Whether or not to get married is an intensely personal decision, but there may be significant cognitive consequences for many who opt to never say their vows (or cannot do so), according to new research.

A broad analysis tracking the health of some 800,000 people has found individuals who never get married stand a dramatically higher risk of developing dementia.

The research, led by psychiatrist Andrew Sommerlad from University College London, pooled data from 15 studies involving participants across the US, Europe, South America, and Asia.

By focussing on the marital status of the 812,047-strong cohort, the team found lifelong singletons were 42 percent more likely overall to develop dementia than married individuals, after taking into account factors such as age and sex.

Widowers too stood a greater risk – being 20 percent more likely to end up with dementia than people who remained married during the course of the studies.

Interestingly though, divorcées didn't show any greater association with the condition, but this could have been due to lower amounts of divorced people taking part in the research.

It's not the standalone act of getting wed that confers some kind of protective benefit to your cognitive health, the team thinks, but rather everything that goes along with married life.

"Married people tend to have healthier lifestyles and are more socially engaged, which may explain why they're less likely to develop dementia," says Sommerlad.

The observational nature of the research means we can't draw any firm conclusions with respect to causation, but if the lifestyle hypothesis is correct, the findings provide a fit in with other studies looking at the impact of social and familial engagement on our overall mental and physical health.

"We have known for some time that social factors play a role [in] dementia," says molecular pharmacologist Ian Musgrave from the University of Adelaide in Australia, who wasn't involved with the study.

"People with rich social interaction networks have a lower risk of dementia than those who are socially isolated."

The reasons for that could be many. In addition to the overall enjoyment and stimulation of living with a partner, married life comes with other not-so-obvious benefits.

"People who are married tend to be financially better off, a factor that is closely interwoven with many aspects of our health," says Laura Phipps from Alzheimer's Research UK, who wasn't involved in the study.

"Spouses may help to encourage healthy habits, look out for their partner's health and provide important social support."

Looking out for your loved one's health could also have unintended consequences for the diagnosis of dementia, which may also explain the trends seen in the data.

Whether they mean to or not, people living with partners experiencing the onset of dementia may end up preventing diagnoses from taking place simply by helping their partner live their lives – a duty that becomes more burdensome as the disease progresses.

"Just being married may reduce dementia risk but may only be because of delayed recognition, diagnosis and treatment because there may be shielding of their behaviour by either the person with dementia or their carer," says palliative care researcher Sandra L Bradley from Flinders University in Australia, who wasn't involved with the study.

"[A]lthough being married is a supportive strategy for those who have dementia, it may not be for the carer who later becomes the widow or widower."

Regardless of the causes, the protective benefit of marriage may also be something that's diminishing with time.

In the latest studies analysed by the researchers, the increased risk of developing dementia by being single was only 24 percent – significantly less than the 42 percent overall.

Figuring out why that's changing is another puzzle for scientists to look into – along with examining how these protective lifestyle-derived benefits can be extended to people who, for whatever reason, never end tying the knot.

"We hope that our findings could be applied to support dementia prevention among unmarried people," says Sommerlad, "as maintaining physical health and ensuring mental stimulation through social engagement among unmarried older people may be beneficial."

The findings are reported in Journal of Neurology, Neurosurgery & Psychiatry.