Inventing Ourselves

The Secret Life of the Teenage Brain


By Sarah-Jayne Blakemore

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A tour through the groundbreaking science behind the enigmatic, but crucial, brain developments of adolescence and how those translate into teenage behavior

The brain creates every feeling, emotion, and desire we experience, and stores every one of our memories. And yet, until very recently, scientists believed our brains were fully developed from childhood on. Now, thanks to imaging technology that enables us to look inside the living human brain at all ages, we know that this isn’t so. Professor Sarah-Jayne Blakemore, one of the world’s leading researchers into adolescent neurology, explains precisely what is going on in the complex and fascinating brains of teenagers — namely that the brain goes on developing and changing right through adolescence–with profound implications for the adults these young people will become.

Drawing from cutting-edge research, including her own, Blakemore shows:
  • How an adolescent brain differs from those of children and adults
  • Why problem-free kids can turn into challenging teens
  • What drives the excessive risk-taking and all-consuming relationships common among teenagers
  • And why many mental illnesses — depression, addiction, schizophrenia — present during these formative years

Blakemore’s discoveries have transformed our understanding of the teenage mind, with consequences for law, education policy and practice, and, most of all, parents.



Adolescence isn’t an aberration

WHEN I TELL PEOPLE I study the adolescent brain, the immediate response is often a joke–something along the lines of: ‘What? Teenagers have brains?’ For some reason it’s socially acceptable to mock people in this stage of their lives. But when you think about it, this is strange: we wouldn’t ridicule other age groups in the same way. Imagine if we went around openly sneering at the elderly for their poor memory and lack of agility.

Perhaps part of the reason why adolescents are mocked is that they do sometimes behave differently from adults. Some take risks. Many become self-conscious. They go to bed late, get up late. They relate to their friends differently.

The author, aged 15

We now know that all these characteristics are reflections of an important stage of brain development. Adolescence isn’t an aberration; it’s a crucial stage of our becoming individual and social human beings. I find teenage behaviour fascinating, but not because it’s irrational, inexplicable–quite the opposite: because it gives us an insight into how natural changes in the physiology of our brains are reflected in the things we do, and determine who we will become as adults.

In this book, I want to tell you what we know about the adolescent brain. I will show you how we study the way the brain develops during these years, how that development shapes adolescent behaviour, and how it ultimately goes on to define the people we become. This is the time during which much of our sense of ourselves, and of how we fit in with others, is laid down. The development that adolescents go through is central to human experience.

So what is adolescence? It’s not a straightforward question to answer. Some people think of adolescence as equivalent to the teenage years. Scientific studies often define it simply as the second decade of life–this is the World Health Organization definition. On the other hand, many people believe that adolescence should not be tied to a particular chronological age range. The first psychologist to study adolescence as a period of development was Stanley Hall, who at the beginning of the twentieth century defined adolescence as starting at puberty, around 12 or 13 years, and ending between 22 and 25 years. Many researchers today define adolescence as the interval between the biological changes of puberty and the point at which an individual attains a stable, independent role in society.* In this definition, the start of adolescence is measured biologically while the end is described socially, and is rather arbitrary. In many industrialized cultures the end of adolescence, defined in this way, is constantly being extended as it has become acceptable for young people to stay in full-time education, and live with their parents, into their twenties or even later. Thus, adolescence in the West is often defined as beginning at puberty, now roughly around age 11 or 12, and ending at some point between the late teens and the mid-twenties. In other cultures, things are very different, and children are expected to become financially and socially independent as soon as they reach puberty. In some of these cultures, adolescence isn’t seen as a period of development and there’s no word for it. Indeed, people often ask whether the concept of adolescence is a recent, Western invention. But it isn’t.

There are three main reasons why we can confidently say that adolescence is an important, distinct biological period of development in its own right, in all cultures. First, you can see behaviours that we typically associate with adolescence, such as risk-taking, self-consciousness and peer influence, in many different human cultures, not just those in the West.

A study led by Laurence Steinberg from Temple University in Pennsylvania, and involving scientists from around the world, investigated sensation-seeking and self-regulation in more than five thousand young people from eleven different countries (China, Colombia, Cyprus, India, Italy, Jordan, Kenya, the Philippines, Sweden, Thailand and the United States). Participants aged between 10 and 30 years completed a number of experimental tasks and filled in questionnaires. Two tasks were combined with a questionnaire to provide a measure of sensation-seeking, the desire to seek out novel experiences, which often involves risk-taking. A measure of self-regulation was also taken–that is, the ability to control yourself and make decisions. Not all cultures showed identical developmental trajectories, but there was remarkable similarity across them. Sensation-seeking increased between age 10 and the late teens (peaking at age 19), and then fell again during the twenties. In contrast, self-regulation increased steadily between 10 and the mid-twenties, after which it levelled out. So, while societal expectations differ between cultures, adolescent-typical behaviours can be seen across cultures.

The second reason why we can consider adolescence a unique period of biological development is that there is also evidence of adolescent-typical behaviour in non-human animals. All mammals undergo a period of development between puberty and becoming fully sexually mature that we can think of as adolescence. There’s a lot of research on this period in mice and rats, which are ‘adolescent’ for about thirty days. Research has shown that, during the month or so of adolescence, these animals take more risks and are more inclined to seek out novel environments than either before puberty or in adulthood. A study published in 2014, carried out by Steinberg and his colleagues, showed that, if given access to alcohol, adolescent mice drink more of it when they are with other adolescent mice; this isn’t the case for adult mice.

Graph showing that adolescent mice spend more time drinking alcohol when with their cage-mates than when alone, whereas adults drink around the same amounts when alone and when with other mice

We come across adolescent-like behaviour in animals in all sorts of settings. A newspaper article in August 2016 described an incident in which a woman was attacked by an adolescent wombat. In an interview with the Guardian, Martin Lind, from the Australian wildlife service, had this to say about the creatures:

As babies, they’re clingy, they’re adorable, they’re with mum 24 hours a day, they’re in a soft, snuggly sleeping bag all the time listening to a heart beat. When they start to mature and hit puberty, they just hate everybody and everything. They go from running between your legs and cute as a button to being absolute little–can I swear?–little shits. They nip you, they wreck, they bite. I won’t look after wombats because you kiss goodbye to your flooring and everything. They just destroy everything.

So, adolescent-typical behaviour is present across human cultures and across species. And, third, such behaviour is also typical across history. One of the earliest descriptions of adolescents I’m aware of is said to come from Socrates (469–399 BC): ‘The children now love luxury. They have bad manners, contempt for authority; they show disrespect for elders and love chatter in place of exercise.’ A hundred years or so later, Aristotle described ‘youth’ as ‘lacking in sexual self-restraint, fickle in their desires, passionate and impulsive’.*

Move on a thousand years and more, and we find the Shepherd in Shakespeare’s The Winter’s Tale (1611) complaining (to knowing laughter in the stalls): ‘I would there were no age between ten and three-and-twenty, or that youth would sleep out the rest; for there is nothing in the between but getting wenches with child, wronging the ancientry, stealing, fighting.’ A century after that, differentiating adolescents from children, Jean-Jacques Rousseau (1712–78) described adolescence thus: ‘A change in humour, frequent anger, a mind in constant agitation makes the child almost unmanageable. His feverishness turns him into a lion. He disregards his guide; he no longer wishes to be governed.’

For millennia, then, adolescents have been painted in the same stereotypical terms often used of them today. This suggests that adolescent-typical behaviour is not a recent, Western phenomenon.

What is it that makes adolescents behave in a recognizably ‘adolescent’ way? Adolescents have long been blamed for their apparently errant ways; some have put their behaviour down to changes in hormones at puberty; others attribute it to social changes following on from puberty and the new importance of peer relationships, or associated with the shift from small primary schools to large secondary schools in early adolescence. Now, though, armed with new knowledge from brain scans and experimental studies, we can try to understand adolescent-typical behaviour in terms of the underlying changes in the brain that happen during these years. Studying changes in brain structure and function reveals a huge amount about why teenagers do what they do, and more broadly about how the architecture of the brain relates to the behaviour we display, and how brain development–as well as hormones and the social environment–shapes who we become as we emerge into adulthood.

There is more at stake here than the advancement of scientific knowledge. Understanding brain development in adolescence has profound implications for social and education policy. Public health advertising aimed at young people, for example, often focuses on the long-term health outcomes of risky activities such as smoking. But there is evidence that this is unlikely to work. Adolescents aren’t stupid–rationally, they already understand the risks. But in the heat of the moment, when they’re offered a cigarette or an Ecstasy tablet, many adolescents care far more about what their peer group thinks of them than about the potential health risks of their choice. Often, their decisions are driven by the fear of exclusion by their friends, rather than by a dispassionate consideration of the consequences. This isn’t true for all adolescents–some young people are not particularly influenced by what their friends think or do–but many are.

Having said this, the new studies on the adolescent brain are also fascinating for their own sake. Contrary to the received wisdom up to the late twentieth century, we now know that our brains are dynamic and constantly changing into adulthood, and that the transformation they undergo in early life continues for far longer and has much bigger implications than was previously thought. Modern brain-scanning technology like magnetic resonance imaging (MRI) is ushering in a new era of understanding of the physiological mechanisms that underpin our sense of who we are, the sense of self that develops during adolescence. In the chapters that follow, I will describe how these technological developments, building on earlier ground-breaking work done with painstaking dexterity on brain tissue samples under microscopes, have revealed the changes that take place in the adolescent brain.

We shouldn’t demonize adolescence–it is fundamental to who we are. The adolescent brain isn’t a dysfunctional or a defective adult brain. Adolescence is a formative period of life, when neural pathways are malleable, and passion and creativity run high. The changes that take place in the brain during this period offer us a lens through which we can begin to see ourselves anew.

The moment I decided that I wanted to spend my life studying the human brain was the moment I first held one in my hands. I was studying experimental psychology at the University of Oxford. The first year of the degree was split between courses in psychology and neurophysiology, and the latter involved learning about the anatomy of the brain. It included a neuroanatomy practical course during which we were able to study and dissect a human brain. I needed a lab coat and I didn’t have one, so I borrowed one from a friend of mine, who was doing a medical degree. Wearing the lab coat, aware that it gave me a new and different identity, I walked briskly into the Anatomy Department and into the large central hall, where students have been dissecting bodies for centuries. I was met with an overwhelming stench of formaldehyde, the liquid used to preserve and store body parts.

In front of each one of us seated around the benches was a large white bucket, closed with a lid, containing a human brain. After a preamble, the lecturer asked us to put on latex gloves and lift the brain with both hands out of the bucket. At that moment, as I held the brain of a stranger–I presumed it to have been that of an elderly man, though I had no evidence for this–I decided that this 3lb mass of tissue had to be the most fascinating and complex object in the universe. I already knew, intellectually, that this was the case, but now I felt it, too. The first thing I noticed was how heavy it was, and it astonished me that we all carry one of these around in our heads. I also noticed the colour–greyish pink; and the texture–smooth and shiny, but with folds all over the surface. Simultaneously, I was struck by the realization that the matter this object was made from was synonymous with the person who owned it. I was holding someone in my hands. Over the course of a lifetime, this now disembodied brain had stored all of their memories, had generated all of their feelings, emotions and desires, had formed their personality, their aspirations and their dreams. All of this, all of you, your self, is contained in your brain. That was what I was holding in my hands. And I knew then I had to–somehow–spend my life studying it.

By the time I was an undergraduate, I’d already had some acquaintance with what happens when the brain goes wrong.

When my friend Jon* was young, he was a typical kid, if there is such a thing. That’s the way his parents expressed it: he had lots of friends, moved in the same circles as his elder brother and sister. At secondary school, he still had a small circle of friends; he played in a band, went out at the weekend, had a girlfriend and got on with his school work.

When Jon was 16, his elder brother, Ben, dropped out of his first year at university because he’d developed schizophrenia. He was back living at his parents’ house, so Jon saw him most evenings. Ben was constantly talking to himself. He hardly ever came out of his room. A few times he flipped out and was violent towards his parents. Jon was scared of him and couldn’t believe this was his brother–they used to be so close.

Jon left school after his A-levels. He took a gap year and travelled to Asia. All the while, he kept thinking about Ben. And then one day, sitting in a hotel room in Bangkok, Jon heard a voice talking to him, completely clearly, as though he had tuned into a radio station no one else could hear.

One of my main interests as an undergraduate was schizophrenia. Perhaps my interest stemmed from the knowledge that my school-friend Jon had developed this condition just a few years after his brother Ben was diagnosed with the same illness. Schizophrenia is a devastating psychiatric disorder in which the patient loses touch with reality. It runs in families, but is not completely genetic–the environment plays a role in triggering the illness in people who have a genetic predisposition, although we don’t yet understand how exactly this happens.*

People with schizophrenia often have auditory hallucinations, such as hearing voices–usually negative and threatening voices–inside their head. They also often have delusions (false beliefs), such as being paranoid, believing that people are out to get them. A common false belief is that an intelligence agency like MI5 is following their every move. What interested me most was how the human brain actually generates these often terrifying experiences–what goes wrong? And why are most of us protected from them?

Or are we? Maybe ‘normality’ is a fragile state that can be disrupted by taking a drug or by a particularly stressful life event. When I had fevers as a child, my mind sometimes used to play tricks on me. I hallucinated and heard voices. It was frightening, but what I was suffering from is fairly common. It’s called fever-induced delirium: the brain heats up because of the fever, which causes neurons to fire in ways that produce unusual, false perceptions. The brain, it turns out, is a delicate ecosystem: nudge things out of balance, and the entire system can be pushed over the edge into the unusual and sinister. So what pushes it?

It was that question–why some people experience delusions and hallucinations, and why most of us don’t–that prompted me to apply to do a PhD on schizophrenia. What is it about how our brains work that means that most of us don’t hear voices or think that the Secret Service is after us? During my PhD, at University College London (UCL), my supervisors Chris Frith and Daniel Wolpert and I found that the brain has a system for labelling self-produced sensations and distinguishing them from sensations produced externally. It turns out that the mechanism for distinguishing between what the world does to you and what you do to yourself isn’t working normally in people with schizophrenia. This is probably the reason why they hear their own thoughts as voices, like Jon did, or believe, for example, that their arm movements are being controlled by someone else. Why do people with schizophrenia hear voices that are often negative and critical? Why do they experience paranoid delusions? Why are people with schizophrenia also often depressed, their emotions and demeanour flattened? These were the kinds of questions I was interested in studying.

During my PhD, I collaborated with psychiatrists in Edinburgh and collected data from patients with schizophrenia in psychiatric hospitals there. When I had completed my doctorate and moved on to my post-doctoral research, I also collected data from patients in a psychiatric hospital in Versailles, just outside Paris. As I worked with these people, I was struck time and again by the same observation. Every patient I asked, regardless of age, race or gender, told me that the first time they experienced their frightening and debilitating symptoms was between the ages of 18 and 25–that is, in what is generally considered late adolescence or emerging adulthood.

It was always the same story. They were pretty regular children, and as teenagers they were OK–some of them started to drop out or take drugs, but not all. There were variations in how fast the disorder appeared–in some cases it had been quite gradual, whereas in others it had happened quickly–but for all of the patients I talked to, it was in their late adolescence that the symptoms started to emerge. This is interesting because it tells us that schizophrenia is a developmental condition, but one that starts much later than other developmental conditions such as autism or attention deficit hyperactivity disorder (ADHD).

The average age range for the onset of several psychological disorders is in adolescence or early adulthood

What is it about late adolescence that makes some brains particularly vulnerable to debilitating delusions and hallucinations? What is it about brain development in the teenage years that goes wrong in people who develop schizophrenia? These seemed to me to be critical questions; they also seemed obvious ones to ask, so I thought I would find the answers in the existing scientific literature. But after a concentrated effort delving into journals, to my surprise I discovered how little was known about how even the healthy human teenage brain develops, let alone how it does so in teenagers who go on to develop schizophrenia–and not just schizophrenia: many psychological and psychiatric conditions start at some point in adolescence, as the chart opposite shows. So I became deeply interested in finding the answers for myself.

That was in the year 2001. It was an exciting time to be a researcher. Most neuroscientists still assumed that the human brain doesn’t change much after mid- or late childhood. That was what the textbooks said. However, a small handful of studies, published in the late 1990s, suggested that the dogma was completely wrong and that the human brain continues to develop throughout the teenage years, and even into the twenties.

So, after my time in France, I changed the focus of my research from studies of adults with schizophrenia to developmental studies of the teenage human brain. In retrospect, it was a risky manoeuvre, because I’d never done a single developmental study, and there are many aspects of developmental research that are specific to studies of children and adolescents, and to theories of the developing mind, which I had never learned or experienced. Nor did I have experience of the technicalities involving recruiting and testing children and adolescents. It was the encouragement of my friend and mentor Uta Frith which gave me the confidence to make the leap into this new field. Frith is a professor of psychology in London and a world expert on developmental conditions such as autism and dyslexia. I had already known her for several years, having done work experience in her lab when I was 15.* My career jump into adolescent brain development was further aided by a research fellowship from the Royal Society, which started in 2004. I have been working on the adolescent brain ever since.

A window is opened on the adolescent mind by teenage diaries, as vividly demonstrated on the BBC Radio 4 programme My Teenage Diary, in which famous people read out their (occasionally painfully embarrassing) diaries from their teenage years.* I recently discovered my own. My parents were clearing out the loft of the house they had been living in for almost forty years and presented me with a large number of musty boxes containing all sorts of things, from old school books to records. One box contained letters I had received as an adolescent–from friends, pen-pals, boyfriends–and the diaries I had written in those years. I’d forgotten that I’d even kept my teenage diaries, and felt a mixture of intrigue and trepidation at the prospect of reading them. As I anticipated, they revealed a fairly typical teenage girl, preoccupied by clothes, music, friends and boys, with occasional heartfelt interjections about the horrors of war (the Gulf War was happening at the time).

So, I found, I was a typical teenager. Perhaps most of us were. Sometimes I wonder whether we forget our own adolescent years when thinking about teenage behaviour today. Adults are quick to criticize teenagers for their moodiness, self-absorption and risky decisions–but, as we’ve already seen, they have been doing so for over two thousand years at least: Socrates and Aristotle were just as dismissive and critical of young people in ancient Greece as any twenty-first-century parent or teacher. Adolescent-typical behaviour–at least as viewed by adults–goes back a long way.

However, there was another side to my teenage years that distinguished me from my peers. While I spent a lot of time thinking about what clothes to buy from the army surplus store, whether I could afford to go to a gig on Saturday night and who my friends had crushes on, in the background of my life something more serious and threatening was happening. My father, Colin Blakemore, did medical research involving animals, and was targeted by animal rights groups in the UK–in fact, for several years he was their number one target. This meant we lived our lives under the constant threat of attack. The animal rights activists threatened to kidnap me and my two younger sisters, resulting in the three of us, aged between 6 and 11 at the time, being followed to and from school each day by undercover police in an unmarked car. It was a period of my life that left a particularly deep imprint on my mind. For some reason, the police were dressed in leather jackets with piercings, and stuck out like a sore thumb in our quiet, leafy neighbourhood of north Oxford. My sisters and I would walk the few blocks to school, and the police would follow us slowly in their clapped-out car. We found it amusing, but I also remember being intensely embarrassed and hoping none of my friends would spot them.

Our house was under twenty-four-hour guard and was like a fortress, with electric gates, multiple alarm systems, cameras and panic buttons that directly alerted the police. Each time we wanted to drive anywhere, my parents would check under the car with a bomb-detector mirror. Getting into a car that might explode as soon as the ignition was turned on was not an experience I enjoyed much.

The threats and the violence got worse and worse throughout my teenage years. Large numbers of animal rights activists would meet on Saturdays in Oxford, often right outside my parents’ house. People would shout abuse through a megaphone and argue vociferously with my dad, who would bravely and calmly confront them. (My 80-year-old grandmother, who lived with us at the time, had a more abrupt approach to the situation, and simply told them to ‘bugger off’.)

Sometimes the police got wind of these gatherings beforehand–it’s hard to imagine how it was all organized so rapidly and how the police found out, because this was long before everyone had mobile phones and internet access. When this happened, there was a squad car outside the house, and uniformed police with dogs patrolling in the street. One year all the animal rights groups in Britain chose our street as the meeting point for their annual day of protest. Our suburban road was filled with police on horseback, who didn’t allow anyone to enter it. Even our neighbours weren’t allowed to drive into their own driveways. Again, this was incredibly embarrassing for my teenage self. Interestingly, I didn’t really worry too much about the danger of these confrontations, or about the inconvenience for others, not even my parents. I was mostly focused on what other people, and particularly my friends, would think of me: something different was happening to me that didn’t happen to anyone else, and when it comes to our teenage lives, ‘different’ is often bad.

Things got serious. Windows were smashed in our house, bricks thrown through them by a gang wearing balaclavas. Several times our car was doused in paint-stripper, which, aside from the obvious cosmetic damage, also caused chemical burns on the paws of our cat, who liked to sleep on the car. Letters packed with razor blades were sent to my dad through the post.


On Sale
May 15, 2018
Page Count
256 pages

Sarah-Jayne Blakemore

About the Author

Sarah-Jayne Blakemore is a Professor of Cognitive Neuroscience at University College London. She has published over 120 papers in scientific journals, and won multiple major awards for her research, including the Turin Young Mind & Brain Prize 2013, the Royal Society Rosalind Franklin Award 2013, the Klaus J. Jacobs Research Prize 2015, the Presidents’ Award for Distinguished Contributions to Psychological Knowledge from the British Psychological Society in 2018, and the Royal Society Prize for Science Books in 2018. She was named in the Times Young Female Power List 2014, was one of only four scientists on the Sunday Times 100 Makers of the 21st Century 2014, and is a Fellow of the British Academy.

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