Image caption Intellectual performance can both improve and deteriorate in adolescence

The mental ability of teenagers can improve or decline on a far greater scale than previously thought, according to new research.

Until now the assumption has been that intellectual capacity, as measured by IQ, stays quite static during life.

But tests conducted on teenagers at an average age of 14 and then repeated when their average age was nearly 18 found improvements - and deterioration.

The findings are published in the journal Nature.

They have implications for how pupils are assessed, and the age at which decisions about their futures are made.

This study involved 19 boys and 14 girls, all undergoing a combination of brain scans and verbal and non-verbal IQ tests in 2004 and then in 2008.

The results show that a change in verbal IQ was found in 39% of the teenagers, with 21% showing a change in "performance IQ" - a test of spatial reasoning.

The findings are seen to have greater validity because for the first time the variations in IQ correlated with changes in two particular areas of the teenagers' brains.

An increase in verbal IQ corresponded with a growth in the density of part of the left motor cortex - a region activated during speech.

And an increase in non-verbal IQ correlated with a rise in the density of the anterior cerebellum - an area associated with movements of the hand.

The work was led by Professor Cathy Price of the Wellcome Trust Centre for Neuroimaging at University College London and is published in the journal Nature.

The paper suggests that the results could be "encouraging to those whose intellectual potential may improve and… a warning that early achievers may not maintain their potential".

Media playback is unsupported on your device Media caption Professor Cathy Price explains how teenage brains and IQs can change over time

Professor Price said: "We have a tendency to assess children and determine the course of their education relatively early in life.

"But here we have shown that their intelligence is likely to be still developing.

"We have to be careful not to write off poorer performers at an early age when in fact their IQ may improve significantly given a few more years."

The research did not seek to understand the causes of the changes.

One explanation is that teenagers mature at relatively different ages - with "early" and "late" developers - while relative standards in education may play a part too.

One of the participants, Sebastian Friston, now aged 23, recorded a marked increase in IQ between the two tests - from average to one of the highest categories.

Educated in the state sector, he told me he had struggled in his early years, needing remedial maths tuition, but is now planning a doctorate in computer engineering.

"I think the change came in school I started doing subjects that really interested me, that I was engaged in, then I found it easier and far more interesting."

The research was funded by the Wellcome Trust, one of many projects supported under its programme of Understanding the Brain.

Future work may focus on how adaptable the brain may be beyond teenage years, and the implications for tackling mental diseases and other neurological conditions.