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When the hormones start to
arrive by the truckload at puberty, something very strange happens to
children. They can turn overnight from sweet, adorable creatures into an
unpredictable and combustible blend of know-it-all arrogance and furious leave-me-alone
vulnerability. They are spotty, moody, truculent and can’t concentrate for
more than two minutes at a time. They also become hugely self-conscious,
suddenly finding everything, including their parents “sooo embarrassing”.
And there is a darker side
too. Soaring rates of death, three quarters of which result from accidents
or “misadventure”, illicit use of drugs or alcohol, risky sexual behaviours
and the first signs of emotional disorders which may be lifelong. Hormones
have a lot to answer for — or have they? Puberty is undoubtedly an
extraordinary hormonal event and humans are lucky that they on have to go
through it only once, unlike most animals which go through this hormonal
onslaught with every breeding season.
The first hormone event takes
place, unseen, between age 6 and 8 and involves the adrenal glands, which
sit atop each kidney. They step up production of male hormones,
particularly one called DHEA, which the body uses as construction material
for other hormones. These androgens prime follicles for pubic hair growth
and make the skin greasy.
The next big step is when the
brain begins production of a key hormone called GnRH (gonadotrophin
releasing hormone). This is the true onset of puberty, although what
triggers it is unknown. It’s not just age because age at puberty varies
worldwide. Nutritional status is important, with percentage body fat
especially so for girls. Pulses of GnRH then make the pituitary gland
produce the hormones which will act on ovary and testes to produce sperm
and eggs.
The effect is dramatic. In
boys, up to 50 times more testosterone is available than before puberty. It
sculpts their bodies and jawlines, increases their muscles and makes them
think about sex every other minute (as little as that? is the reaction of
most 13-year-old boys). In girls, oestrogen rearranges body fat, and
stimulates the growth of womb and breast. They begin to have periods and to
ovulate, although very irregularly at first. In both sexes, body-hair
growth is promoted.
A range of teen traits is
directly influenced by hormones. Spots, for instance, are caused by skin
sensitivity to testosterone. Fridge-raiding is caused by higher levels of
the hormone cortisol, which sharpens the appetite and makes adolescents
seek the food that they need for growth. Not getting up until lunchtime is
caused by alterations in the secretion of melatonin (see page 11).
We can see what surges of
reproductive hormones do to rutting stags or nesting birds in the mating
season, so there’s no doubt that these hormones can affect behaviour, but
they have never seemed adequately to explain the complexities of human
teenage behaviour. Neither has anyone managed to correlate degree of
teenage angst with hormone levels. But recently a whole new explanation has
emerged.
It was always thought that
the brain stopped developing within a couple of years of birth. During
pregnancy and early life, a huge number of nerve connections (synapses) are
formed, but these are then pruned radically. “It’s a way of making the
brain more efficient,” says Dr Sarah-Jayne Blakemore, a research fellow at
the Institute of Cognitive Neuroscience at University College London and an
expert on the adolescent brain. She gives the example that, worldwide, all
babies can distinguish the difference between the sound of the letters R
and L. In the Japanese language however there is no difference and, after
about a year, Japanese babies lose the ability to distinguish these sounds.
They don’t need it.
This example relates to the
sensory areas of the brain and was long assumed to be true of the entire
brain — that “plasticity” as it is called, was lost by about 3 years old.
But post-mortem work in the Eighties on adolescent brains suggested
something very different. It wasn’t confirmed until just a few years ago,
when MRI scans of adolescent brains revealed the stunning truth. Not only
is there major reorganisation in the teenage brain but it continues to
develop until the early twenties.
Puberty coincides with two
brain events. A process called myelination, which massively increases brain
activity. There is also a pruning exercise among synapses which have
proliferated during childhood as the brain is fine-tuned in response to the
environment: strengthening synapses used frequently, ditching the rest. The
pruning takes place mainly in the pre-frontal cortex. This part of the
brain is responsible for executive action — a shopping list of the things
that teenagers struggle with: priority setting, impulse inhibition,
planning and organisation.
The changes in the adolescent
brain primarily affect motivation and emotion, which manifest themselves as
mood swings and conflict with authority. The combination of a hormone such
as testosterone, which drives bravado, with an impaired ability to reason, is
an explosive one.
The pre-frontal cortex is
also responsible for our self-identity and for socialisation and empathy.
Research has already shown that one effect of this brain reorganisation is
a 20 per cent dip at puberty in the ability to gauge emotions from faces.
This is likely to make teenagers less able to read social situations or
recognise when they are treading on dangerously thin ice with authority
figures.
Dr Blakemore is currently
researching empathy in teenagers, and her work suggests that this also
seems to dip at puberty. “It would mean they are less able to put
themselves in other people’s shoes and imagine how they feel.”
One aspect of teenage brains
is that they get a bigger reward from nicotine and alcohol than adults. As
a result, those who begin drinking before the age of 15 are four times more
likely to develop alcohol dependence than than those who begin drinking
later.
Teenagers are by turns
maddening and glorious. But, as they are caught in limbo between adult and
child, we should treasure and understand them.
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