Being single when you reach middle age could mean more than having the house to yourself - it could increase your risk of dementia.
Swedish research, presented at a US conference, found that marriage or having a partner halved the risk of developing dementia.
Scientists believe social interaction between couples may ward off illness.
The Alzheimer's Research Trust said the results were worrying, given the high divorce rates in the UK.
The study by the Karolinska Institute suggested that the problem might be even greater for some people.
Divorcees who remained single, they noticed, had a trebled risk of dementia, while those widowed at a young age who stayed single faced a six times greater chance.
The research looked at 1,449 people from a Finnish database, who were asked about their relationship status in mid-life, then revisited 21 years later to see if they had developed dementia.
In total, 139 of them had some sort of cognitive impairment, and Alzheimer's had been diagnosed in 48 of these.
Even after other factors which might have an impact on dementia were adjusted for, the study consistently showed people with partners as less prone to the illness.
Advice for the single
Dr Krister Hakansson, who led the study, said: "Living in a couple relationship is normally one of the most intense forms of social and intellectual stimulation.
"If social and cognitive challenges can protect against dementia, so should living as a couple.
"This study points to the beneficial effects of a married life."
Rebecca Wood, from the Alzheimer's Research Trust, said that more research along these lines was urgently needed.
"These findings are particularly worrying for the UK - a society with a high divorce rate, marriage at an all-time low, and ageing population.
"This is the first study of its kind to examine the link between midlife marital status and dementia, adding to previous research suggesting that social interaction reduces dementia risk."
However, Susanne Sorenson, from the Alzheimer's Society, had some words of cheer for partnerless people.
She said: "Singletons shouldn't worry - there are many other ways to reduce your risk of dementia that don't involve popping the question.
"The best evidence is around eating a Mediterranean diet, exercising regularly and not smoking."
She also said that the findings were consistent with other research showing social interaction could be beneficial.
She added: "Whether it's reaching for the vacuum cleaner or going for long romantic walks, lifestyle factors associated with being married may also help."