Teenage brains aren't undeveloped, they're just doing something else

August 22, 2020

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There's a trope we often see about the brains of young people. The idea that the teenage brain is undeveloped, or that the brain isn't fully developed until the age of 25. Something along these lines. It's annoying, doesn't make a lot of sense, and encourages a view of the brain that doesn't really tell us anything helpful about the brain or the behaviour of young people.


There's a trope we often see about the brains of young people. The idea that the teenage brain is undeveloped, or that the brain isn't fully developed until the age of 25. Kids are dumb because their brains aren't done. Something along these lines. It's annoying, doesn't make a lot of sense, and encourages a view of the brain that doesn't really tell us anything helpful about the brain or the behaviour of young people.

Let's start with what should be an obvious question. What does it mean for any part of the brain to be 'developed'? The 'developed' adult brain, for example, undergoes extensive changes during learning, something we should be very grateful for, or else we wouldn't be able to learn. Yet, given that the development of the brain also involves extensive changes, one must wonder what the difference is.

Cognitive decline begins well before mid-life, perhaps as early as our 20's (scroll to the figure and the para just above it). This is also associated with changes in the brain that mean we are less and less good at doing tasks that require certain kinds of thinking. Are these changes characteristic of developed brains? Developing? Regressing?

Brains _never_ mature, nor are they ever finished 'developing'.

The point of course, is that while many people discuss the 'immaturity' of the teenage brain, no one discusses the fact that the brain is a structure constantly undergoing a process of change. There is no single point of maturity, because the brain never finishes changing.

What is different about young brains?

The particular facet of brain development that people are referring to with the trope adolescent brains are not developed is specifically related to the front of the brain. A wrinked sheet of brain tissue that is pressed up against your forehead and wraps around above your eyeballs. This is called the 'frontal lobe', or the 'prefrontal cortex'. Neither of which, I should point out, are anything but a vague term that literally means 'a bit of brain near the front'. You could point to any part of your brain and call it a 'cortex'.

Young people's 'frontal' or 'prefrontal' cortex is the thing that is not as 'developed' as adult brains. So let's explore first what the prefrontal cortex is. Then we can explain how it isn't developed. Then we can talk about how this idea is the kind of simplification that doesn't help anyone understand anything; parents, teenagers, or young people alike.

A little neuroanatomy

What does the frontal cortex do? Well, the frontal cortex is the front part of the neocortex. This is the technical term for the wrinkly sheet that's wrapped around the outside of the brain. There are more brain structures inside and underneath the wrinkly sheet, but this is the one most people are familiar with because most images of the brain show the outside of it. It's also the most well-studied in humans because it's easiest to get to.

A diagram of the brain The neocortex is like a big sheet of paper, crumpled up and wrapped around other brain structures. Or rather, like two sheets of paper—one for each side of the brain. Illustration by Levent Efe

The neocortex seems to be involved in most things (as in fact most brain areas are). But the most useful summary is that it's responsible for processing input (e.g. perceptions like sound or touch) and helping turn those inputs into output (e.g. making an action happen), and in the process it takes cues from other regions of the brain.

So for example, the bit of neocortex at the back of the brain takes in visual information. At the sides, you have a strip that processes touch information and another, bigger patch that seems to generate movements. While it does this, maybe it wants to know how emotionally charged that input is, so it asks the amygdala. Or perhaps which of two perceptions are more valuable so it asks the basal ganglia.

An obvious over-simplification, but it's good enough to explain my next point.

The frontal lobe is a mystery

For the longest time we've thought that the frontal cortex was responsible for what are typically called executive functions. These are things the brain does to control other processes in the brain. For example, when you're texting and driving, you can't do both things at once. You only have one pair of eyes. That's what makes it so dangerous. The brain has to be doing something that flicks your attention to the phone, then to the road, then back again. Or the psychology classic, the Stroop task. Reading the word 'red' is much easier than naming the colour of the ink of this word: blue. You want to read the word blue, but your brain is overriding that so you can name the colour 'red'.

We think the frontal cortex is responsible for this because parts of the frontal cortex are active during brain scanning when people are doing these kinds of tasks. The problem is:

  1. although we know that something like executive functions must exist because of things like attention switching and the Stroop task, we don't actually have any idea what they are; and
  2. the frontal cortex is involved in most things, and other parts of the brain are also active during 'executive function tasks'. As such, there's still plenty of debate as to the actual role of the front part of the brain.

The upshot is that we have called the prefrontal cortex the part of the brain responsible for 'controlling' the rest of the brain for ages, while actually not truly knowing what it does.

The orbitofrontal cortex is the part that's throwing everyone off

The orbitofrontal cortex is the part of the brain just above your eyes. The eye cavities are 'orbits', hence 'orbitofrontal'. It's also called the ventromedial prefrontal cortex though, because like I said before naming bits of brain is a usually vague and haphazard process. If you're ever in the habit of reading neuroimaging papers and have time on your hands, look up the coordinates of any brain region. You'll notice that the same region's coordinates differ from paper to paper. Very annoying.

That said, if you damage the bit of brain above your eyes, you become an animal. You'll say and do profane or vulgar stuff and engage in sexual behaviours more frequently. You appear to become more impulsive, you see. Like you don't have any self-control. The most enduring example of this is that of Phineas Gage. This poor man graces the lectures of any first year psychology course. He took a huge metal rod in through the eye and out the top of his head and survived. Afterwards, as noted by researchers at the time:

he was gross, profane, coarse, and vulgar, to such a degree that his society was intolerable to decent people

Between these alarming behavioural changes associated with damage to the orbitofrontal cortex and the role of the rest of the front of the brain in 'controlling the rest', you end up with this idea that the frontal lobe is the key to all the smart, rational behaviour of humans. If the frontal cortex isn't developed in young people, then this handily explains why they're impulsive little idiots, right? Obviously not, or we wouldn't be here.

The 'front' of the brain is often thought of as the part responsible for intelligent rational behaviour. But this is the result of two quasi-mythological conclusions about that part of the brain.

A bit of truth

Firstly, Phineas Gage, while "gross, course, and vulgar" shortly after his injury, at a minimum became normal enough later on and at a maximum wasn't even all that different in the first place. Both things are explainable. The former because, like I mentioned before, adult brains are also in the habit of developing, and after injury may use other uninjured parts of the brain to do the things that got wiped out. The latter, because we like to hype things up. Like for example, how undeveloped teenagers are.

A huge amount of damage to any region of your brain is going to make you behave like an animal. Damage to smaller parts has very confusing and unpredictable results. The orbitofrontal cortex does seem to play a role in helping identify what contexts are appropriate for which behaviours. In particular, something to do with determining what contexts reward some behaviours and punish others. But beyond that, it's as much of a mystery as the rest of the frontal lobe. Note this very anaemic three line Wikipedia entry.

The frontal lobe is probably involved in at least some of whatever executive functions might turn out to be. But so are many other regions of the brain. The research group I work in currently is interested, for example, in the 'multiple-demand cortex'. This is a collection of regions spread all over the brain that also appear to be involved in executive functions.

The front of the brain isn't the smart bit. The brain is the smart bit.

Development of the frontal cortex in young people

So, what part of the prefrontal cortex is 'under-developed' in the young? Well it turns out that they have less white matter than adults over the age of 25 or so. The brain consists largely of two kinds of tissue. Grey matter and white matter. Grey matter refers to the actual neurons: cells that send signals and in doing so, process information. White matter fits all around the grey matter and takes care of the grey matter. It protects it, repairs it, and improves its performance.

If young people had less grey matter than adults, the claim that young people were less developed would be a little more understandable. Less grey matter means less ability to process information. The density of grey matter in humans is thought to be one of the reasons we are so different from other animals.

But that isn't the case. They have less white matter. In actual fact, we know very little about white matter (again, to illustrate how little I'll send you to Wikipedia for a two line summary). It has been mostly neglected for the history of brain science. Recent research has commenced investigating the extent of the role of white matter in brain function and it may turn out to be more important in function that we typically suspect.

However, until we determine that, we are left with what we do know. White matter certainly improves performance. It can speed up signals in the brain and contribute to the precision of those signals. This means faster and more accurate information processing. But there's a cost to this.

Young people have less 'white matter', although the same amount of neurons as older people. White matter improves the speed of the signals between neurons, but the brain becomes less flexible.

One of the reasons for the boost in performance is that white matter acts as a kind of insulation for grey matter. White matter cells wrap themselves around the connections between neurons like insulation. This means the signals passed along the connections don't leak, and move faster. White matter also cleans up the communications. When a signal gets to the end of a connection between one neuron and another, it has to jump across a little gap. The white matter cells keep this gap clean and uncluttered. Finally, white matter appears to help integrate signals across longer distances. A neuron can only signal to its neighbours, but white matter can help tell neurons that aren't nearby what's going on.

This is all wonderful for speed and performance. But it means that these neurons, now surrounded by all this construction, can't change their connections very easily anymore. It means these neurons are quicker at doing something, but much less good at doing other stuff. They are less flexible.

Language as a case study

This is what learning does to the brain. The more you engage in a particular activity, the more white matter grows around those neurons. Consider language learning. If you learn a language when you're young, you can take on the accent. The older you get, the less likely you are to be able to take on the accent. This is (at least in part) because your brain practiced speaking in a particular kind of way. White matter grew to improve performance. Now, when you try to speak a different way, your neurons can't create new connections to move your mouth parts around in an unfamiliar way. You can read fine, understand fine, and even say the words fine. But you can't quite say the words in the right way.

It also explains findings that the later you learn a language, the more likely it is that you'll have a different part of the brain involved for the new language than for the old language. This is as opposed to younger people, who have a similar region of the brain handling both. The brain finds it easier to assign the new kind of input and output to a more flexible region of the brain, rather than integrate it with the old built up part. Like building a house in the country, rather than finding space in the city.

Younger brains aren't less developed, they're just doing a different thing

We could say something like 'young people are still learning'. That's why they're idiots. But that's also probably not sufficient. Rather, what's happening is that the brain is preferring to maximise flexibility, rather than emphasising rigid forms of behaviour. We know that it's not 'just learning' because the brain cycles through these two stages—flexibility and rigidity—a couple of times. From the age of about two to the age of about seven the brain is growing as many connections between neurons as possible. Then it begins cleaning up these connections and deleting the unused ones. If it wanted, it could just go ahead and finish up. Seven years old, you're done, let's build all the white matter. But it doesn't. It slows down the clean up, and doesn't start again in earnest until between 20-30 years of age. There is something very deliberate about it.

We could have a crack at some explanations for this. The most straightforward would be something like, evolution prefers our young people to be out doing things and older people are more useful for settling in to make babies. So, young brains are more flexible to adapt to changing environments. Older brains are more rigid because they need to do fewer things but better. This might explain the findings that younger people are more likely to engage in risky behaviour.

Another could concern the fact that people greatly prefer familiarity. Older people have discovered what kinds of behaviour are necessary to maximise rewards and minimise punishments, and so they overutilise those behaviours. This encourages white matter growth, as these things become more practiced. Younger people are still working out what behaviours are most efficient. This could explain the apparent 'reward focus' of young people.

In fact there could be any number of likely explanations, and any number that are actually true. But what they should all emphasise to us is that young people are not underdeveloped. They are just doing a different thing. They're being more flexible. And for whatever reason, that's handy.

We might focus on the aspects of this flexibility that are harmful. But that is only one, misleading aspect of the phenomenon. If you truly want to understand young people, all you need to know is that they aren't mercurial and volatile. They're just more likely to try new things.

When I was young?—Ah, woful When! Ah! for the change 'twixt Now and Then! This breathing house not built with hands, This body that does me grievous wrong, O'er aery cliffs and glittering sands, How lightly then it flashed along

Coleridge, Youth and Age

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