In a study covering more than 800 000 people, they found that walking through life alone increased the chances of Alzheimer’s and other forms of dementia by 40%.
Being widowed after extended co-habitation also took a toll, boosting the odds of mental slippage by about 20%.
“There were fairly well established health benefits of marriage, so we did expect there to be a higher risk in unmarried people,” said lead author Andrew Sommerlad, a psychiatrist and research fellow at University College London.
“But we were surprised by the strength of our findings.”
Couples living together without having formally tied the knot were still considered as being married for the purposes of the study, he added.
Interestingly, elderly people who had divorced were no more likely to suffer from dementia that married couples.
Across the different categories, there was also no detectable difference between men and women in the rates of mental decline.
To explore the links between marriage and dementia, Sommerlad and colleagues reviewed data from 15 earlier studies covering 812 000 people from a dozen countries.
The vast majority were from Sweden, but there were enough from other nations — including France, Germany, China, Japan, the United States and Brazil — to confirm surprisingly little variation across cultures.
The finding were detailed in the Journal of Neurology, Neurosurgery & Psychiatry. But even if the results were robust, the question remained: why?
Because the study was observational rather than based on a controlled experiment — something scientists can do with rats or mice but not humans — no clear conclusions could be drawn as to cause and effect.
Still, the evidence suggests at least three mutually compatible explanations.
“We don’t think it is marriage itself which reduces the risk, but rather the lifestyle factors that accompany living together with a partner,” Sommerlad explained.