The tendency of more intelligent people to live longer has been shown, for the first time, to be mainly down to their genes by new research published in the International Journal of Epidemiology today.

By analysing data from twins, researchers found that 95 per cent of the link between intelligence and lifespan is genetic.

They found that, within twin pairs, the brighter twin tends to live longer than the less bright twin and this was much more pronounced in fraternal (non identical) twins than in identical twins.

Studies that compare genetically identical twins with fraternal twins -- who only share half of their twin's DNA -- help distinguish the effects of genes from the effects of shared environmental factors such as housing, schooling and childhood nutrition.

Rosalind Arden, a research associate at the London School of Economics and Political Science (LSE), said: "We know that children who score higher in IQ-type tests are prone to living longer. Also, people at the top of an employment hierarchy, such as senior civil servants, tend to be long-lived. But, in both cases, we have not understood why.

"Our research shows that the link between intelligence and longer life is mostly genetic. So, to the extent that being smarter plays a role in doing a top job, the association between top jobs and longer lifespans is more a result of genes than having a big desk.

"However, it's important to emphasise that the association between intelligence and lifespan is small. So you can't, for example, deduce your child's likely lifespan from how he or she does in their exams this summer."

The researchers looked at three different twin studies from Sweden, the United States and Denmark where both intelligence and age of death was recorded, and where at least one twin in each pair had died. Only twins of the same sex were included in the analysis.

On the reasons for the findings, Rosalind Arden said: "It could be that people whose genes make them brighter also have genes for a healthy body. Or intelligence and lifespan may both be sensitive to overall mutations, with people with fewer genetic mutations being more intelligent and living longer. We need to continue to test these ideas to understand what processes are in play."

This is the first study to test for a genetic association between intelligence and lifespan.