Stress and depression linked with watching a spouse deteriorate could increase the risk of dementia, according to the study on 1221 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.
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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 on shared 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 caregivers.
"Having a loved one with dementia is stressful regardless of age, but the burden for spouses may be even greater because of close emotional ties to their partner, their own medical comorbidity, greater risk for functional limitations, 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.
"By keeping a healthy weight, getting regular exercise throughout your life and managing blood pressure and cholesterol from 35 onwards you can reduce your risk of developing dementia by up to 20 per cent."
