Decisions and the teenage brain

20 April 2020

Interview with 

Professor Sarah-Jayne Blakemore, Cambridge University




Once we’re born into the world, we grow into youngsters and early adults, and as teenagers we start making an awful lot of decisions about the person we’re going to be. Sarah-Jayne Blakemore is a Cambridge University psychology professor and an expert in the teenage brain, and she spoke to Katie Haylor.

Sarah-Jayne - So there's a lot of change across the entire brain throughout adolescence. The cortex, which is the surface of the brain, undergoes very substantial changes in, for example, the volume of grey matter it contains. We know that in childhood gray matter volume increases and then it peaks in late childhood or early adolescence, around 9, 10 years and then it undergoes a really substantial decline throughout the whole of adolescence and only starts to stabilise in the mid twenties. And at the same time, the amount of white matter in the brain increases linearly throughout childhood, adolescence and even into the twenties and thirties.

What we know less about is what cellular processes underlie these structural changes, but we can make educated guesses based on research on animals and also on postmortem human brain tissue. And we know from that research that a whole host of neurodevelopmental processes are going on throughout childhood and adolescence, including the fact that axons - the long fibers that connect up neurons in the brain - grow in diameter and become myelinated. That is they have a fatty substance added to them. And that increases the amount of white matter in the brain. And at the same time it results in a decrease in grey matter. Another neurodevelopmental process that occurs during adolescence is a reorganisation of synapses. That is the connections between neurons. And we know that synapses hugely expand in number during childhood. And then what happens is that the excess synapses get pruned away during adolescence.

And the really interesting thing about synaptic pruning is that synapses that are being used in a particular environment or the synapses that remain and grow stronger. While synapses that are not being used in a particular environment are the synapses that get pruned away. So in that way, brain development is partly dependent on the environment that you're growing up in.

Katie - It sounds like there's a fantastic amount of change that occurs during these years. Do these changes impacts our decision making behaviour?

Sarah-Jayne - Yeah, so at the same time we know for example that the ability to plan actions, to inhibit inappropriate responses, to remember things, also to take other people's perspectives, and certain forms of self-awareness are all undergoing quite a lot of change and development during adolescence as well. What we don't know is how much that's related to the changes in the brain. We assume that these changes in behaviour are related to the changes in the brain, particularly because the brain regions that undergo the most substantial and protracted changes are in areas like the prefrontal cortex and the inferior parietal cortex that are known to be involved in high level cognitive processes like decision making and planning.

Katie - There's a stereotype that teens make risky decisions. Is this just a stereotype or is there developmental stuff going on to back this up?

Sarah-Jayne - Well, is it a bit of a stereotype in that you can't really generalise about teenagers, just like you can't really generalize about humans. On top of that stereotype, there is evidence to suggest that risk-taking is heightened in adolescence and adolescents do show an increased propensity to take risks. We often worry about risks, the risks that adolescents might take, and that's completely justified worry because sometimes those risks can be dangerous. On the other hand, we learn by trial and error. We learn by taking risks, we explore our environment by taking risks and ultimately we become independent adults by forging our own way through our adolescence and making our own decisions and taking risks along the way. Of course it has to be constrained and we have to educate young people about the potential negative consequences of risk taking. But often it's not a negative thing.

Katie - How significant is the social influences of what your friends' are doing, in this conversation?

Sarah-Jayne - Adolescent risk-taking often depends on the context of the decision making, and adolescents are far more likely to take risks when they're with their friends compared to when they're on their own. So if you think about the risks like smoking and drinking and um, taking drugs or even dangerous driving, those are risks that even adolescent takes them, they're much more likely to take them when they're with their peers than when they're by themselves. We know from lots of studies in labs and also from real life data that risk-taking increases in adolescence when they're with their friends compared with when they're on their own.

Katie - When you say "adolescent", in my head, I'm thinking teenager, I'm thinking 13 to 19. Is that what adolescence is?

Sarah-Jayne - Actually, well the definition of adolescence is not clear cut, but the most recent agreed upon definition is the period of life between 10 and 24 years. So really a very long period in humans! And this definition was developed a couple of years ago by adolescent scientists in Australia at Susan Sawyer and George Patton. And it's partly based on the new knowledge about how the brain develops so substantially across that entire period of life between 10 and 24. And that's the definition of adolescence that I now go with and most of my colleagues do.

Katie - Blimey! So as an expert in the teenage brain then, how arbitrary do you feel the decision is that we are legally an adult at 18 then?

Sarah-Jayne - All these age cutoffs like you know, whatever it might be. The age of criminal responsibility, which by the way is 10 in this country in England and Wales - much younger than most other European countries - age of consent, age of legal smoking, drinking and of course voting and legal adulthood, these are all really quite arbitrarily chosen. Most of these age cutoffs have not been based on what we know about brain development, because they were decided way before we knew anything about how the brain develops during adolescence. So what I would say is that those kinds of decisions about age cutoffs should incorporate the new knowledge about brain development during adolescence.

On the other hand, this is a question I'm asked often, I don't think the neuroscience can provide an age for you. We can't say, "Oh, the neuroscience shows that the brain becomes adult at age 18 or 24" or whatever it might be. It's much more complex than that. I mean, first of all, different brain regions develop at different rates and mature at different rates. So there are big individual differences in the speed of brain development and when things start to stabilise. So what I would say is that what we know from neuroscience is the kind of age range, the very broad age range when the brain becomes mature and adult. And that's much later than 18, between 20s and 30s for most people. So of course that cannot generate an age at which you become legally adult.

Brain development is not, you know, the only factor when considering things like rational decision making and adulthood. One area of discussion is should the voting age be reduced to age 16? And actually there, brain development - it’s relevant, but it’s not critical. What is critical is more psychological factors and ethical factors. Actually, there isn't much in the cognitive development literature that I know of that would argue against voting age being reduced to 16.

Katie - Are there significant gender differences in decision making through adolescence? Because from what I understand, boys can develop a little bit later than girls. Does this have an impact?

Sarah-Jayne - Um, yes. Well boys go through puberty later, on average, later than girls, probably about 18 months on average later than girls. And there is some evidence that the big changes in sex hormones and physical maturity that we all go through in our early adolescence is partly responsible for some of the changes in the brain, rather than your chronological age, how many years you've been alive. And so in some ways you might expect boys, especially in early adolescence, to be developing slightly later than girls. However, interestingly, the evidence is really unclear. Although initially when the first studies of brain development during adolescence were published, there seemed to be some evidence that boys' brains developed a little bit later than girls' brains, actually more recent evidence hasn't replicated that finding. More recent studies with more sophisticated analysis that takes into account, for example, overall brain size and overall cranial size. Once that's taken into account, the gender differences between brain development seem to be much reduced. So actually there isn't really clear cut evidence for gender differences in brain development in adolescence.


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