This autumn, the elderly judges of the United States Supreme Court will don their spectacles and ponder the issue of adolescence. The 80-year-old chief justice and his eight colleagues must decide whether 16 and 17-year-olds should be exempt from the death penalty. Those fighting to save teenagers condemned to the electric chair cite the gathering mass of scientific evidence about the brain.

“Our objection to the death penalty is rooted in the fact that adolescents’

brains function in fundamentally different ways than adult brains,” says David Fassler, a university psychiatrist determined that the lawyers should listen to the scientists.

“Teens have increased difficulty making mature decisions and understanding the consequences of their actions.”

So says the American Bar Association, which supports abolishing the death penalty. No surprise there, but the big question is why? Just what is going on inside a teenager’s head? Whatever it is, it isn’t finished.

Scientists once thought our brains were mature by the age of 12. Certainly Jean Piaget, the pioneering guru of child development, believed children reached cognitive maturity by about around that age - or not at all.

But thanks to Dr Jay Giedd, an American scientist who has been scanning the brains of hundreds of children for more than a decade, we now know that is not the case. Between the ages of six and 12, a process undergone in babyhood is repeated. Once again, great quantities of electrical connections are produced. Synapses transmit messages between brain and body and in the brain itself, and we need a lot of them. There are about 100 trillion inside every two-year-old’s head.

By five, many of these connections are being shed as the brain adjusts to its world. This loss, known as “pruning”, is crucial to a child’s development, say neuroscientists, because it gets rid of superfluous mental wiring, allowing youngsters to acquire specific knowledge, such as language.

Dr Giedd has found that, just as in babies, synapse-building is followed by pruning, a mental restructuring that lasts at least until 20. The “neural secateurs” are working flat out on an adolescent brain that is usually floundering amid a flood of sex hormones.

But how does this manifest itself in the average teenager? Many neuroscientists dislike linking what they deduce from their magnetic resonance imaging (MRI) scans with the 13-year-old lounging sulkily on the sofa. But Professor Robert McGivern of San Diego state university has said that this brain remodelling makes it harder for adolescents to process basic information, rendering them socially and emotionally clumsy.

In 2002, Professor McGivern asked 250 people aged between 10 and 22 to identify the feelings on pictures of faces. The slowest were the 12-year-olds, who answered the questions at about one-fifth of the speed managed by 10-year-olds. Only by about age 15 were response speeds back up to those achieved by the younger children.

Professor McGivern believes that this difficulty in reading facial expression could help to explain why teenagers often think things are stacked against them, that life is unfair. His work is backed up by that of Harvard neuropsychologist Deborah Yurgelun-Todd, who found that young adolescents often misread images of fear as ones of anger, confusion and sadness. As a result, they have an unfortunate talent for seeing hostility where none exists.

Other teenage talents - that challenging mix of high emotion, apathy and recklessness - can also be partly explained by brain science. Remember the next time you encounter a chaotic, impulsive and weepy adolescent - let’s call her Jenni - that her mental personal organiser is not yet fully installed.

Neuroscientists say that the brain develops from back to front. The bits right at the front behind your eyebrows - the last to mature - are your frontal cortices. One of these, the prefrontal cortex, is sometimes called the “area of sober second thought”. Research on people with brain damage in this area shows that it is involved in organising, prioritising, controlling and expressing emotion, concentration, and weighing up the consequences of actions.

To put it crudely, the brain operates on a use-it-or-lose-it principle.

Connections that Jenni uses regularly are strengthened, the neglected ones pruned. So how the girl’s frontal cortices are wired up is likely to be influenced by what she is doing and thinking. A top priority for the 13-year-old is who her best mates are and what they think of her. She spends quite a lot of time in bed pondering who said what to whom and why.

John Geake, professor of education at Oxford Brookes university, says that for adolescents, those thought processes and that kind of logic are often more engaging than what they do at school.

“Obviously, there are exceptions,” he says. “Lots of kids really blossom in maths, chess and reading. They can get their logical priming from the school curriculum. But for many kids the main priming comes from their social lives, which may explain why social life and peer group become so important.”

Lurking deep behind our eyebrows is the cingulate cortex, connected to the limbic region, the brain’s feeling and emotional part.

“Broadly, this inhibits most of the impulses we get from our emotion centres. If it wasn’t for our cingulate cortex we’d be in a mess,” says Professor Geake, who is also convenor of the Oxford Cognitive Neuroscience and Education Forum.

“We know that when you get drunk that depresses that inhibitory function, and it could be speculated that the sudden surge of testosterone that comes with puberty may also depress that inhibitory function through elevating levels of dopamine, one of the brain’s feel-good chemicals. So you see a growth in loud talking, skylarking on buses, being obnoxious in public places.”

More evidence for the emotional skyrocket of adolescence comes from Deborah Yurgelun-Todd at Harvard. When she asked under 14s to identify feelings in images, they relied on a part of the brain which is home to primal emotions such as fear and rage. Called the amygdala, it has been described as a “general-purpose defence-response control network”. Adults doing the same task used their prefrontal cortex. Similarly, the over-20s were able to enlist their mature prefrontal cortices when taking part in research on motivation. They were more prepared to seek rewards than the adolescents, who found it very hard to be bothered.

So, young teenagers are dealing with a developing brain and body. Some scientists now believe that humans evolved an adolescence. Dental evidence suggests that prehistoric man did not go through this long and painful transition to adulthood. They propose that adolescence occurred to give young humans more time to learn how to use their brains. As neurons connect and disconnect, and hormones ebb and flow, maybe teachers will evolve even greater stores of patience. The alternative, says John Geake, is a good bottle of wine on Friday night.