Sarah-Jayne Blakemore: how to understand teenage brain development

05It’s not easy being a teenager - we’ve all been one. But somehow, they universally get a bad rap. We think of them as moody, inscrutable and hyper sensitive, but, as neuroscientist Professor Sarah-Jayne Blakemore explains, it’s not their fault. Their brains are the culprit.

Her life’s work has been dedicated to studying teenage brain development, which she explains in her book Inventing Ourselves: The Secret Life of the Teenage Brain, the winner of the 2018 Royal Society Insight Investment Science Book Prize. We spoke to her about what makes teenagers’ brains different, the best way to nurture them, and what it’s like to win the Royal Society Book Prize.

A lot of us just don't understand teenagers - what's happening in their brains?

The teenage brain is undergoing a huge amount of change, but interestingly we never used to think that was the case. Until about 20 years ago, neuroscientists assumed the brain stopped developing after childhood, but because we are able to scan the living human brain with MRI scanning, we now know that is not true at all. The human brain develops right throughout childhood, through the teenage years and even into the 20s.

So at what point do we start considering a child’s brain to be a teenage brain?

It's not a sudden change, it's quite a steady development between childhood and the teenage years in terms of the brain's structure, but very substantial changes go on during adolescence.

What sort of changes are they?

For example, the brain is made up of grey matter and white matter. Grey matter contains cell bodies like neurons, and the connections between neurons called synapses. White matter contains the fibres that connect up different regions of the brain and allow different regions to communicate with each other. During adolescence there are huge changes in the amount of both grey and white matter.

We know that white matter increases during the teenage years, and the interesting consequence is that it probably means the brain becomes faster. More white matter means that the brain is quicker to generate signals and transmit them between neurons and brain regions.

At the same time there is a decrease in grey matter. We think that's partly because brain tissue is changing from grey to white, but also possibly because the number of connections, or synapses, in the brain is decreasing. That's important because this process is called synaptic pruning, and it partly depends on the environment. Synapses that are being used are the ones that will remain and grow stronger, and the synapses that are not being used in a particular environment will get pruned away. During childhood and teenage years, the brain is being shaped by the environment that the child or teenager is in.

Is it just the environment that changes the shape of the brain or are there other factors?

Well we don't know, but we think that all environmental factors are involved. It could be the learning environment, the education environment, family and social experiences, but also things like nutrition, exercise and alcohol play a part too. All these external environmental experiences, in theory, can influence the way the brain develops.

Does that explain some of the behaviour we tend to associate with teenagers?

Possibly yes. When we think about teenagers we stereotypically think of behaviours like increased risk taking, self-consciousness, being easily influenced by their peers, and being very embarrassed, particularly by their parents. But the first thing to say is that it's not all teenagers that display these behaviours, there are huge individual differences between them. And of course, lots of adults and younger children also take risks or are influenced by their friends.

But it is true that there is an increase in those behaviours during the teenage years, and that's in non-human species of animals too. It's true also across different cultures and even across different centuries or even millennia of history.

So there is something about adolescence that makes it a unique period of development and behavioural change. We used to put the behaviours that we associate with the teenage years down to changes in the level of hormones, sex hormones at puberty, and also social changes, like going to a new school, from a small primary to a big secondary for example. But we now know that in addition to hormones and social effects, the brain is also undergoing a huge amount of change, and it’s the development of the brain that probably explains some of the typical teenage behaviours.

Is there an optimal way the brain should develop?

The science of brain development during adolescence is actually very new, and we don't know the answer to those kinds of questions yet. But one question I often get asked by parents and teachers, and even teenagers themselves, is what does digital technology do to the development of the brain?

This generation of teenagers are growing up completely surrounded by mobile phones and social media. Is it good or bad for brain development? We don't know the answer to that, there's just no information, but that kind of question is now being looked at in lots of different studies around the world.

There's a really exciting study called the Adolescent Brain and Cognitive Development study (ABCD), happening in the US. They are following 10,000 children aged 9-10 over the next 10 years, tracking them and testing them every year and acquiring a whole load of measures. Measures of hormones, their genetics, brain measures, behavioural measures and also their environment, like how much they use their mobile phone, their mental health. They will look at how all these different factors influence the way a young person develops.

Will this give us a good idea of the best environment for the positive and strong development of teenage brains?

I think so. The study is going to take 10 years, but by the end of it I think we will have a very rich and detailed data set, and that will inform us about the optimal environment for brain development.

Is there a difference between boys’ and girls’ brains or are they identical?

I'm always asked this question about gender differences and I think it's because boys and girls do seem quite different when you know them. Anecdotally, teenage boys and teenage girls do seem like different people, but what's interesting is that, actually, most of the research on teenage brain and cognitive development doesn't really show much in the way of gender differences.

I think that might be partly because there is so much overlap between the genders that to see a difference between them would require huge numbers of people in each study. In our studies we include maybe a few hundred young people, which is actually relatively large in the field, but to see gender differences maybe we'd need to include several thousand, and that's not really possible.

But even though at a crude level, in everyday life, yes, there clearly are gender differences, but actually, when you test them at a much finer, more sensitive level, with really carefully designed experimental tasks, then maybe those gender differences are not robust, they just don't come out. But the jury is out, this is a new field and the larger studies, like the big ABCD study, are likely to show any gender differences in brain development, if indeed there are any.

Are there some things that the teenage brain is better at learning compared to a child's or an adult's brain?

There’s not a lot of evidence on that, but we found that teenagers in their late teenage years learn information like non-verbal reasoning (being able to understand and analyse visual information), better than younger teenagers. It's actually quite surprising, as most people think that young children are better at learning things than teenagers because they have very plastic, sponge-like brains, but we found that's not the case. That said, they aren’t better than adults, so it's not like teenagers were the best at learning - they're exactly the same as young adults.

There is some evidence from the Netherlands that teenagers are more creative than either children or adults and they have more novel responses on creativity tasks. It kind of makes sense. If you think back to your teenage years, this is a time that you're not only good at learning but also very imaginative, and you have ideas that maybe decline a bit as you get older.

Is that why I spent so much time playing guitar in a garage as a teenager?

Exactly, that's a really good example, you were just being your creative late teen.

Finally, your book Inventing Ourselves: The Secret Life of the Teenage Brain recently won the Royal Society Insight Investment Science Book Prize – how does that feel?

It's an honour I never dared to dream of - and a dream come true! I started working on the adolescent brain about 15 years ago, and it was thanks to Royal Society funding that I was able to get to where I am now, so I am very appreciative of their support.

I’m enormously grateful to my research group, both current and former members, and the teenagers I work with, as there would have been no research to write about without them, and everyone else who has helped me with this book.