Chris Rees was fifteen when he first broke the law. Growing up in an impoverished high-rise in north London, Rees joined a gang and quickly fell into a life of petty crime. He’s been in and out of prison (a place he says only encourages criminal behaviour) and has now been diagnosed with anti-social personality disorder (ASPD), a mental health condition that causes a person repeatedly to engage in reckless, impulsive and often violent behaviour.
‘Being social wasn’t an option,’ Chris, now twenty-four, told me during our long conversation in London’s Regent’s Park, a place he said makes him feel calm and at ease with the world. ‘I saw everyone as a threat, but also as a kind of opportunity. I was always just thinking about myself. I couldn’t relate to other human beings.’
Chris is intelligent and charming. His worst crimes, however, include knifepoint muggings and assaulting his brother. Like other personality disorders, ASPD exists on a spectrum, with minor offences at one extreme and sociopaths at the other. Chris is somewhere in the middle. He sees a counsellor once a week and is receiving psychological therapy.
I had first heard of Chris a year earlier through a research colleague. I’d asked if she knew anyone whose brain condition could shed light on the evolution of social minds. The field was still very much in its infancy, so I wasn’t entirely sure what I was looking for. But when I called Chris to arrange our meeting, I knew he could provide insight. He said he was tired of being perceived as a criminal and wanted a deeper understanding of his condition.
‘Talking to someone definitely helps,’ he said, as we strolled through the park. ‘They tried a bunch of random medications, but they just put me to sleep. You can’t be social when you’re a zombie. The social part of my brain, which I guess is malfunctioning or just hasn’t really developed, needs people, not pills.’
The causes of ASPD are partly social themselves: inequality, poverty, lack of social mobility, time spent in dysfunctional youth prisons, addict parents – all are strongly linked to ASPD. But research is also identifying genetic risk factors for the condition, often in genes linked to aggression and impulsivity. Variations in the gene MAOA (monoamine oxidase A), nicknamed the ‘warrior gene’ because it’s thought to drive aggression, have been found to contribute to ASPD in maltreated children.1 Other candidate genes including COL25A1 (collagen type 25 alpha 1 chain), CDH13 (cadherin 13) and LINC00951 (long intergenic non-protein coding RNA 951) have been linked to ASPD, though their mechanism of action remains unknown.2 Chris considered having his genome sequenced to learn if he possesses these genes, but said he isn’t ready for a psychological battle with genetic determinism.
I know genes aren’t destiny – at least, that’s what I read – but I still feel like knowing that kind of information would set me back in my struggle with this thing. I want to get better. I need to get better. And I think understanding my brain and how I got here will get me there faster than just saying, ‘Oh well, you’ve got these bad genes, but we have no idea what they do.’
In the brain, ASPD wreaks havoc on the ventromedial prefrontal cortex (vmPFC), which controls social cognition and the perception of threats. While little is known about how the vmPFC performs these tasks, we know that neural networks within this region are essential, and acutely sensitive to change. Damage one neural network in the vmPFC and a person may never be able to learn from their mistakes; damage another and a person may be easily misled and unable to make their own decisions (something Chris said his old gang exploited ruthlessly in him). As part of the cortex, the brain region that expanded to occupy 80 per cent of the human brain (compared to only 28 per cent in rats), the vmPFC represented a critical evolutionary leap. The vmPFC is very ancient, and probably evolved in apes more than 15 million years ago.
When I told Chris that I was writing a book about brain evolution and that I wanted to understand what his condition tells us about our social brains, his response was astonishing:
We hear about evolution a lot – it was definitely drummed into my head at school – but we still don’t think of humans as a messy, imperfect result of it. We still think we’re above it. Separate from it. Well, we’re not, and I’m sorry if my brain doesn’t work the way it should, but nature’s been changing human brains for how long?
‘Seven million years,’ I said.
‘Jesus. Seven million years. How are any of us supposed to be “normal” after that?’
As unfortunate as Chris’s situation is, his condition provides testimony to this ancient brain history. For we are all only as social as evolution has allowed us to be. One of the greatest accomplishments of our evolving minds is their ability to connect with other conscious beings. Be it through family or friendship, bonding at work, team sports or religious affiliation, Homo sapiens are known for manifesting intense social relationships not seen in any other primate including our early-human predecessors. Only humans form complex communities in which an outsider is welcomed, given food and shelter, and allowed to integrate for the benefit of the entire group. Only humans will allow someone completely unrelated to hold their new-born babies. Only humans organise themselves into networks capable of such staggering feats as sequencing their own genome and building the Large Hadron Collider. Quite a difference, given that we share 98 per cent of our DNA with apes who would fight to the death to avoid carrying a log together.
To understand how our social brains came about, we must again travel back to East Africa more than 4 million years ago.
Our human forebear Ardipithecus holds many of the clues to understanding our social brain.3 Standing four feet tall, with a strikingly large toe to grasp branches and long curving fingers to climb trees, she was a forest-dwelling primate who developed social thinking primarily to avoid predators and gather food. She preferred wooded or forested habitats, where she lived alongside leaf-eating monkeys, parrots and peacocks. Fossils from Aramis in the Middle Awash region of Ethiopia suggest that she lived in groups at least thirty strong. Fossils of her teeth suggest she was a fruit eater, rather than a plant and leaf eater, and studies show that apes tend to forage for fruit as a cohesive social unit, relying on each other to find fruit in the sporadically distributed fruit-bearing trees, mark its location and remember when it will ripen.
The logic of social harmony is simple enough: if a band of hunter-gatherers (at first a single family which then expanded into a group of several families) works together to outwit predators, everybody eats. The loner who eschews this strategy isn’t going to last long, presumably taking his anti-social genes with him. By working together towards the same goal – what biologists call mutualistic cooperation – humans were able to rise above their selfish impulses and in turn create new methods for hunting, fishing, foraging and threat avoidance.
These practices were then passed to future generations using basic gesturing. A member of the group would point to the treetop harbouring a predator, or to a lone gazelle sheltering in the forest glade. Only humans, domestic dogs and, perhaps, elephants understand the unspoken signal of pointing; chimps do not. And though pointing may sound primitive, this type of gesture laid the foundation for abstract thinking: it signified the ability to think about something other than one’s self. Where once humans thought solely in terms of individualistic needs and desires – a ruthless dog-eat-dog struggle for survival – now they understood the power of good collaboration based on the concept of shared intentionality. Where once humans understood only themselves, now they understood the power of the collective.
Before long an entire culture based on these instincts was born, and social minds eventually mushroomed into armies, religions, governments, free markets and a worldwide matrix of social media. Needless to say, to study something so entwined with the social norms and cultural conventions of all of human history is an enormous challenge. Fortunately, humans alive today provide excellent clues. In addition to stories of patients such as Chris Rees, we can use advanced neuroimaging techniques to observe brain activity during a particular task. The neuroimaging I’m most familiar with is functional magnetic resonance imaging (fMRI). Based on conventional MRI, which uses a strong magnetic field to create detailed images of the body, fMRI goes a step further by looking at blood-flow changes in the brain. Such changes then tell us – or at least strongly indicate – which areas of the brain are most active in response to different stimuli, because blood flow and brain cell activity are intimately linked. Today, fMRI is used to investigate vital functions such as speech, attention, vision, touch, memory, pain and emotion. When I was doing research into Alzheimer’s disease, I often used fMRI data to see how the brain stores and retrieves memory, the results usually showing the brain’s hippocampus and cortex lighting up like a Christmas tree. Like all neuroscience techniques, fMRI has its limitations: the resolution is low and it can only be used on a single person remaining perfectly still. But it is still the only technique that offers a tantalising glimpse into the human brain, and is the closest we can come to watching the brain at work.
In the context of the social brain, fMRI studies show that our ability to connect with others depends on a network of ‘neural modules’, a constellation of circuits that links the amygdala to the parietal and prefrontal cortex. There is no single site controlling social interaction; rather, each module is activated depending on the social activity we engage in. For example, modules in the prefrontal cortex are activated when we have a conversation with someone about something we are both familiar with; modules in the amygdala are activated when we are pondering whether we like someone; and modules in the parietal lobe are activated when we simply observe someone’s body language. As Matthew Lieberman observes in his remarkable book Social: Why Our Brains Are Wired to Connect,
Just as there are multiple social networks on the internet such as Facebook and Twitter, each with its own strengths, there are also multiple social networks in our brains, sets of brain regions that work together to promote our social wellbeing. These networks each have their own strengths, and they have emerged at different points in our evolutionary history moving from vertebrates to mammals to primates to us, Homo sapiens.4
To understand how the first social brain operated, let’s rewind the clock and imagine performing an fMRI on our 4-million-year-old ancestor Ardipithecus. Granted, she’d likely be somewhat displeased by the prospect and, despite being gentler than a chimpanzee, would probably try to kill her Homo sapiens experimenters. But let’s pretend our Ardipithecus ancestor plays along. Ardipithecus had a small brain (about 400–550 cm3), similar to that of a gorilla and about a third of the size of Homo sapiens, but one that nonetheless contained sophisticated social brain circuitry. She possessed a cerebral cortex, albeit a shrunken version of ours, which probably stored microcircuits of neurons with a similar basic structure to our own. Our fMRI scan might, therefore, detect a flash of activity in Ardipithecus’s frontal cortex during an activity requiring social cognition, such as seeing pictures of her kin. She also possessed an amygdala, which would have strengthened her social ties by imbuing them with emotions such as love, happiness and fear. This region of Ardipithecus’s brain would also light up in our fMRI machine in response to the sound of her offspring in distress. But the connections between Ardipithecus’s social brain networks were not as advanced as ours, making each flash of brain activity a relatively isolated event.
One way to understand how these social networks evolved is to think about the earth’s natural history. When continents drift across the ocean bed they produce a constellation of landmasses, each unique and each containing enough geological evidence to reveal how they moved over time. So it is with the social circuitry of the brain. Here, however, landmasses can be represented by the neural circuitry for a variety of social behaviours, which neuroscientists can then use as building blocks to understand how human social circuits evolved over time.
To bring this to life, consider the cetaceans, a group made up of dolphins, whales and porpoises which evolved from land-living mammals that re-entered the ocean over 50 million years ago. They have astonishingly rich social lives – hunting together, babysitting one another’s young and cooperating with other species (famously helping Brazilian fisherman with their catches). This appears to be due to the expansion of the cetaceans’ brains, an evolutionary process known as encephalisation. This suggests that social circuits were once buried deep within the brain, bunched together like the innards of a golf ball. Then, like the Pangaea supercontinent breaking apart, the social brain began to expand, spreading its cellular tentacles into every corner of the brain, establishing new connections to oversee every aspect of social life.
We still don’t know precisely how social genes and the social environment came together to shape the social brain, but research indicates that a person’s genes and their environment interact to influence their social lives.5 No surprises there, you may think. But you would be surprised at how many people still contend that one is more important than the other. To those who pitch their tent on only one side of this debate, I am afraid you have two incontrovertible facts to confront. 1. A person raised by gay parents in San Francisco is going to have a radically different social attitude towards homosexuality than a person raised by members of the Taliban in Afghanistan and Pakistan. Ergo: nurture matters. And 2. Although Homo sapiens crammed together on an airplane can suppress any feelings of irritation and behave altruistically, if replaced by a planeload of chimpanzees, as primatologist Sarah Hrdy has pointed out, ‘Bloody earlobes and other appendages would litter the aisles.’6 Ergo: nature matters as well.
Among the first to recognise our brain’s susceptibility to the social environment was the sociologist Gustav LeBon, who published a book in 1885 called The Crowd: A Study of the Popular Mind. Inspired by Darwin’s zeal and intrepidness, LeBon travelled throughout Europe and Asia to study the different peoples and civilisations of the world, eventually proposing an idea that he called the ‘crowd mind’. He argued that human behaviour is largely guided by external influences and that every person’s mind is merely ‘a grain of sand amid other grains of sand, which the wind stirs up at will.’
Both Hitler and Lenin are known to have read The Crowd. They knew only too well that people could ignore their sense of right and wrong to be included in the tribe; that people often behaved irrationally to fit it. LeBon knew it too. In 1871, at the age of thirty, he witnessed the Paris Commune when, for two months, crowds of far-left extremists set fire to buildings and staged executions by firing squad. They killed more than 15,000 people targeting farmers, workers, priests, the rich and social democrats. The French poet Anatole France, himself a socialist, described it as ‘a government of crime and madness’. The crowd mentality, LeBon came to realise, was a terrifyingly fragile ecosystem.
Among the first to show how genes can influence our social brains was the honeybee (Apis mellifera). Though most types of bee are solitary, honeybees are decidedly social insects – living together in large, complex societies consisting of a single queen, hundreds of male drones and anything from 10,000 to 80,000 female worker bees. They’re so social, in fact, that they’re referred to as ‘eusocial’: an advanced level of social existence in which the animal lives in multigenerational family groups, with one animal (in this case the queen) reproducing, and the others working to care for the next generations. They communicate using elaborate social signals including the waggle dance, round dance and shaking signal, as well as touching antennae (antennating) and exchanging food (trophallaxis). When the honeybee genome was finally sequenced in 2006, Nature ran a cover declaring, ‘A blueprint for sociality’.
By all appearances, honeybees and humans are radically different organisms, yet we share 44 per cent of our DNA with honeybees. And when viewed as a single superorganism, honeybees behave in the same way that neurons in the human brain react to the outside world, obeying psychophysical laws linked to perception and our response to external stimuli. For this reason, researchers are investigating the honeybee in search of shared social genes. One that stands out is an ancient gene called Fushi Tarazu Factor 1 (FTZ-F1), which codes for a nuclear receptor: a molecule that binds DNA to regulate the expression of multiple genes. Another is a set of genes encoding a protein called heat-shock protein 90 (HSP90), which helps stabilise and fold other proteins in the cell. While we don’t yet know how these molecules regulate social behaviour, the overall conclusion is inescapable: our social brains and the social brains of other animals, especially the honeybee, share deep evolutionary roots.
Over time evolution enhanced the connections between the brain’s social circuits, allowing the brain to adapt to whatever social and cultural milieu it found itself in. As more complex social systems emerged in the world, the brain developed neural circuitry in the temporoparietal neocortex: a region crucial for understanding people’s intentions, beliefs and personality traits. A million years later, this circuitry gave rise to empathy and the realisation that others may have different perspectives from our own.
Darwin, for his part, was fully persuaded that empathy lay at the heart of human sociability. In another of his lesser-known works, The Descent of Man, a brilliant and trailblazing piece of evolutionary psychology published in 1871, eleven years before his death, he did a U-turn on the notion that humans are opportunistic, competitive and ultimately selfish animals. Instead, he declared, we humans are naturally equipped with a compassion and empathy (derived from the German Einfühlung, ‘feeling into’) that often values the welfare of others – including nonhumans – more than our own survival and reproduction.7 Why else would humans engage in the biologically senseless act of altruism, doing things for others with no guarantee of reciprocity?
The primatologist Frans de Waal believes that primates are uniquely predisposed to empathy. He has shown that when two monkeys are placed side by side and one monkey is offered one of two tokens – one that rewards only itself with a slice of apple, the other that rewards its partner also – the monkey nearly always opts for the pro-social option, even when paired with a monkey he or she has never come across.8 De Waal adds that fear doesn’t come into the equation because dominant monkeys are in fact the most charitable.
Empathy is hard to spot in the brain. Just like the neural hardware governing social interaction, the brain has not evolved with a specific empathy centre or empathy neurons. What appears to have evolved instead is a distinct network of cells encompassing the temporoparietal junction (which lets us think about others), the dorsolateral prefrontal cortex (which lets us think about others’ wellbeing), and the orbital and ventromedial cortex (which lets us determine an appropriate response). In humans, the ability to intuit what another person is thinking and feeling develops early, at a point that psychologists call the ninth-month revolution. From then on infants will look where their parents point and follow their gaze, offering the first signs of an emotional and social connection. Avoiding eye contact at this stage can be a symptom of autism: a different kind of mind characterised by differences in social interaction. The National Institutes for Health describes autistic children as ‘indifferent to social engagement’, stating that some ‘only interact with others to achieve specific goals.’ Yet as we will see in chapter 8, autism has played an important role in brain evolution and there are fascinating evolutionary reasons for its existence.
The ninth-month revolution is only the beginning. Well into adulthood human minds use culture to understand others’ states of mind. Studies show, for instance, that children living in Australia and the United States understand that people have different opinions on a subject before they understand that some people are simply ignorant about a subject; but for children growing up in China and Iran, it is the other way round.9 In Samoa, where a culture based on Fa’a Samoa (‘the Samoan way’) considers it taboo to question the truthfulness of local beliefs, children understand what false beliefs are five years later than children raised in Europe and North America.10 Japanese children recognise the importance of criticism more than Italian children.11 Fijian children appreciate the value of experience over education more than children living in the West.12 Everywhere one cares to look, cultural practices wire our social brains in astonishingly diverse ways. And these differences appear to depend largely on what a culture does and does not decide to talk about.
These findings suggest that social brains evolved at least twice – first as a way to recognise another’s feelings through empathy, second as a way to share another’s feelings through culture – and that both adaptations were critical to the survival of our species. But while those traits were one source for the evolution of social minds, another was even more surprising.
Rats like to play. Place two in a cage and they will rough and tumble like there’s no tomorrow. Place two differently sized rats in a cage and the same thing happens, only the larger rat will assert its dominance by winning the play fight. Given the opportunity, the large rat will do this repeatedly. But let the experiment run for long enough, as many neuroscientists have, and something interesting happens. If the larger rat doesn’t let the smaller rat win at least 30 per cent of the time, the small rat no longer wants to play. No one enjoys a game they constantly lose. It’s just not fair.
Likewise, when humans play an ultimatum game in which one player decides to distribute £10 between himself and another unevenly – keeping, say, £9 for himself while giving just £1 to the second player – the second player almost always rejects the offer despite being worse off as a result. One might think it’s better to get something instead of nothing, but this doesn’t play out in human interactions. We prize fairness over material gain. The question is: why?
Fair play probably evolved in the social brain to balance power. Vampire bats are a brilliant example. When a hungry bat fails to suck the blood of its prey (usually other mammals but occasionally humans), it will return to its cave and beg one of its fellow bats to share from their own bloody spoils. Remarkably, the request is granted and the fellow bat regurgitates some of its meal of blood for the hungry bat to devour. Because the bats remember which bats have shared blood with them in the past, they reciprocate if the charitable bat one day finds itself struggling for food. The behaviour provides hunger insurance as well as a sense of cam-araderie. Cleaner fish and their hosts (or ‘clients’) are another example of this. In exchange for chasing off possible dangers to the cleaner fish (including not eating it itself), the client gets its ectoparasites removed, which if left can cause serious injury. Most cleaners, however, prefer to eat the client’s mucus or healthy skin and will do so if they can. They cheat, in other words. The client then has to decide if the cleaner fishes’ services outweigh the occasional act of cheating. Sometimes a cleaner fish will gently rub itself against a client to gain trust and form a memorable bond, signifying that it wants to play fair.
Unsurprisingly, evidence of fair play in humans is harder to find. At the time of writing, the richest eight people own half the world’s wealth. Barack Obama called this ‘the defining challenge of our time’. Yet the hunter-gatherer societies that have survived into the twentieth century suggest that we have evolved the brain hardware for fair play. The Kalarahi bushmen, African pygmies, Andaman islanders, Greenland eskimos, Australian aborigines, Paraguayan Indians and Siberian nomads all behave in an egalitarian manner. Food is shared among the tribe and members are treated equally. And while individual autonomy is encouraged, a strong emphasis is placed on collaboration and humility. When the anthropologist Richard Lee asked the indigenous people of Botswana about a practice they call ‘insulting the meat’, an elder replied,
When a young man kills much meat, he comes to think of himself as a big man, and he thinks of the rest of us as his inferiors. We can’t accept this. We refuse one who boasts, for someday his pride will make him kill somebody. So we always speak of his meat as worthless. In this way we cool his heart and make him gentle.13
Fair play suppresses the desire to dominate. It’s what stops the large rat tearing the small rat to shreds. It’s what keeps the bat roost committed to distributive justice. It’s what stabilises the human conflict between individual and collective thinking.
The reason human brains have evolved a fondness for inequality is because our minds intuitively draw a distinction between unfair equality (all students receiving the same exam grades regardless of merit) and fair inequality (the doctor earning more than the cleaner). When push comes to shove, humans nearly always prefer fair inequality to unfair equality. This is what allowed us to work together in large groups. As the cognitive scientist Mark Sheskin notes:
Wouldn’t you prefer to team up with someone who puts in at least a fair share of the effort and takes at most a fair share of the reward, rather than somebody who is lazy or greedy? Likewise, others will prefer to interact with you if you have a reputation for fairness. Over our evolutionary history, individuals who cooperated fairly outcompeted those who didn’t, and so evolution produced our modern, moral brains, with their focus on fairness.14
What happens in the brain when we engage in fair play may surprise you. It produces a surge of neural activity in our reward centres, a set of brain structures called the striatum, the ventral tegmentum and the ventromedial prefrontal cortex. When the brain is viewed through neuroimaging, these areas light up when money is distributed evenly among participants in an ultimatum game. They light up during friendly behaviour in so-called trust games. They even light up if the winner of a cash prize sees another person win a prize that reduces the value of her prize. In other words, fairness feels good. When the brain’s reward centres are activated by fair play, they release a potent cocktail of chemicals that influence our happiness including dopamine, oxytocin, serotonin and endorphins. Individually, these chemicals offer a slight buzz. But together, they can make us ecstatic.
In his book The Expanding Circle, the philosopher Peter Singer argues that Homo sapiens are gradually widening their circles of compassion to include all people and all sentient life.15 I think he’s right. Humans are increasingly participating in what scholars call the ‘humanitarian revolution’: an astonishing decline in violence in both the long and short term. While social brains began their evolutionary journey by connecting our biological kin, they are now connecting all social groups. They have taken what was once purely genetic and transformed it into a consciously chosen ethic. Despite all the doom and gloom about social media and how it’s making us lonelier, our brains are evolving to be more social, not less. Nevertheless, there may be a limit to how social we can be.
In the late 1980s a British anthropologist named Robin Dunbar noticed a curious relationship between the size of a primate’s brain and the size of its social group – the bigger the brain, the bigger the social group.16 He spotted this after measuring the size of the orbitofrontal cortex (a region at the front of the brain involved in higher thinking and advanced cognitive functions) in monkeys and apes. He found that it correlated with a variety of social behaviours: a large orbitofrontal cortex was linked to more social play and social learning, more elaborate grooming networks, higher rates of deception and coalition, a greater number of females in the group, and even an increase in male mating strategies. This led to a spellbinding new theory of human brain evolution: the social brain hypothesis.
Simply put, it states that humans need large brains to manage their remarkably complex social systems. However, there is a constraint on the number of individuals a person can maintain a stable relationship with, which Dunbar calculates to be about 150 people – Dunbar’s number, otherwise known as a ‘clan’. It turns out that a striking number of human organisations from factories to villages to armies operate around units of about 150 people. And the vast majority of Facebook users list around 150 friends.
On the surface, Dunbar’s hypothesis did seem to reflect a truth about the social brain. Other brainy mammals, such as sperm whales – which have a brain six times larger than that of humans – display unusually sophisticated social behaviours. They travel in pods 500-strong, avidly fool around and play with one another, and communicate using a fascinating pattern of clicks called ‘codas’, some of which are believed to represent names of particular individuals. Conversely, many small-brain animals (owls, foxes, koalas, sea turtles) spend the majority of their lives completely alone. Moreover, archaeologists now believe that Neanderthals disappeared not because of their lack of intelligence but because they tended to be anti-social, limiting themselves to groups of twenty to fifty people in small territories while Homo sapiens travelled 100 kilometres or more to find new friends.17
Also important is the shape of one’s social network – that is, whether your friends know each other directly or through you alone. Ronald Burt, a sociologist at the University of Chicago who studies how social networks create advantages in the workplace, is particularly interested in the latter shape. In the realm of sociology research, those who befriend people who wouldn’t otherwise know each other are called information brokers. In 2004 Burt wondered whether information brokers might possess an edge over people who only connect with members of their own group. To find out, he and his colleagues looked at data describing 673 managers in the supply chain of Raytheon, America’s largest electronics company based in Waltham, Massachusetts. Each manager was asked to write down how they think business performance could best be improved. Executives at the company then rated their ideas.
The most valuable ideas, Burt found, came from managers who connected with people outside their work group – the information brokers. They offered smarter solutions to problems and tended to be promoted more often, earning higher wages as a result.18 ‘People who live in the intersection of social worlds are at higher risk of having good ideas,’ Burt told the New York Times. Bridging social worlds gives brokers what he calls a ‘vision advantage’. Where others see limited options for handling difficult situations, they see fresh perspectives and divergent interpretations. Where others restrict themselves to the narrow opinions, phrasing and behaviour of a single group, they branch out and open themselves up to unorthodox views and novel ways of thinking.
Neuroscientists think that this kind of behaviour changes the way the brain works: that the brain evolved both to influence and be influenced by the social circles we keep. When we look more closely at the regions being used, we see that they overlap with what psychologists call the mentalising network: a dense labyrinth of neurons and fibres crucial for understanding other people’s intentions, beliefs and desires.19 Researchers studying this network are particularly interested in von Economo neurons, a strange type of brain cell only found in whales, elephants and apes – including humans. We believe they evolved in highly gregarious animals specifically to deal with complex social behaviours.
Of course, the size and shape of a group are not the only factors determining the evolution of social minds. A flock of birds or a swarm of bees is a large group taking various forms, yet still little more than a cluster of individuals all doing the same thing, a hive mind built from a few simple social interactions. ‘If it were only the size of social groups that mattered, wildebeests would be wizards,’ declared the social anthropologist Joan Silk.20 What makes human brains special is that they have evolved to handle complex social situations within groups as well. And this arises from a deeper human necessity, a social contract written into the fabric of our DNA that set all forms of social interaction in motion. It is a phenomenon universal to all human societies, but we call it the nuclear family.
Biologically speaking, monogamy (pairing for life) is hard work. The monogamous brain consumes more energy because it has to consider a partner’s perspective as well; for our ancestors, this would have concerned everything from successful hunting to deciding where to seek shelter and how to outsmart predators. Monogamy also requires all of those social intelligence skills that we take for granted: the ability to listen and understand others, to lead or follow when necessary, to validate other people’s feelings, to recognise that different people hold different beliefs. All of these demand a tremendous deal of energy that only humans and a few other brainy species can afford to expend. Only 9 per cent of mammals pair up for more than one breeding season, and only 15 per cent of primates live together as couples (and even they refuse to be sexually exclusive).
No one knows when monogamy began. The fossil record puts it somewhere between 3 and 4 million years ago, based on a few conspicuous clues. One clue was found in 1975, when the fossilised remains of seventeen members of the species Australopithecus afarensis were found in Hadar, Ethiopia. Thought to be around 3.5 million years old, the group comprised nine adults, three adolescents, and five children. They were found so close together they are believed to be part of the same family, and have since been dubbed ‘the first family’ (though this doesn’t prove the parents were entirely faithful, of course).21
Another clue was unearthed in 2011, when archaeologists discovered that male and female finger lengths started to equalise around 3.5 million years ago.22 This might not sound like much of a find, but finger-length equality is actually a telling sign of monogamous behaviour. In primates there is a connection between mating behaviour and body size differences between males and females of the same species. Biologists call it sexual dimorphism: the more dimorphic the primate, the more likely it will be polygamous (that is, having more than one mate). Male gorillas for example are twice as big as females and nearly always polygamous. Male and female gibbons on the other hand are equal in size and are mostly monogamous.
Some scientists think monogamy is even older, based on, of all things, penis shape. Humans are one of the few species to lack a penis bone, known scientifically as the baculum, which evolved around 90 million years ago and helps polygamous species have sex for longer, a crucial skill if an animal is to fertilise a female successfully amongst stiff competition (pun intended). In fact, the more straight, smooth and, frankly, boring a penis looks, the more likely the species is to be monogamous. The penises of polygamous species are usually far more exciting, with all kinds of twists, turns, kinks, ridges and spikes.
Homo sapiens are by no means a purely monogamous primate, however. According to recent studies, humans have evolved to be a monogamous primate that exhibits tendencies towards polygamy.23 Scientists have devised a clever way of determining this. By using genetic data from three human populations – African, Asian and European – and looking at the frequency of X chromosome genetic material (which only recombines with females), they’re able to measure the ratio of female to male breeding partners and how they have changed over time. A completely monogamous population would have a breeding ratio of 1:1 (one male to one female). The data churns out 1:1 in Asia, 1:3 in Europe and 1:4 in Africa, averaging about two women to every procreating man, which is still considered monogamous but nevertheless leans towards polygamy. Given that we descend from primates that are polygamous, this isn’t surprising. Polygamy still echoes within the human genome and indeed most societies still practise some form of it. Modern monogamy has thus probably only existed for a few thousand years, certainly not long enough to silence the evolutionary impulse of polygamy completely.
How did monogamous brains evolve in the first place? Why were they so advantageous? Three schools of thought have emerged among scientists: female spacing, infanticide avoidance and a need for male paternal care.
Female spacing suggests that females deliberately spread themselves out across the savannah in order to retain a single male partner and obtain exclusive access to greater food resources. This is a win–win strategy for the pair because it means less fighting between competing males and ample security for the female’s offspring since they are also protected from rival males. If our 4-million-year-old ancestor Ardipithecus, for example, had chosen to live an isolated existence, she might have fared better than mammals that reared their offspring in groups. In the brutal, unforgiving realm of the ancient savannah, her children would have been at high risk of being killed by other, more violent humans. By occupying a smaller territory that didn’t overlap with other females, she could attract a male to guard her children; in exchange, the male secures a mate he can monopolise, ensuring paternity. This might sound harsh by the behavioural standards of twenty-first-century Homo sapiens, but it’s important to remember that safety in numbers wasn’t always a wise option for early humans, making female spacing an attractive alternative. As the Cambridge University zoologist Peter Brotherton observes in a paper supporting the female spacing idea, ‘Provided that a single female can be monopolized successfully, monogamy can be viewed as a risk aversion strategy.’24 But while this scenario is certainly possible, scholars admit that prehistoric females might have been too socially dependent on one another to spread themselves out in this fashion.
Infanticide avoidance posits that females needed monogamous partners solely to save their children from the aggressive and often murderous intentions of other males. Infanticide was an ever-present danger in the wild and is still practised by more than 40 per cent of primates: it’s common among chimpanzees, gorillas and baboons; absent among orangutans, bonobos and mouse lemurs. Why it happens is unclear, but most scientists agree that it isn’t blind violence. In primate society, infanticide might occur when food resources are too thinly spread, when a sick infant becomes a burden, or when the infant becomes food themselves and is cannibalised. A reliable male companion to defend a female’s progeny was therefore essential. This too is a credible idea, especially since humans are also infanticidal primates. The ancient Romans, Greeks and Chinese killed both male and female babies in droves, as did medieval Europeans and Australian aboriginals. In fact, research conducted by anthropologist Laila Williamson reveals that infanticide has been practised on every continent and by every kind of people.25 The reasons are similar to those of other primates: poverty (i.e. scarce resources), family planning (albeit a brutal form), perceived illegitimacy and/or the birth of infants with physical deformities. Still, the idea that infanticide pushes a species towards monogamy is probably false, since it remained the rule rather than the exception in monogamous societies; plus, many infanticidal primates have remained polygamous to this day.
Far more persuasive is the idea that monogamy evolved from the need for male paternal care. If a male redirects his energy from bellicose tribalism to pulling his weight at home, both he and his children are not only more likely to survive in the long run, but to develop into powerful individuals as well. Fundamental to paternal care is the notion of male guarding, that is, warding off rival males who may be desperate to find a reproductive-age female, especially if there is a shortage due to our male-biased sex ratio – there are roughly 105 males born for every 100 females (no one really knows why). If our ancestors experienced a scarcity of females, compounded by menopause and our long lifespan, male guarding was probably a crucial first step in the evolutionary pathway to paternal care, since it ensures paternity – which then provides a selection pressure for paternal care. In the brain, paternal care relies on the same structures that support maternal caregiving, namely the insula, the temporal and frontal cortex, the amygdala, the hypothalamus and the nucleus accumbens, which together form the ‘human caregiving network’. This ancient network is rich in oxytocin receptors (receptors for one of our love hormones, discussed in the previous chapter), making it especially sensitive to caregiving behaviour.
For thousands of years, monogamous arrangements have been facilitated by the institution of marriage. While the rules and traditions of marriage have changed over time, one constant appears to be the pacifying effect it can have on males. For example, testosterone, a hormone known to promote male aggressiveness and poor decision-making, declines when males get married and have children. The sociologists Robert Sampson, John Laub and Christopher Wimer analysed data from a study that followed 1,000 low-income Boston teenagers for forty-five years and found that getting married made them 35 per cent less likely to commit crime (even after factoring in things that make marriage more likely: increased intelligence, financial stability, a healthy family background, etc.).26 Today, no one would deny that males are just as able to raise children as females and gender non-binary people; indeed, 40 per cent of gay, lesbian, bisexual and transsexual people have children, and 60 per cent of those are biological. Splitting the responsibilities of childcare between two parents also lets the brain retain enough energy to pursue other activities, such as problem solving and creative thinking.
A touching example of paternal care is seen in the owl monkeys of South America. Male owl monkeys are involved in most of the childcare – carrying their infants on their back, feeding them and playing with them from the time they are two weeks old. A genetic study of seventeen pairs of owl monkeys and their thirty-five offspring revealed that every pair were their offspring’s biological parents. They are completely monogamous, a trait that has been reported in only five species (the Malagasy giant rat, the California mouse, the oldfield mouse, the Kirk’s dik-dik antelope and urban coyotes).27 To preserve their emotional bond, the lifelong pair are often found with their long bushy tails draped around each other, snuggling in the treetop.
Dual parenting paves the way for others to pitch in. Anthropologists call them alloparents: unrelated group members as well as family members (think teachers, nannies, nursery staff, social workers and the kind strangers who help mothers with their pushchairs on the underground). No other species takes social thinking this far. But it makes perfect sense when we consider the big picture. With the help of alloparents human minds could get on with the business of building cities and civilisations. Alloparents are perhaps best encapsulated by the insightful African proverb, ‘It takes a village to raise a child.’ Popularised by the eponymous title of Hillary Clinton’s 1996 book, it conveys the shared responsibility of parenting and the importance of community. It reminds us that no matter how small we feel as individuals, we all have a part to play. The Sukuma tribe of Tanzania have their own version: ‘One knee does not bring up a child.’ We only have to substitute ‘knee’ for ‘brain’ to unveil the metaphor. The fact is our social brains have evolved not just for our nearest and dearest but for the wider, extended family of humankind as well.
The philosophy of liberal parenting and the tendency for humans to act in loco parentis fashioned new kinds of social thinking. Like a great oak growing from a seed, human relationships became remarkably complex compared to those of other primates, and magnificent societies, from the bustling streets of Manhattan to the sprawling markets of Mumbai, burst forth. We may never know exactly how these forces shaped the evolving brain, nor the extent to which our brains have contributed and responded to the social advantages of monogamy. But new research shows that human monogamous brains are more active in reward-related areas (including the thalamus, nucleus accumbens, caudate, pallidum, putamen, insula and prefrontal cortex) than non-monogamous brains. Moreover, non-monogamous brains tend to have a special kind of dopamine receptor gene called DRD4, which is linked to promiscuity and infidelity. Of course, such findings face a chicken or egg conundrum since we still don’t know which came first: the behaviour or the biology. As always, the answer is likely to be a complex interplay of both.
All told, human societies are remarkably complex compared to those of other animals. As we have seen, over the course of evolutionary history our social brains have equipped us with the perfect hardware for living in an increasingly crowded world. In our present age of globalisation, we can only hope that our minds are capable of extending and intensifying the social bonds that unite us. I for one am cautiously optimistic. The endless adaptability of the human brain should not be underestimated, and our increasing desire for social change is a powerful force.
‘The arc of the moral universe is long,’ proclaimed Martin Luther King Jr, ‘but it bends towards justice.’ That arc started with our 4-million-year-old ancestor Ardipithecus, and is limited only by the degree to which we are willing to cooperate with others. Yet as our brains evolved the traits of empathy, mutualism, compassion and fair play, this social complexity then became critically dependent on something else: memory.