Are adolescents really 'emotionally incontinent' or is the troubled teenager more a figment of the marketing manager's mind? Jerome Burne reports.

It has always been a topic of much frustration to parents - why are the exams on which much of their children's lives hinge timed to coincide with the period when they are at their most rebellious, and what is it that makes adolescence such a problem time anyway? For years the standard explanation has been that they are filled with raging hormones. But now it seems there is another culprit - their brains.

Researchers at Cincinnati College of Medicine recently said they had found that "pyramidal neurons" - brain cells that integrate information - were 15 per cent smaller in the brains of some 12 to 17-year-olds, compared with a few 17 to 24-year-olds. This comes at a time when the issue of adolescent brains is a topic of fierce debate in the US.

Several recent books, with titles such as Yes Your Teen Is Crazy and The Secret Life of the Brain , have picked up on research reported in Nature Neuroscience in 1999 that found that adolescent brain cells go through a sudden growth spurt, followed by a pruning of neurons as they reach 18. It is the same sort of pattern seen in newborn babies and toddlers, and it is taken to mean that intensive learning is in progress.

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What has propelled these findings into the public eye is that this is happening in the frontal cortex, the region of the brain associated with executive functions such as planning, judgement and self-control. The link is obvious. Teenagers cannot plan their homework properly because they do not have the necessary brain hardware to do so.

"The big surprise is how late very drastic brain growth takes place," says Paul Thompson, one of the neurologists who did the original research. "Teens often change personality through those years, from being emotional risk-takers to being very self-controlled. It is possible the two sets of changes match up."

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Upbringing experts have seized on the notion that there is a physical explanation for adolescents' "emotional incontinence", claiming it shows they need more parental hand-holding than is often assumed.

But not everybody is convinced. More cautious scientists have pointed out that we do not know enough to go from brain-cell activity to mental states. For instance, a brain scan cannot tell you if you are looking at the brain of someone with an IQ of 80 or 160. What the research showed was that grey matter thickens between ten and 12, peaks around puberty and then thins out in late adolescence. What the commentators usually ignored is that this process was also found to be going on in areas specialised for "visuo-spatial ability". So do teenagers need help with catching too?

The notion has been dismissed as pop psychology by cognitive neuroscientist John Bruer. "Anything neuroscientists have to say about how the maturity of the frontal lobe relates to behaviour is just speculation," he says. "We just do not know." A few years ago, Bruer very publicly laid into the "Zero to Three" movement, which claimed that failure to take advantage of this window of brain development when babies' brains are wiring up could doom your offspring to a life of unfulfilled potential. "We have to be very careful about looking for purported biological explanations for very local social and cultural phenomena we may not like," he says.

All this argument assumes is that there is something wrong with adolescents. "As every parent knows," intoned the trailers for the recent US TV documentary The Teenage Brain , "adolescence is a time of rolling emotions and poor judgement." But is it?

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Many researchers have looked at the "troubled teenager" theory and found no evidence for it. Daniel Offer, a psychiatrist at Northwestern University, in the US, followed 30,000 youths for three decades and found "no support" for the adolescent turmoil theory. "Decision-making for adolescents is no different from decision-making for adults," he concluded. All the same, from an evolutionary perspective, humans take a long time to mature compared with other primates. We have a four-year gap between sexual maturity and prime reproductive age. So what might be the advantage of this?

John McCrone, author of several books on the brain, has suggested that it is to do with our complex cultural and social lives. "Slower development of the brain," he says, "gives the plasticity that allows the human child to be stamped with the rhythms of grammatical speech and even the rhythms of culturally evolved habits of thought and behaviour." What is striking is that boys and girls follow a very different developmental path in this period. While boys become fertile and hormone-driven from about 13, their bodies stay puny and sexually unappealing until they put on muscle in their late teens. Girls, on the other hand, develop their adult shape much earlier but are not fertile until later. McCrone's explanation is that males' early sexual drive and aggression allows them to learn their adult role without being too threatening to their elders. For girls, looking like women allows them to be included in baby-care routines to learn about mothering.

But such evolutionary theorising, combined with brain-scan studies, sets alarm bells ringing. There is a long and disreputable history of finding biological underpinnings for social distinctions. It was only 100 years ago, for instance, that scientists were confidently pontificating about women's "underdeveloped brains", which resulted in an "absence of thought and an incapacity to reason". At around the same time, the "less developed posterior lobe" of African-Americans was declared responsible for their "irrationality and violent behaviour". So why now should it be adolescents whose brains condemn them to an irrational limbo?

A clue about how these brain-scan studies fit into a wider social picture comes from a review of scientific theories of teen behaviour conducted by researchers at the University of Wisconsin in 1987. They found that in times of war and employment booms, scientists pronounced adolescents as being "capable and adult-like", but during peace time and economic downturn they characterised them as "psychologically incapacitated and slow to develop".

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Maybe the simultaneous demonising and infantilising of adolescents tells us more about our society's need for good consumers than it does about the nature of our children.