Scientists have discovered that the brain's centre of reasoning is among the last areas to mature.
The finding, by a team at the US National Institute of Mental Health, may help to explain why teenagers often seem to be so unreasonable.
Researchers used imaging techniques to show "higher order" brain areas do not develop fully until young adulthood.
The research is published in the journal Proceedings of the National Academy of Sciences.
The decade-long study used magnetic resonance imaging to follow the development of the brains of 13 health children every two years as they grew up.
Dr Judith Rapoport
The researchers found that grey matter - the working tissue of the brain's cortex - diminishes in a back-to-front wave over time.
They believe this is a key part of the maturation process, whereby unused and unneeded connections between brain cells are gradually destroyed.
They found the first areas to mature were those with the most basic functions, such as processing the senses and movement.
Next came areas, such as the parietal lobes, involved in spatial orientation and language.
Last to mature were areas such as the prefrontal cortex with more advanced functions such as integrating information from the senses and reasoning.
The sequence of maturation seen by the researchers in the developing brain roughly parallels the evolution of the brain from lower order mammals to the highly complex organ that is found in man.
For instance, the prefrontal cortex emerged late in evolution and is among the last to mature.
Researcher Dr Judith Rapoport told BBC News Online: "Maturation starts with more basic facilities such as vision and hearing and then goes on to the ability to integrate and organise many inputs, to weigh consequences of behaviours and to relate to others.
"It is a smart sequence in terms of evolution and individual development."
It has long been thought that the brain produces too much grey matter during the first 18 months of life, and that this is followed by a steady decline as unused circuitry is discarded.
Several years ago the NIMH team discovered a second wave of grey matter over-production just prior to puberty, followed by a second bout of "use-it-or-lose-it" pruning during the teen years.
In a previous study, the same team also found that teenagers who became psychotic prior to puberty lost four times the normal amount of grey matter in their frontal lobes.
This, they argued, suggested that childhood onset schizophrenia may be due to an exaggeration of the normal maturation process, possibly leading to the destruction of potentially useful brain circuits.
By contrast, autism has been associated with an increase, rather than the normal decrease, in grey matter.
Published: 2004/05/18 11:04:33 GMT
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