Sydney: Genetic factors only partially affect our lifelong intelligence levels, while environmental causes seem to exert the largest influence.

In a number of studies since early 2000, researchers have shown that when people took intelligence tests as children and then again in old age they tended to keep about the same relative score.

However, there was also some change: some who did well early on went down a bit, and some who scored poorly as children did better in old age. The researchers are keen to understand what drives these changes in lifetime cognitive ageing.

"Identifying genetic influences on intelligence could help us to understand the relationship between knowledge and problem solving and an individual's outcomes in life, and especially to understand why some people age better than others in terms of intelligence," said study co-author Peter Visscher.

"We excluded people with dementia," added Visscher, professor at the Queensland Brain Institute and the University of Queensland Diamantina Institute. "This research was only possible because of remarkable detective work by Professor Ian Deary and his team at the University of Edinburgh, and Professor Whalley and his team at the University of Aberdeen," he said, according to a Queensland statement.

In June 1932 and June 1947, intelligence tests were carried out on almost all children born in Scotland in 1921 and 1936, respectively. Ian Deary and colleagues successfully tracked down 2,000 of these people who, then aged from 65 to 79, agreed to be re-tested and to give samples for DNA analysis. They then examined more than half a million genetic markers to work out how genetically similar the individuals were, even though they were not related.

"Until now, we have not had an estimate of how much genetic differences affect how intelligence changes across a lifetime," said Deary of the University of Edinburgh's Centre for Cognitive Ageing and Cognitive Epidemiology.

"The results also strongly suggest how important the environment is helping us to stay sharp as we age. Neither the specific genetic nor environmental factors were identified in this research. Our results provide the warrant for others and ourselves to search for those," said Visscher.