The test can be done on a computer and could be used to alert doctors and psychologists to intervene early, said Barbara Sahakian a professor of clinical neuropsychology at Britain's Cambridge University.

Ian Goodyer, a child and adolescent psychiatrist who worked with Sahakian on a study published on Wednesday, said screening 11- to 12-year-old children could reveal those who have "low resilience" - putting them at higher risk of developing mental illnesses such as depression.

Mental health problems are common in young people. Some 10 percent of children aged between five and 16 in Britain are assessed as having a mental disorder of some kind.

Adolescence is also a critical period for the development of major depression - an illness that exacts a heavy toll on people and economies worldwide with patients unable to hold down jobs or needing repeated long stretches of time off work.

The World Health Organization says more than 350 million people worldwide have depression and predicts that by 2020, the disorder will rival heart disease as the illness with the highest global disease burden.

Sahakian said testing children at school age could help health authorities get in early and offer therapy to prevent people descending into more serious, hard to treat conditions.

DETECT RISK EARLY

"When you think that the burden of mental illness is more than cancer, more than heart disease - so why on earth don't we try to do something more proactive," Sahakian told Reuters after presenting her results at a briefing in London. "Why are we not doing anything to pick it up early? To me it's a no-brainer".

Goodyer's and Sahakian's test, details of which were published in the PLOS ONE journal, involves a computer assessment designed to gauge how teenagers process emotional information. It includes asking whether certain words, such as "joyful", "failure" or "range", are positive, negative or neutral.

For their initial study, 15- to 18-year-olds also underwent genetic testing - an exercise that would be too expensive for routine use but which validated a connection between genes and upbringing in determining mental health risks.

The researchers found that adolescents who had a variation of a certain gene linked to the brain chemical serotonin and who had also experienced regular family arguments and parental rows for longer than six months before the age of six, had significant difficulty evaluating the emotion in the words.

This, said Goodyer, suggested those children suffered from an inability to process emotional information - a factor which previous studies have established is linked to a significantly increased risk of depression and anxiety.

"The evidence is that both our genes and our early childhood experiences contribute," said Goodyer. "Before there are any clinical symptoms of depression or anxiety, this test reveals a deficient ability to ... perceive emotion processes ... which may lead to mental illnesses."

Experts are concerned about the early onset of mental disorders - a factor they say many policy makers and members of the public have not yet grasped.

Hans Ulrich Wittchen, a psychologist at Germany's University of Dresden, said in a major European study of mental illness last year that he too thought governments should consider screening adolescents to try to reduce the number who go on to suffer major and recurring bouts of depression.

But other mental health experts advised caution.

"Early screening in the service of early intervention to try to prevent later mental health problems undoubtedly has allure," said Felicity Callard of London's Institute of Psychiatry.

"But to grow up with the knowledge that you are 'at high risk' of future mental health problems can affect the very way in which you grow up - and thereby ... embed a sense that you are mentally vulnerable, with potentially untoward consequences."

(Editing by Alison Williams)

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