means that I care not only about myself but also about others with whom I share genetic material.

Our drive to come together into groups yields a survival advantage — but it has a dark side. For every in-group, there must exist at least one outgroup

However, people still co-operate irrespective of kinship. That observation leads to the idea of “group selection”. If a group is composed entirely of people who co-operate, everyone in the group will be better off for it. On average, you’ll fare better than other people who aren’t very co-operative with their neighbours. Together, the members of a group can help each other to survive.

Although humans are competitive and individualistic much of the time, we spend quite a bit of our lives co-operating for the good of the group. This has allowed human populations to thrive across the planet and to build civilisations — feats that individuals, no matter how fit, could never pull off in isolation.

So our drive to come together into groups yields a survival advantage — but it has a dark side. For every in-group, there must exist at least one outgroup.

A row of human skulls from the genocide in Rwanda

This understanding is critical to understanding our history. All across the globe, groups of people repeatedly inflict violence on other groups, even those that pose no direct threat. The year 1915 saw the systematic killing of more than a million Armenians by the Ottoman Turks. In 1994, over a period of 100 days, the Hutus in Rwanda killed 800,000 Tutsis, mostly with machetes.

After the Holocaust, Europe got into the habit of vowing “never again”. But between 1992 and 1995, during the Yugoslav war, more than 100,000 Muslims were slaughtered by Serbians. One of the worst events of the war was in Srebrenica in July 1995 when, over the course of 10 days, 8,000 Bosnian Muslims — known as Bosniaks — were shot and killed. They had taken refuge inside a United Nations compound after the town was surrounded by siege forces. But on July 11, the UN commanders expelled the refugees from the compound, delivering them into the hands of their enemies. Women were raped, men were executed, and children were killed.

After the war, in Sarajevo, I met a tall, middle-aged man called Hasan Nuhanović, a Bosnian Muslim who had been working at the compound as a UN translator. His family were among the refugees but they had been sent out to die. Only he had been allowed to stay because of his value as a translator. His mother, father, and brother were killed that day. What haunts him the most, he told me, is this: “[That] the continuation of the killings, of torture, was perpetrated by our neighbours — the very people we had been living with for decades. They were capable of killing their own school friends.” There are, he added, “universal values and these values are very basic: ‘Don’t kill.’ In April 1992, this ‘don’t kill’ suddenly disappeared — and it became ‘go and kill.’”

It would normally feel unconscionable to murder your neighbour. So what suddenly allows hundreds or thousands of people to do exactly that?

Traditionally we examine warfare and killings in the context of history, economics, politics; but for a complete picture, we need also to understand this as a neural phenomenon. It would normally feel unconscionable to murder your neighbour. So what suddenly allows hundreds or thousands of people to do exactly that? What is it about certain situations that short-circuits the normal social functioning of the brain? To understand something like violence or genocide, we need to drill down one step further, to dehumanisation.

Lasana Harris of the University of Leiden in Holland has conducted a series of experiments that move closer to understanding how this happens. Harris is looking for changes in the brain’s social network, in particular the medial prefrontal cortex (mPFC). This region becomes active when we’re interacting with, or thinking about, other people — but it’s not active when we’re dealing with inanimate objects, such as a coffee mug.

Harris showed volunteers photographs of people from different social groups, for example, homeless people, or drug addicts. And he found that the mPFC was less active when they looked at a homeless person. It’s as though the person was more like an object.

As Harris puts it, by shutting down the systems that see the homeless person as a fellow human, one doesn’t have to experience the unpleasant pressures of feeling bad about not giving money. In other words, the homeless have become dehumanised: the brain is viewing them more like objects and less like people. As Harris explains: “If you don’t properly diagnose people as human beings, then the moral rules that are reserved for human people may not apply.”

Genocide is only possible when dehumanisation happens on a massive scale. The perfect tool for this is propaganda. It keys right into the neural networks that understand other people, and dials down the degree to which we empathise with them. We’ve seen how our brains can be manipulated by political agendas to dehumanise other people, but is it possible to programme our brains to prevent this?

Education can play a key role in preventing genocide. If children are taught that systems of rules are arbitrary; that the truths of the world aren’t fixed, and moreover are not necessarily truths, this gives them the power to see through political agendas and form their own opinions.

In this age of digital hyperlinking, it’s more important than ever to understand the links between humans. You might assume that you end at the border of your skin but there’s a sense in which there’s no way to mark the end of you and the beginning of all those around you. Your neurons and those of everyone on the planet interplay in a giant, shifting superorganism. What we demarcate as you is simply a network in a larger network. If we want a bright future for our species, we’ll want to continue to research how human brains interact — the dangers as well as the opportunities. Because there’s no avoiding the truth etched into the wiring of our brains: we need each other.

David Eagleman is a neuroscientist and writer at Baylor College of Medicine, where he directs the Laboratory for Perception and Action. This is an edited extract from his book ‘The Brain: The Story of You’, published on November 5 by Canongate, £20. For information on Eagleman’s UK events, visit bit.ly/davideagleman

Stitched illustration by Evelin Kasikov

Letter in response to this article:

Do we mirror our dogs? / From Burgess Davis