London, May 20 (IANS): Teenagers with lower cognitive ability run higher risk of developing depression 20 years later, new research has found.

The results add new evidence that lower mental ability may be a contributor to depression, rather than a consequence of it.

"Adolescents with slower processing speed may be at increased later risk of anxiety and depression," said researcher Catharine Gale from the Edinburgh University in Scotland, Britain.

In the study, teenagers with slower performance on a test of "cognitive processing speed" were found to have depression and anxiety symptoms as adults.

The researchers analysed data from 705 Scottish participants in a study including follow-up from adolescence into adulthood.

At age 16, the participants were evaluated on a simple test of cognitive processing speed -- reaction time in pressing keys corresponding to numbers (one to four) flashed on a screen.

At age 36, the participants completed standard questionnaires assessing depression and anxiety symptoms.

The relationship between reaction time in adolescence and mental health in adulthood was assessed, with adjustment for a wide range of other factors (education, lifestyle habits, etc).

Slower cognitive processing speed--that is, longer reaction time -- at age 16 was associated with increased anxiety and depression symptoms at age 36.

The findings appeared in the journal Psychosomatic Medicine: Journal of Biobehavioral Medicine.