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Now we know adolescence is ‘a clever way to grow a brain’

Deeper understanding of how our brains develop could inform changes in our schools and penal systems - and explain teenagers’ assorted hairstyles and musical tastes

Technology that allows researchers to observe the living human brain in action has transformed our understanding of what happens to it during adolescence, according to Sarah Jayne Blakemore, professor of psychology and cognitive neuroscience at the University of Cambridge.

Until about 20 years ago it was thought that brain development ended at around the age of eight, when the human brain reaches its full overall volume, but it is now known, courtesy of MRI scans, that the brain goes through a period of “heightened neuroplasticity” that continues through to our mid-20s.

While many people think of teenager and adolescent as synonymous, in fact the latter continues until the early to mid-20s. Adolescence is now most often defined by experts as the period between the ages of 10 and 24 years, says Prof Blakemore, though “it is actually quite difficult to define adolescence” and­­­­, she emphasises, it is different for different individuals.

Puberty is key to its onset, she says, with some people entering puberty at age eight, others at 13 – all being within the normal range. But what is clear is that when adolescence ends in the early to mid-20s, the brain is very different from what it was like in late childhood, both in terms of physical appearance and the way it works. “The brain undergoes massive development across adolescence both in terms of its structure and function,” says Prof Blakemore.


In adulthood the brain is still plastic, and individuals can learn new skills and accumulate new knowledge, but development and change no longer occurs in the dramatic way it does during adolescence.

Asked how the brain changes during adolescence, Blakemore talks about grey and white brain matter. White matter contains long fibres that allow different parts of the brain to communicate with each other. During adolescence, the amount of white matter increases, as the fibres are increasingly coated with a substance called myelin that speeds up the transmission of neuronal signalling between neurons (and shows up white under a microscope).

Grey matter is made up of neurons, synapses (connections between the neurons), uncoated fibres, and other material. These proliferate until late childhood, and then decline during adolescence, mostly due to the grey matter being changed into white matter through myelin coating. “At the same time there is synaptic pruning, where synapses or connections are being pruned away,” says Prof Blakemore.

Humans produce an excess number of synapses during the period from gestation to late childhood. Then, during adolescence, the synapses that are not being used are pruned away, while those that are being used are strengthened. The object of the exercise is optimal brain function.

Research shows that the pruning occurs in the prefrontal cortex, an area of the brain that is much bigger in humans than in any other species, when body size is considered, and which is particularly late in reaching full development during the human adolescent period.

“It is responsible for all sorts of high-level executive functions such as planning and decision-making, and working memory, and inhibiting inappropriate behaviour, as well as things such as empathy and self-awareness and understanding other people’s minds, [and] social interactions. These are the kind of processes that are being moulded and shaped by environmental experience during adolescence.”

[There are] biological explanations for some adolescent behaviours that otherwise don’t make sense ... [such as] the intelligent young person who knows all about the harm of smoking or illegal drug-taking and yet, when they are with their friends, they go and do these things

—  Prof Sarah Jayne Blakemore

Asked why she thinks evolution brought about a process of brain development where an abundance of synapses are made and then pruned back, Prof Blakemore says “it is actually quite a clever way to grow a brain. It means that any individual brain will be able to adapt to the specific environment in which it finds itself. It is not stuck before it knows what environment it is going to be growing up in.

“It creates all the synapses that it will need to cope with any kind of environment, any kind of language, any kind of social environment, any kind of cultural environment, any environment that requires any kind of skills or training or experience [and] it waits. We are talking about 25 years of waiting, just to find out what environment each individual finds him or herself in, and [the brain] then decides which synapses will stay and which will be eliminated.”

She finds it’s interesting that the development period for the human brain takes so long relative to other species, given the fact that for most of history and in most cultures, humans become parents and join the world of work by their mid-teens.

“I wonder if it is because we have to form such complex social societies and complex social networks, and to become completely independent from our families we need to figure out where we are in the social hierarchy and we need strong social allegiances and we need to know how to do that and all that takes time as we gradually become more autonomous and more independent.”

Research has shown how adolescents are more inclined to be focused on their peers than is the case with children or adults, something that can explain such phenomena as their being afraid to put their hands up in class or acting in ways that lead them to get into trouble with the police when out in company.

Knowing that adolescents have brains that are very different from the adult brain can be useful for parents, she says. “It gives biological explanations for some adolescent typical behaviours that otherwise don’t make sense.”

“The classic example is the intelligent, well-educated young person who knows all about the harm of smoking or illegal drug-taking and yet, when they are with their friends, they go and do these things.”

“We think of adolescents as risk-takers, taking physical risks, or health risks, or legal risks, but really they are quite risk averse when it comes to taking any kind of social risk that might result in them being excluded from their peer group. And that has a biological basis.”

This adolescent focus on peers can explain why so many adolescents who engage in criminal behaviour cease doing so when they reach adulthood, and the different nature of the adolescent brain is something that might be relevant to the issue of sentencing in the criminal justice system, says Prof Blakemore.

Another aspect of adolescence that parents might find it useful to know about has to do with sleep. The brain produces a substance called melatonin that, in children and adults, prompts the individual to start to feel sleepy at the end of the day (changes in daylight influences melatonin production). However, melatonin production occurs a few hours later in adolescents, so they have different body clocks and, classically, stay up later.

“But we make them get up and force them to go to school and that results in them walking around sleep deprived.”

A lot is now known about the important role sleep plays in facilitating the processing of recently acquired information in the brain, so having sleep-deprived students “is not a good thing”. In her 2018 book, Inventing Ourselves, the Secret Life of the Teenage Brain, Prof Blakemore suggests that school starting times might be adjusted to take account of the adolescent brain. Doing so might improve learning and motivation, and make for happier young people.

Riittakerttu Kaltiala, professor of adolescent psychiatry at Tampere University Hospital in Finland, defines adolescence as “a special developmental period between childhood and adulthood [that] starts with physical puberty and closes roughly 10 years later with the consolidation of adult personality structures.”

Different cognitive changes that occur during this period happen at different rates, or at different times, and it is the “unsynchronized” aspect of adolescence development that, she says, makes it so interesting.

“All kinds of desiring and wanting and sensation-seeking has developed quickly, but the ability to control emotions and consider in a balanced way the consequences of one’s choices, only develops later.”

The capacity for “complex abstract thinking that is characteristic of adulthood thinking and allows for the development of emotional controls and being able to [find the] balance between emotional cues and reality” – comes near the end of the process. “The slowest to develop is emotional control.”

When talking about adolescence, Prof Kaltiala makes frequent use of the phrase “identity work”, which she says is the most important aspect of adolescence.

“Adolescence is the period when an individual starts to ask, who am I, and what should I be?” Clothes, hairstyles, musical taste, can all form part of this identity exploration, something that is familiar to everyone who has been through the process.

When adolescents are exploring different identity options, Prof Kaltiala says, their identities are actually “fragmented and context-related”, so that with one group of peers, the adolescent can feel they are one kind of person, and when in another group, they can feel like they are another kind of person. This is not just a case of individuals presenting themselves in different ways when they are with different groups. Adolescents genuinely feel differently in the different contexts. And not only is this not a problem, she adds, it is the nature of adolescence.

Sometimes, Prof Kaltiala says, when adolescence strikes, and their children change, parents can grow scared. Adolescents can be “a bit like a shell that closes, and they don’t want to talk.”

But she says it is best if parents don’t lose their nerve, and continue talking to their adolescent children.