Being single or widowed increases the risk of dementia, research suggests.
A review of 14 studies presented at the Alzheimer's Association International Conference in London found that, when compared with married people, those who were single had a 42% increased risk of dementia.
Meanwhile, people who were widowed saw their chance of dementia go up by a quarter.
However, the research, carried out by experts from University College London and Camden and Islington NHS Foundation Trust, found no increased risk for divorced people.
Experts suggested that some habits, such as staying active and social interaction, may be behind the findings.
Dr Laura Phipps, from Alzheimer's Research UK, said: "There is compelling research showing married people generally live longer and enjoy better health, with many different factors likely to be contributing to that link.
"People who are married tend to be financially better off, a factor that is closely interwoven with many aspects of our health.
"Spouses may help to encourage healthy habits, look out for their partner's health and provide important social support.
"Research suggests that social interaction can help to build cognitive reserve - a mental resilience that allows people to function for longer with a disease like Alzheimer's before showing symptoms.
"While people who are unmarried or widowed may have fewer opportunities for social engagement as they age, this certainly isn't always the case.
"This research points to differences in levels of physical activity and education underlying much of the differences in dementia risk between single, married and widowed people.
"Staying physically, mentally and socially active are all important aspects of a healthy lifestyle and these are things everyone, regardless of their marital status, can work towards."