Does your spouse sometimes drive you crazy? Think again: A study published in Friday's edition of the British Medical Journal finds that people who lived with a partner during middle age had much lower odds of developing dementia or Alzheimer's disease.

Researchers in Sweden and Finland collected data on 1,449 people as part of a large study on cardiovascular risk factors, aging and dementia. They were first interviewed in the 1970s and 1980s, when their average age was 50, and again nearly 21 years later. On both occasions, they were asked whether they were single, married, divorced or widowed. In the second interview, their cognitive function was assessed.

The lowest rates of dementia and Alzheimer's were found in people who were married at midlife, according to the study. By comparison, people who were single at midlife or divorced were 1.5 times more likely to develop dementia and 1.8 times more likely to develop Alzheimer's. Those who were widowed were 3.3 times more likely to be diagnosed with dementia and 2.5 times more likely to have Alzheimer's, the study found.

The findings are in line with previous studies that have found that mental stimulation -- in the form of education, work or intellectual leisure activities — has a protective effect against cognitive decline. People who develop rich social networks also seem to reduce their risk.

But those studies had data going back only about 10 years before a dementia diagnosis was made. By following participants for more than two decades, the new study makes a convincing case that the intellectual stimulation provided by a spouse is a key factor in long-term mental health, French epidemiologist Catherine Helmer wrote in an editorial accompanying the study.

But the study's authors said things are probably more complicated. If living with another person were the key factor, people who were never married would have had a higher incidence of dementia than people who were widowed, they wrote. Because widows and widowers were most at risk, the researchers speculated that the "psychosocial trauma" associated with the death of a spouse could initiate a detrimental immune system response that triggers the onset of dementia in people with a genetic predisposition to Alzheimer's.

In the study, the people at highest risk were those who were widowed and also had the e4 variant of a gene called ApoE, which has been linked to the disease.