But before you head to the chapel, consider a couple of caveats. The researchers posited that the association may be the other way around; that those destined to have trouble thinking show symptoms decades before and therefore have trouble with relationships.

There's even more evidence to consider. Another study, this one from Israel, suggests that ruminating about life could actually protect your brain. If you're alone, then, perhaps you should worry about it. In the study of partnered and non-partnered people, said to be the first of its kind, Swedish researchers examined 1,449 Finnish people who were questioned in midlife and then again in 1998, an average of 21 years later.

The results of both studies were expected to be released Wednesday at the Alzheimer's Association 2008 International Conference on Alzheimer's Disease in Chicago.

Almost 10 percent of those in the study were diagnosed with some form of cognitive impairment in 1998; 48 had Alzheimer's disease. Those who lived with a partner in midlife were less likely to be cognitively impaired than all the others (including those who were widowed, single, divorced or separated).

After the researchers adjusted their figures to take into account the effects of factors such as weight, physical activity and education, those with partners still had a 50 percent lower risk of showing signs of senility in later life compared to those who lived alone. Those who stayed single their whole lives had a doubled risk of dementia, while those who were divorced from midlife onward tripled their risk.

It's not clear why being single is riskier for the brain. Cognitive and intellectual stimulation has been reported to be protective against dementia in general. Living in a couple means that you are confronted with other ideas, perspectives and needs. You have to compromise, make decisions and solve problems together with someone else, which is more complicated and challenging. It is probably easier to get stuck in your own habits and routines if you live by yourself. But this theory only partially explains the results since those who were widowed and didn't remarry had a much higher risk of dementia.

In the other study, Israeli researchers looked at about 9,000 participants in a multiple-year study of heart disease among male civil servants in Israel. Those who reported that they were most likely to not "ruminate" about family difficulties in midlife were more likely to suffer from dementia in old age. More than one in five of those who ruminated the least had signs of senility, compared to 14 percent of those who "usually ruminate."

Those who ruminated the most about work difficulties were also the least likely to suffer from dementia. Why the difference? One possible explanation could be that some forms of rumination may be associated with effective problem-solving and are a form of cognitive activity. Cognitive activity has been demonstrated to be associated with decreased risk for dementia.

References
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