People living in northern countries could be more likely to develop dementia, according to researchers.
An Edinburgh University study suggested that environmental factors, such as lack of sunlight, could increase the risk of developing the illness.
Scientists mapped the disease in Scotland among 37,000 people born in 1921.
A second study involved more than 26,000 Swedish twins.
Researchers found that the further north people lived, the more likely they were to suffer from dementia.
Exposure to vitamin D, which is made in the skin by the action of sunlight, has been shown to be linked to healthy brain function.
In Scotland, the study revealed a substantial change in disease risk depending on where people lived as an adult, but there was no change in risk linked to where people lived as children.
In Sweden, they found that twins living in the north were two or three times more likely to develop dementia compared with those in the south, after they accounted for factors such as age, gender, and genes.
Dr Tom Russ of the University of Edinburgh's Alzheimer Scotland Dementia Research Centre said: "If this geographical variation in dementia risk is the result of one or more environmental risk factors, and if these could be improved in the whole population, our findings suggest that it might be possible to halve dementia rates."