A north-south divide may influence a person's risk of developing Alzheimer's, research shows.
Scientists found that dementia rates were higher among people living in northern parts of Scotland and Sweden.
They believe this suggests a possible link with environmental factors such as levels of sunlight and vitamin D.
Understanding the association could help halve rates of dementia, which affects 850,000 people in the UK, say the experts.
Researchers carried out two dementia studies, one involving more than 37,000 Scottish people born in 1921 and the other among more than 26,000 Swedish twins.
Twins living in the north of Sweden turned out to be two to three times more likely to have the disease than those in the south, after accounting for factors including age, gender and genes.
Likewise in Scotland, where people lived as an adult had a significant impact on the chances of developing dementia.
There was no change in risk related to where people lived as children.
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 and dementia.
Dr Tom Russ, from the University of Edinburgh's Alzheimer Scotland Dementia Research Centre and the Division of Psychiatry, 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."
The research, reported in the journal Epidemiology, was supported by Alzheimer Scotland.
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