The husbands and wives of people with dementia are six times more likely to develop the condition themselves, research suggests.
The stress and depression linked to watching a spouse deteriorate could increase the risk of dementia, according to the study of 1,221 married couples. Husbands also appear to be at higher risk than wives, researchers said.
Scientists 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 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 research was needed on lifestyle factors and environment, which could have an effect. They noted that people caring for a loved one with dementia report more stress and the need to provide more support than those who are caring for somebody with a physical disability.
The authors concluded: "The chronic and often severe stress associated with dementia caregiving may exert substantial risk for the development of dementia in spouse care-givers. The burden for spouses may be even greater because of close emotional ties to their partner and greater likelihood of fatigue with physical exertion."
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 and more is needed to determine which people are the most vulnerable."