Scientists have used brain scans to map the changes that occur in teenagers.
The research could help explain why the first signs of mental health problems emerge in adolescence, according to experts from the University of Cambridge.
Nearly 300 14-24 year-olds had MRI scans to study the structure of their brains during a study published on Tuesday in the Proceedings of the National Academy of Sciences.
Professor Ed Bullmore, head of psychiatry at Cambridge said: "Adolescence can be a difficult transitional period and it's when we typically see the first signs of mental health disorders such as schizophrenia and depression.
"This study gives us a clue why this is the case: it's during these teenage years that those brain regions that have the strongest link to the schizophrenia risk genes are developing most rapidly."
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By comparing the brains of teenagers of different ages, the researchers found that the outer regions of the brain shrink in size, becoming thinner.
As this happens, levels of myelin - the sheath that "insulates" nerve fibres, allowing them to communicate efficiently - increase within the outer regions
Previously, myelin was thought mainly to reside in the "white matter", the tissue that connects areas of the brain, but this study shows that it can also be found in the outer regions and that levels increase during the teenage years.
Dr Kirstie Whitaker from the department of psychiatry at the University of Cambridge, the study's first author, said: "During our teenage years, our brains continue to develop.
"When we're still children, these changes may be more dramatic, but in adolescence we see that the changes refine the detail.
"The hubs that connect different regions are becoming set in place as the most important connections strengthen. We believe this is where we are seeing myelin increasing in adolescence."
