The Incredible Shrinking Brain

Over the last 20,000 years, the human brain has shrunk by about the size of a tennis ball.1 Palaeontologists found this out when they measured the fossilized skulls of our prehistoric ancestors and realized they were larger than the modern brain. This is a remarkable discovery by any standards, since for most of our evolution the human brain has been getting larger.2 A shrinking brain seems at odds with the assumption that advancing science, education and technologies would lead to larger brains. Our cultural stereotypes of large egg-headed scientists or super-intelligent aliens with bulbous craniums fit with the idea that smart beings have big brains.

Small brains are generally not associated with intelligence in the animal kingdom; this is why being called ‘bird-brained’ is regarded as an insult (though in fact not all birds have small brains). Animals with large brains are more flexible and better at solving problems. As a species, humans have exceptionally large brains – about seven times larger than should be expected, given the average body size. The finding that the human brain has been getting smaller over our recent evolution runs counter to the generally held view that bigger brains equal more intelligence, and that we are smarter than our prehistoric ancestors. After all, the complexity of modern life suggests that we are becoming more clever to deal with it.

Nobody knows exactly why the human brain has been shrinking, but it does raise some provocative questions about the relationship between the brain, behaviour and intelligence. First, we make lots of unfounded assumptions about the progress of human intelligence. We assume our Stone Age ancestors must have been backward because the technologies they produced seem so primitive by modern standards. But what if raw human intelligence has not changed so much over the past 20,000 years? What if they were just as smart as modern man, only without the benefit of thousands of generations of accumulated knowledge? We should not assume that we are fundamentally more intelligent than an individual born 20,000 years ago. We may have more knowledge and understanding of the world around us, but much of it was garnered from the experiences of others that went before us rather than the fruits of our own effort.

Second, the link between brain size and intelligence is naïvely simplistic for many reasons. It is not the size that matters but how you use it. There are some individuals who are born with little brain tissue or others with only half a brain as a result of disease and surgery, but they can still think and perform within the normal range of intelligence because what brain tissue they do have left, they use so efficiently. Moreover, it’s the internal wiring, not the size, that is critical. Brain volume based on fossil records does not tell you how the internal microstructures are organized or operating. Relying on size is as ridiculous as comparing the original computers of the 1950s that occupied whole rooms with today’s miniature smartphones that fit into your pocket but have vastly more computing power.

Structural arguments aside, why would such a vital organ as the human brain, one that has been expanding for the majority of our evolution, suddenly begin to shrink around 20,000 years ago? One theory is related to nutrition. As we shifted away from being hunter-gathers living off meat and berries to subsistence farmers who cultivated crops, the change in our diet over this period might have been responsible for the brain change. However, this seems unlikely. Farming only recently arrived in Aboriginal Australia and yet they also experienced the same decline in brain size over this period. Also, agriculture first appeared in Asia around 11–12,000 years ago, well after the human brain had begun to change.

Environmental scientists point out that once the climate warmed up around 20,000 years ago, marking the beginning of the end of the Ice Age, we no longer needed large bodies to carry heavy loads of fat reserves. This might have led to a corresponding decline in brain size. Big brains require lots of energy, so a decrease in body size would have enabled our ancestors to downsize the brain, too. But that explanation fails to address similar periods of climate change that also took place during the 2 million years when hominid brains were still increasing in size.

One other theory about the reason for the shrinking brain may seem quite absurd – that the human brain is smaller because we have become domesticated. Normally, the word ‘domestication’ brings to mind washing machines, ironing, mortgages, weekend barbecues and family. While domestication has come to refer to all of these facets of modern home life, it was originally a biological term to describe artificial selection and breeding of plants and animals. Charles Darwin was fascinated by domestication and indeed many of his arguments for his theory on the origins of species were based on the effects of selective breeding by man of plants and animals, as an insight into the way that natural environments select to favour some individuals for reproduction over others. But unlike natural selection, domestication is not blind: with the invention of farming and animal rearing some 12,000 years ago, humans deliberately manipulated the selection process of both plants and animals to eventually modify the various species they wanted to exploit. We wanted animals to be more docile so that we could rear them more effectively. We bred aggression out of them by selecting individual animals that were easier to manage and, in doing so, we changed the nature of their behaviour.

This is how we also began to domesticate ourselves to live together in larger cooperatives. It was self-domestication because, unless you believe in divine intervention, humans have not been reared by a higher being who selected only some of us for reproduction. Rather, we have been self-regulating so that certain traits that were more acceptable to the group than others proliferated because individuals who possessed them were more successful in surviving and having children. In this sense we have been self-domesticating through the invention of culture and practices that ensure that we can live together.

Something about the domestication process produces profound and lasting physical changes. When wild animals are domesticated, their bodies and brains change along with their behaviour.3 The brains of all the roughly thirty animals that have been domesticated by humans have decreased in volume by about 10–15 per cent in comparison with their wild progenitors – the same reduction observed over the last 1,000 generations of humans.

This effect on the brain has been observed in a set of experiments into selective breeding. In the 1950s, Russian geneticist Dmitri Belyaev began a programme of research to see if he could domesticate the Siberian Silver Fox.4 Unlike modern dogs that are descendants of a strategy of selective breeding of wolves, most foxes have remained wild. Belyaev thought that domestication depended on temperament. Only those foxes that were less aggressive and less likely to run away when approached by the experimenter were chosen for breeding. These animals were tamer because, coded in their genes, they had slightly different brain chemistry regulating their behaviour. After only about a dozen or so generations of selective breeding, the offspring were markedly more docile. But they experienced significant physical changes too. They developed a white patch on their foreheads, were smaller than wild foxes and, like many dogs, had floppy ears. As Darwin noted in On the Origin of Species, ‘not a single domestic animal can be named which has not in some country drooping ears’. They also had smaller brains.

Breeding for tameness as opposed to aggression means selecting for physiological changes in the systems that govern the body’s hormones and neurochemicals. One possible mechanism to explain smaller brains is that individuals who are more passive may naturally have lower levels of the hormone testosterone. Testosterone is associated with aggression and dominance behaviour in animals, but its anabolic properties also play a role in body size by making muscles and organs larger and stronger. It also increases brain size. Sex-change individuals undergoing hormone treatment to facilitate the change to the opposite sex were found to have either increased or decreased brain volume depending on which hormone they were taking.5

Not only does domestication in animals lead to smaller brains, but it also changes the way they reason. Brian Hare, a leading expert on animal behaviour at Duke University, has shown that domesticated dogs in comparison to wild wolves are much better at reading the social signals of others. We humans can easily read the direction of another person’s gaze to indicate where their focus of attention is. As we shall see in later chapters, it is a social skill that is present in young infants but becomes more sophisticated with the more social interactions we have as we develop. Domesticated dogs can also read human social signals6 such as gaze and even the uniquely human gesture of pointing with the hand, whereas wolves and most other animals are generally baffled or indifferent.

Most fascinating is the change in dependence. Wolves will persist in trying to solve a difficult task through cunning, using different solutions, whereas the dog will typically give up earlier and try to recruit the help of its master. Domestication not only makes the animal more socially skilled but also more dependent on others. Over the years, several of the domesticated foxes from the breeding farms in Russia escaped back into the wild, only to return days later, unable to survive on their own.7 They were dependent on those that had raised them.

Could domestication apply to human evolution as well? As a young researcher at Harvard, Hare went to a dinner where Richard Wrangham, a distinguished primatologist in the anthropology department, described how bonobos, the pygmy chimpanzee species famous for their sexual promiscuity when resolving disputes, were an evolutionary puzzle with a set of unusual traits not found in chimpanzees. Hare realized this was true of silver foxes as well. The more he looked at the similarities between domesticated animals and bonobos, and the way they differed from chimps, the more the evidence seemed to support a hypothesis that this subspecies of primate had become self-domesticated. The way their social groups had evolved placed a greater emphasis on social skills and conciliation rather than aggression. If this was true for bonobos, then why not humans?8 After all, humans are also primates that have evolved the most remarkable capacity for social interaction. Hare would later write, ‘human levels of flexibility in using others’ social cues may have evolved in the human lineage only following the emergence of species-specific social emotions that provide motivation to attend to other individuals’ behaviour and, subsequently, their communicative intent during purely cooperative interactions’.9 In other words, the need to be more sociable by cooperating had altered the operations of the early hominid brain.

This is an old idea that has lately been re-seeded with new research and potential mechanisms. It first appeared under the guise of Social Darwinism in the nineteenth century – the idea that there were selective pressures that emerged from living together that changed the nature of the individuals. At first glance, it seems a bizarre hypothesis that living peacefully together caused the human brain to change, let alone shrink. After all, humans have been civilized for much longer than 20,000 years, with many earlier examples of societies, religions, art and culture. The recent discovery of stone artefacts on the Indonesian island of Flores that date to one million years ago tentatively indicate that an early hominin ancestor, Homo erectus, had inhabited the island.10 If correct, that means that Homo erectus must have had considerable seafaring skills that would have required the cognitive capacity and social cooperation to coordinate such a voyage on early rafts because the land masses were separated by substantial amounts of open sea.11

Clearly our ancestors were cooperating and communicating well before the end of the last Ice Age. But there was a rise in the population around this time that could have increased pressure to adapt to cohabitation in larger groups.12 Analysis of our species’s history reveals that the world’s population rose significantly in three continents well before the Neolithic period began around 12,000 years ago.13 When the ice sheets covering the northern continents began to melt around 20,000 years ago, the demographics of our species changed rapidly, creating social environments that required increased levels of skills to navigate. The process of selection for social traits must have started when our hominid ancestors first began cooperating hundreds of thousands of years ago, when domestication first began to appear, but it could have undergone a sharp acceleration when they settled down to live together after the last great Ice Age.

Strength and aggression were advantageous for hunter-gatherer existence, but in these settled communities cunning, cooperation and trade were necessary. Humans now would have had to keep cool heads and even tempers. Those who prospered in this new selective environment would pass on the temperaments and social abilities that made them skilled at negotiation and diplomacy. Of course, there have been extreme violence and wars in the modern era and we have developed technologies to kill each other in vast numbers, but modern combat is typically orchestrated by groups; brute individual aggression was more prevalent in the smaller hunter-gatherer tribes of our prehistory.

By self-domesticating, we have been changing our species by promoting genes that produce relatively slowly developing brains in comparison to bodies. This would mean longer periods of development and social support that would have incurred more parental investment. It would require mechanisms that modulate temperaments and teach children how to behave in socially appropriate ways. Humans who lived together more peacefully in settled communities reproduced more successfully. They acquired skills that enabled them to cooperate, share information and eventually create our cultures.

Modern civilization arose not because we suddenly became more intelligent as a species, but rather because we learned to improve upon technologies and knowledge that we inherited by sharing information that was a by-product of domestication. Long childhoods were useful for transferring knowledge from one generation to the next, but they originally evolved so that we could learn to get along with everyone in the tribe. It was the drive to learn to live together in harmony that enabled collective intelligence to thrive, not the other way around. By sharing knowledge we became more educated, not necessarily more intelligent.

In 1860, two intrepid Victorian explorers named Robert Burke and William Wills set out on an expedition to cross the Australian continent from Melbourne in the south to the Gulf of Carpentaria in the north – a distance of 2,000 miles. They were successful in reaching the north coast, but on the return journey they both succumbed to starvation. Burke and Wills were educated modern men, but they did not know how to survive in the Outback. They were living on a plentiful supply of freshwater shellfish and a plant known as ‘nardoo’ that the local Aboriginals ate. However, both contain high levels of an enzyme that destroys vitamin B1, which is a vital amine (hence ‘vitamin’) essential for life. By ignoring the traditional Aboriginal method of roasting the shellfish and wet grinding and then baking the nardoo, which neutralizes the toxic enzyme, Burke and Wills had failed to capitalize on the ancient cultural Aboriginal knowledge. They did not die because of a lack of things to eat, but of beriberi malnutrition. Aborigines did not know about vitamin B1, beriberi or that intense heat destroys enzymes; they just learned from their parents the correct way to prepare these foods as children – no doubt knowledge that was acquired through the trial and error of deceased ancestors. Their cultural learning had provided them with critical knowledge that Burke and Wills lacked. As the two explorers’ fates show, our intelligence and capacity for survival depends on what we learn from others.

Learning through domestication entails the transfer of knowledge and practices that are not always evident, either in terms of their purpose or origin. In the case of roasting Australian ‘bush tucker’, this practice was how to safely prepare food, but other examples include hunting and childbirth, both potentially life-threatening activities associated with folk wisdom. Of course, much folklore also contains superstition and irrational beliefs, but as we will discover in the following chapters, there is a strong imperative to copy what those around you say and do, especially when you are a child.

As a developmental psychologist, it is my view that childhood plays a major role in understanding the cultural evolution of our species. I usually tell my students at the University of Bristol the oft-cited finding that animals with the longest rearing periods tend to be the most intelligent and sociable. They also tend to be found in species that pair-bond for life rather than those that have multiple partners and produce many self-sufficient progeny. So it should be no surprise that of all the animals on this planet, humans spend proportionally the longest period of their lives dependent on others as children, and then as parents investing large amounts of their time and effort in raising their own offspring. This is how our species has evolved.

Of course, that profile of extended parental rearing is not unique to humans, but we are exceptional in that we use childhood to pass on vast amounts of accumulated knowledge. No other species creates and uses culture like we do. Our brains are evolved for it. As the leading developmental psychologist Michael Tomasello once quipped, ‘Fish are born expecting water, humans are born expecting culture.’ Other animals have the capacity to pass on learned behaviours such as how to crack nuts or use a twig for prodding a termite hill, but none have the same ability to transmit wisdom that increases with complexity from each generation to the next. Our ancient ancestors may have taught their children how to make a simple wheel, but now we can teach our children how to build a Ferrari.

The ability to transmit knowledge requires communication. Other animals can also communicate, but only limited and rigid information; humans, with our unique capacity for generating language, can tell limitless stories – even fantastical ones that are literally out of this world. We can also speak, write, read and use language to reflect on the past or think about the future. And it is not just the complexity and diversity of human language that is unique. Language had to build upon an understanding and desire to share knowledge with others who, like us in the first place, were willing to learn. It required understanding what others were thinking. Communication is part of our domestication – we had to learn to live peacefully and cooperatively with others for the collective good by sharing resources that include knowledge and stories. We do not just educate our children – we also socialize them so that they become useful members of society governed by all the rules and behaviours that hold it together.

Of course, this does not mean that our species is necessarily peaceful. There are always tensions and struggles in a world of limited resources and individuals will group together to defend their position against members from another tribe. However, for the conflicts that arise between groups and among individuals, modern societies govern with a greater level of control through our morality and laws than previously experienced in our history. To be an accepted member of society, each of us must learn these rules as part of our domestication.

We are such social animals that we are completely preoccupied with what others think about us. No wonder reputation is paramount when it comes to feeling good about ourselves. The social pressure to conform involves being valued by the group because, after all, most success is really defined by what others think. This preoccupation is all too evident in our modern celebrity culture, and especially with the rise of social networking, where normal individuals spend considerable amounts of time and effort in pursuit of recognition from others. Over 1.7 billion people on this planet use social networking on the Internet to share and seek validation from others. When Rachel Berry, a character in the hit musical series Glee, about a performing-arts school, said ‘Nowadays being anonymous is worse than being poor’, she was simply echoing our modern obsession with fame and our desire to be liked by many people – even if they are mostly anonymous or casual acquaintances.

We have always preferred others for what they can do for us. In the distant past, it may have been the individual attributes of strength to bring home the bacon and fight off competitors or our capacity to bear and raise many children that were selected for, but those attributes are no longer essential in the modern world. In today’s society, it is as much strength of character, intellect and potential financial earning that most regard as desirable traits. Top of the list of qualities most of us would like to possess is high social status, which explains why many individuals who already are well off in every other domain of their life still seek the attention of others.

What others think about us is one of the most important motivations for why we do the things we do. Some of us may have moments of blissful solitude when we escape the rat race of modernity and pressure to conform, but most inevitably return to seek out the company and support of others. Deliberate ostracization can be the cruellest punishment to inflict on an individual, short of physical harm. Like domesticated foxes that escaped into the wild, we invariably need to return to the company of others.

Why is the group so important and why do we care about what others think? The Domesticated Brain shows that we behave the way we do because of how our brains evolved to be social. For humans, being social requires skills of perception and comprehension when it comes to recognizing and interpreting the activity of others but it also requires changing our own thoughts and behaviours to coordinate with theirs so that we can be accepted. This domestication as a species took place over the course of human evolution as self-selecting mechanisms shaped social behaviours and temperaments that were conducive to living in communities, but we continue to domesticate ourselves during the course of our own lives and especially during our most formative years as children.

Our brains evolved for living in large groups, cooperating, communicating and sharing a culture that we passed on to our children. This is why humans have such a long childhood: during this formative period, our brains can become acclimatized to our social environment. The need for social learning requires babies to pay special attention to those around them but also enough flexibility to encode cultural differences over the course of childhood. This enables each child to recognize and become a member of its own group. A child must learn to navigate not just the physical but the social world by understanding others’ unseen goals and intentions. We have to become mind readers.

We need to develop and refine skills that make us capable of reading others in order to infer what they are thinking and most importantly, what they think about us. Where possible, evidence from comparative studies is considered to reveal the similarities and differences we share with our closest biological cousins, the non-human primates. And of course, we focus on human children. Developmental findings that reflect the interplay between brain mechanisms and emergence of social behaviour are the key to understanding the origins and operations of the mechanisms that keep us bound together.

That analysis could rely solely on the costs and benefits of social behaviours, but then we would miss the important point that people are emotional animals with feelings. It is not enough to read others and synchronize with them in some coordinated tango to achieve optimal goals. There is also an imperative to engage with others through positive and negative emotions that motivate us to be social in the first place. Taking that perspective casts a better light on understanding why humans seem to behave so irrationally because sometimes they care too much about what others think.

One of the more controversial issues that The Domesticated Brain addresses is the extent to which early environments can shape the individual and even pass on some acquired characteristics to their offspring. For most Darwinians committed to the theory of natural selection, whereby the environment alone operates to select genes that confer the best adaptations, this idea sounds heretical. Yet we examine the evidence that early social environments leave a lasting legacy for developing our temperaments through what are known as epigenetic processes – mechanisms that change the expression of our genes that can affect our own children.

Every child at some point has been told that they must ‘behave’ and when they do not, they are ‘misbehaving’. What parents really intend when they scold their children for misbehaving is that they must learn how to control their thoughts and actions that conflict with the interests or expectations of others. Self-control is a feature of our developing frontal lobes of the brain and is central to our capacity to interact with others. Without self-control we would never be able to coordinate and negotiate by suppressing the urges and impulses that could interfere with social cooperation. This capacity for self-control is critical when it comes to being accepted and without it we are likely to be rejected – labelled anti-social because we fall foul of the moral and legal codes that hold our societies together.

That danger of rejection is the flipside of the benefit of living in a group and the devastating consequence of becoming an outsider. Ostracism and loneliness not only register as pain in our brain but also make us both psychologically and physically ill. Rejection can make individuals behave in destructive ways not only against themselves when they self-harm, but also against others. We may be more connected through social networking on the Internet, but this digital village also makes it much easier to become isolated.

Considering the vast size of the different territories covered in The Domesticated Brain, from human evolution, brain growth and child development to genetics, neuroscience and social psychology, any attempt to bridge these regions will be ambitious – yet it is a goal worth pursuing. When we recognize the importance of others in shaping who we become and how we behave, we can begin to understand what makes us human.

  • 1.Kathleen McAuliffe (2010), ‘If Modern Humans Are So Smart, Why Are Our Brains Shrinking?’, Discover magazine, September 2010.

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  • 4.Claudio J. Bidau (2009), ‘Domestication through the centuries: Darwin’s ideas and Dmitry Belyaev’s long-term experiment in silver foxes’, Gayana 73 (Suplemento), 55–72.

  • 5.Hilleke Pol et al. (2006), ‘Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure’, European Journal of Endocrinology, 155, S107–S114.

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  • 8.Brian Hare, Victoria Wobber and Richard Wrangham (2012), ‘The self-domestication hypothesis: evolution of bonobo psychology is due to selection against aggression’, Animal Behavior, 83, 573–85.

  • 9.B. Hare (2007), ‘From Nonhuman to Human Mind. What Changed and Why’, Current Directions in Psychological Science, 16, 60–4.

  • 10.Adam Brumm, Gitte M. Jensen, Gert D. van den Bergh, Michael J. Morwood, Iwan Kurniawan, Fachroel Aziz and Michael Storey (2010), ‘Hominins on Flores, Indonesia, by one million years ago’, Nature, 464, 748–52.

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  • 13.H. Zheng, S. Yan, Z. Qin and L. Jin (2012), ‘MtDNA analysis of global populations support that major population expansions began before Neolithic Time’, Scientific Reports, 745; DOI:10.1038/srep00745.

Next chapter

Navigating the Social Landscape

Chapter 1