The husbands and wives of people with dementia are six times more likely to develop the condition themselves, research suggested today.
Stress and depression linked with watching a spouse deteriorate could increase the risk of dementia, according to the study on 1,221 married couples.
Husbands also appear to be at higher risk than wives although this could be down to chance, researchers said.
Experts from Utah State University in the US analysed data from couples where dementia was present and compared it with couples where dementia did not develop.
More than 200 people were diagnosed with dementia over the 12 years of the study, carried out in northern Utah on people aged 65 and older.
A total of 125 cases of dementia only in the husband were diagnosed, 70 only in the wife, and 30 where both spouses were diagnosed (60 people).
The authors, writing in the Journal of the American Geriatrics Society, said stress may play a key role in increasing the risk of dementia although further study was needed.
They noted that people caring for a loved one with dementia reported more stress and the need to provide more support than those who are caring for somebody with a physical disability. Professor Clive Ballard, director of research at the Alzheimer's Society, said: "Two people living the same lifestyle may be exposed to the same risk factors so it could be possible that spouses both develop dementia.
"However there has been limited research in this area."