Teenagers' brains 'linked to recklessness'

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Teenagers may have a good excuse to be anti-social, moody, self-centred and reckless, according to new scientific insights into how the human brain develops.

A teenager's psychological flaws are typically blamed on society, bad parenting or out-of-control hormones.

But the real reason for a teenager's transformation from angel to monster mainly lies way north of the gonads.

Michael Mosley
  • Michael Mosley presents Inside the Human Body - Building Your Brain
  • Thursday 19 May, 9pm, BBC1
  • Catch up on iPlayer

During the teenage years, there are important changes going on inside the brain.

New imaging techniques have surprised scientists by showing that brains take far longer to mature than had previously been believed.

Teenage brains do not suddenly become adult brains when they get the right to vote or drive a car.

The human brain, the most complex thing in the known universe (and one that, incidentally, runs on a very economical 25 watts, the equivalent of a low energy light bulb) is not really the finished item until we reach our early 20s.

The video below allows us to see what happens to the human brain as it matures.

These images were not actually created by following a single individual over time, but by putting together a composite of about 1,000 brain scans carried out on children from the age of three through to 18.

In the video, the size of the brain does not change. Instead the colours represent activity - how densely packed that particular part of the brain is.

Red and yellow means that there are loads of active brain cells and connections in that particular section, while blue and green means there is less activity.

Now you might expect to see much more red appear over time as the brain grows along with the body. And this is certainly what happens, at least initially.

In the womb, you grew an incredible 8,000 new brain cells every second.

By the time you were born, you had all the brain cells you would ever need.

From then on, like an ambitious social climber, it was all about making new connections.

Video: National Institutes of Health

Each of the hundred billion brain cells you were born with makes, on average, 10,000 different connections. It happens so fast that the basics of the brain's architecture are laid down by the time you are six.

From birth until you hit puberty, your brain goes on growing.

And then something truly astonishing happens. Instead of making new connections, from about 12 onwards you start to lose them.

'Ruthlessly pruned'

In the video you can see what looks like a wave of blue travelling from the back of the brain (on the left) towards the front (on the right), as brain connections start to die off.

During the teenage years you lose about one per cent of your brain's grey matter every year.

This might appear to be bad news, a sign your brain is already going downhill. It is anything but.

During the teenage years, your brain is being reshaped. It is a bit like a sculptor who starts off with a great big block of marble. To create a stunning statue, the sculptor has to chip away, slowly turning the uniform block into a thing of beauty.

What is happening in the teenage years is that your brain is being de-cluttered. Unnecessary or unused connections are ruthlessly pruned.

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Whenever a teenager takes a risk, like driving a car too fast, the brain is rewarded with a hormonal rush”

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That is why the teenage years are so critical to your future development. Skills and habits laid down then are likely to persist.

This pruning process will, in time, make the teenager's brain faster and more powerful. But, significantly, the scans show the last bit of the brain to reach full maturity is the bit at the front, the prefrontal cortex.

The prefrontal cortex is the part of the brain which is responsible for things like planning, anticipating, controlling your own emotions and understanding them in others - in short, being grown up.

If you don't have a fully functioning prefrontal cortex you tend to be impulsive, insensitive to other people's feelings and take unnecessary risks.

As well as lacking some of the essential braking mechanisms on impulsive behaviour that the prefrontal cortex provides, teenage brains also seem to have the brain accelerator firmly pressed down.

Whenever a teenager takes a risk, like driving a car too fast, the brain is rewarded with a hormonal rush, a much stronger natural high than an adult would feel.

One explanation why teenagers might be wired to be reckless is that being a risk-taker also encourages us to explore the world, to try out a range of new things.

Modern brain imaging, by enabling us to see inside a teenager's brain, may make parents more sympathetic.

Once we realise that a lot of bad behaviour may be partly a product of the unfinished wiring inside teenagers' heads, perhaps we will be a little bit more forgiving.

Michael Mosley presents Inside the Human Body - Building Your Brain on BBC One at 9pm on Thursday 19 May 2011 or catch up on all 4 episodes on BBC iPlayer

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