June 12, 2008 -- It gets you good grades at school, improves your prospects at work and makes you more attractive to the opposite sex.

And as if all that weren't enough, being brainy could add 15 years to your life, a study suggests.

Researchers found those with rogue copies of a gene linked to intelligence are unlikely to survive beyond the age of 85.

However, those blessed with good versions of the gene could live to be 100.

The gene governs an enzyme which destroys a chemical known to dampen brain activity and cause drowsiness.

The enzyme - succinic semialdehyde dehydrogenase, or SSADH - also detoxifies the brain by getting rid of excess acid, protecting cells from damage which can accelerate ageing.

The gene comes in two common forms, with the so-called 'T' version working 20 per cent less well than the 'C' variety. Previous studies have shown that men and women with two copies of the 'T' version do less well in IQ tests than others.

But only now have scientists made the link to life expectancy, this week's New Scientist reports.

The Italian researchers based their findings on a study of 115 elderly men and women.

They were asked to complete a test of memory and mental dexterity more usually used to diagnose Alzheimer's disease. The volunteers, who were aged between 65 and 85, had given blood samples to determine which version of the gene they were carrying.

Those carrying two copies of the rogue 'T' version of the gene performed significantly worse than the others, echoing the results of the earlier research on young people.

The University of Calabria researchers then used data on how common the versions of the gene were across different age ranges to work out if it was linked with longevity.

This showed those carrying two 'T' variants were unlikely to live past 85 - but those with at least one 'C' could expect to live to up to 100.

The scientists said that having a double dose of the rogue gene appeared to make people more vulnerable to mental decline in old age, as well as affecting their survival.

But they stressed that genes do not tell the whole story.

Professor Giuseppe Passarino said: 'There's no doubt lifestyle such as reading, having challenging work and enriching your cultural life is far more important than having the bad variant.'

Professor Robert Plomin, of the Institute of Psychiatry in London, who in 2004 linked the gene to IQ, said: 'Although the sample size is small...the reported associations with cognitive ability are significant and in line with our previous results.'

He added that further studies were needed to confirm the link with longevity.