Those who wed are 30% less likely to suffer from the condition.
After combining data from more than 800 000 people worldwide, researchers suspect the lifelong interaction of marriage keeps the brain active, warding off the development of the disease.
People in a loving relationship are also more likely to eat healthily, take their medication and go to the doctor if they are sick, all factors which reduce dementia risk.
Being widowed increases the risk of dementia by 20%, suggesting the stress of bereavement hastens cognitive decline, the research team from University College London discovered.
Doctors have long known that being married helps people stay physically healthy.
But this is the strongest evidence yet that it also has such a big impact on the brain. They combined the results of 15 studies conducted in Europe, Asia and the Americas and are urging doctors to keep an eye on unmarried people and intervene to ensure they maintain a healthy lifestyle.
The team, writing in the Journal of Neurology Neurosurgery & Psychiatry, wrote: “Our findings are the strongest evidence yet that married people are less likely to develop dementia. Dementia prevention in unmarried people should focus on education and physical health and should consider the possible effect of social engagement as a modifiable risk factor,” they concluded.
Scientists have called marriage the “most fundamental” form of social support because research has found it also raises the chance of surviving a heart attack, and lowers the risk of high blood pressure, high cholesterol and Type 2 diabetes.
Dr Laura Phipps of Alzheimer’s Research UK, said: “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 social interaction helps to build cognitive reserve, a mental resilience that allows people to function for longer with a disease like Alzheimer’s before showing symptoms.”
Dr James Pickett, head of research at the Alzheimer’s Society, said: “These studies can’t tell us what it is about married life that is important for brain health.
“But the analysis hints that poorer physical health among those who remain single is partly responsible.”