Findings also suggest that many of the genes that affect intelligence in childhood also influence intelligence in old age. The study, by some 20 researchers at the Universities of Edinburgh, Queensland and (left) Aberdeen, published in Nature, suggests that the largest influence on changes in intelligence is probably environmental.

Identifying genetic influences on intelligence could help understanding of the relationship between knowledge and problem solving and individual outcomes in life, and especially to understand why some people age 'better' than others in terms of intelligence.

The researchers combined DNA analysis with data from people who took intelligence tests aged 11 and again aged 65 to 79 examining more than half a million genetic markers in about 2,000 people, to work out how genetically similar they were, despite being unrelated.

The new findings were made possible because Scotland has a rich source of cognitive test data. In June 1932 and June 1947 intelligence tests were carried out on almost all children born in Scotland in 1921 and 1936, respectively. For the study, about 2000 of these people were traced and re-tested in old age.

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

"These new findings were possible because our research teams were able to combine a range of valuable resources.    The results partly explain why some people's brains age better than others. We are careful to suggest that our estimates do not have conventional statistical significance, but they are nevertheless useful because such estimates have been unavailable to date."

Professor Peter Visscher (above right) of the University of Queensland, said: "Unique data and new genome   technologies combined with novel analysis methods allowed us to tackle questions that were not answerable before. 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."

The study was supported by funding from the Age UK (Disconnected Mind project), the UK's Biotechnology and Biological Sciences Research Council (BBSRC), The Royal Society, The Chief Scientist Office of the Scottish Government, the Wellcome Trust, Alzheimer's Research UK, the Australian Research Council, the National Health and Medical Research Council (Australia), and the Royal Society of Edinburgh.