Sarah-Jayne Blakemore, professor in cognitive neuroscience at University College London, is the author of a groundbreaking new book, Inventing Ourselves: The Secret Life of the Teenage Brain, in which she explains the development of the brain during the precarious, enriching and crucial years of adolescence.

In a sense, your book is a defence of adolescents. Why, as a society, do we demonise our teenagers?

Adolescents for ever have had a bad reputation. There are so many negative stereotypes. You can go back as far as Socrates, who said they have “bad manners, contempt for authority, show disrespect for elders and love chatter in the place of exercise”. It is not socially acceptable to mock and demonise other sectors of society. You wouldn’t get away with it, on social media, were you to mock women or a certain race, or elderly people with poor memories. But it is, strangely, acceptable to mock and demonise teenagers. As a society, we don’t like it that our children, who used to do what we said, are rebelling, becoming independent. It is difficult to handle that, and one way of handling it is to mock them.

Why do some well-behaved children become problematic teenagers?

What I’m most interested in is that the brain is undergoing a huge change in adolescence that has a knock-on effect in terms of behaviour, of how you think of yourself, how you interact with other people and the decisions you make. Your little child – and I know because I have children and can see it happening to them – changes rapidly and substantially in adolescence.

What goes on in the teenage brain?

When I was an undergraduate 25 years ago, we knew nothing about how the human brain develops. I was taught that the brain is fully mature by mid-childhood. Since then, we have developed MRI scanning that allows us to look inside the living human brain and track how it functions in terms of structure and activity across the lifespan. We now have a rich, detailed picture of how the human brain develops, and what it shows is that what I was learning in my textbooks was completely wrong. The brain continues to develop through childhood and adolescence and even into the 20s and 30s in some brain regions. White matter increases, grey matter decreases. These changes are thought to be caused by important neurodevelopmental processes that enable the brain to be moulded and influenced by the environment.

Not all teenagers would find it helpful to know about their brains



To what extent is this book written for teenagers as well as about them?

It is for everyone. But, as a teenager, I’d have found it helpful to know that everything I was feeling, the changes, the turbulence, the difficult times, was part of a natural, adaptive biological process that would take a while but which would stabilise. I work with lots of adolescents and it is important to stress they are not all the same. Not all teenagers would find it helpful to know about their brains, but the majority I’ve worked with have found it enlightening and empowering. And they have a right to know. At the moment, teenagers don’t learn about their brains at all at school. It would be good to include this on the curriculum.

What else needs to change in education?

First, we must acknowledge that this is a young science. There is evidence that the circadian rhythm, the body clock, shifts at puberty by a couple of hours. Teenagers need to go to bed a couple of hours later than adults. They can’t make themselves go to sleep earlier just because we want them to – and when we force them to get up for school, it is the middle of their biological night. School needs to start later. Teenagers are exhausted by the weekend and catch up on sleep – they have a shifting time zone, a phenomenon called “social jetlag”. Evidence suggests this is not good for cognition or mood.

Why do some adolescents take more risks than adults?

When you take a risk, the brain’s positive reward system gets activated. In adolescents, that activation is higher during risk-taking than in adults. It is true for non-human adolescents, too. Adolescent mice take more risks. They also drink more alcohol when they are with other adolescent mice.

Is the effect of technology on the teenage brain all bad? Photograph: Burger/Phanie/Rex Features

Your father, the neurobiologist Colin Blakemore, had animal rights protesters gathering outside the house when you were a child growing up in Oxford. What is your attitude to animal experiments now?

The rules for medical research on animals in the UK are the strictest in the world – that’s no bad thing. I don’t do research on animals because my research asks fundamental questions about humans and because technology has replaced animal research. But research on mice and rats in the US and elsewhere has taught us so much about how different neurotransmitter systems work and, critically, about systems that go wrong in mental illnesses.

Why does mental illness often kick off in adolescence?

We don’t know. Three-quarters of mental illnesses appear before the age of 24. Depression, anxiety, eating disorders, schizophrenia. It’s a period of vulnerability. Changes are happening at the same time, a confluence, a perfect storm: hormonal changes, neural changes, social changes and the pressures of life suddenly increase. The changes in the brain mean it is particularly plastic and susceptible to environmental stress. The pressure of schools and exams is increasing. As an adolescent, your mind and cognitive capacity is developing so you’re able to reflect on life, the future and your place in the social hierarchy in a more sophisticated way. You’re starting to look like an adult. People expect you to behave like an adult, too.

You write about teenage impulsivity – something that can be helped by mindfulness?

I’m involved in an enormous trial on mindfulness being led by Oxford University, involving thousands of young people in British schools. We’re looking at whether a 10-week mindfulness course in school improves wellbeing. There is a lot of hype around mindfulness in schools but there has not been a systematic large-scale trial. Even if it shifts the population upwards in terms of wellbeing, mindfulness is never going to be a panacea. It’s not going to eliminate mental-health problems, but it might help some young people.

What is your hunch about the effect of technology on the teenage brain?

This is, again, a new field. Those of us who work in this area assume it affects the teenage brain – but is it all bad? There is a lot of scaremongering around. We have to be careful about newspaper articles that talk about an increase in depression in adolescence and relate it to more access to phones and social media, because that is merely a correlation. We do not yet have causal data to prove that link. But, I must admit, I do use my own phone far too much. As adults, we worry about our children, but what about using our phones in front of them? My children tell me off – I hide my phone from them.

Do some people stay teenagers for ever?

When I give talks to adults and state that the end of adolescence is when a person has a stable independent role in society, half the audience starts laughing as if to say: “I have not got there yet.”

• Inventing Ourselves: The Secret Life of the Teenage Brain by Sarah-Jayne Blakemore is published by Doubleday (£20). To order a copy for £17, go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99