What is it with teenagers? The moodiness, the self-consciousness, the risk-taking, the tendency to be led astray by peers. Traditionally we have blamed hormones. But now scientists are beginning to find that their brains are different.

The brain develops massively in infancy, and part of the pro-cess involves a reduction in the number of synapses, the connections between neurons, or nerve cells. This is a good thing, says Sarah-Jayne Blakemore. “It’s like pruning a rose bush. You don’t want too many branches, because then no one branch can grow really strong. As you prune back it allows others to be strengthened.”

Much of this pruning is genetically programmed, but there are suggestions that environment could play a part. For example, the Japanese language does not have “r” and “l” sounds, so Japanese adults cannot differentiate between them. But Japanese babies do hear the two distinct sounds. Scientists believe that the babies can do this because the synapses that enable them to hear the distinction have not yet been pruned back through lack of use.

In the past decade neurologists have conducted MRI scans on adolescents and found that the volume of grey matter that contains synapses decreases during adolescence. They speculate that if the process is the same as in infancy then environment could be an influence on the pruning process. Blakemore points out that the science is in its infancy and is wary of leaping to conclusions. “It’s a huge statement even to speculate that the environment could affect brain development in adolescence, even though I would be confident that it could do,” she says. Already studies have made links between smoking marijuana and schizophrenia in adolescence.

In the future we may re-examine how we educate adolescents. “At the moment we focus on academic subjects, but we know that parts of the brain that are involved in non-academic things such as social understanding, self-awareness, decision-making and planning are developing. It is possible that lessons could include that kind of learning,” Blakemore says. At the very least she hopes that teenagers will be taught what is happening to their brains.

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