This finding comes from a study by the Norwegian Institute of Public Health (NIPH) (link in Norwegian).

Previous studies suggest that being married later in life protects against dementia, and that being single in old age increases the risk of dementia. The new results support this.

“There's a correlation between being married in midlife and a lower risk of dementia as an elderly person. Our data also shows that divorced people account for a significant proportion of dementia cases,” the first author of the study, Vegard Skirbekk says. He is a senior researcher at the Department of Physical Health and Ageing and the Centre for Fertility and Health at the NIPH.

The researcher’s starting point was looking at the marital status of 8,706 adults in the age group 44-68 who were registered in various Norwegian national registers. They then saw how many of these developed dementia after the age of 70.

One explanation for the lower risk of dementia among married people may be that marriage is an important source of social contact.

“In several studies, it has been shown that social isolation is related to an increased risk of dementia. Marriage has also been shown to be a particularly important protective factor against dementia for men. However, in our study marriage was equally important for both men and women,” Skirbekk says.

The increased risk of dementia for unmarried people could largely be attributed to childlessness.

https://journals.sagepub.com/eprint/AMG4ZUJHMMEFDKDGRIMU/full