Partners do not simply grow old together, they often grow more alike.

Decades of shared routines, environments and experiences leave couples with increasingly similar health profiles, a pattern long recognised for obesity, diabetes and heart disease and stroke.

Could dementia now belong on that list? A new nationwide study from Taiwan suggests the answer is yes – but not for the reason many headlines might imply.

The shadow of shared lives In this study involving almost one million married adults, researchers found that when one spouse developed dementia, the other was significantly more likely to receive the same diagnosis.

This observation itself is not entirely new.

Smaller studies have reported similar findings over the past decade.

What distinguishes this study is its scale and ability to explore how age, income and family circumstances shape that risk.

Researchers analysed health-care records enrolled in Taiwan’s national health insurance program between 1998 and 2022.

The study included more than 190,000 people whose spouse had a new diagnosis of dementia and compared them to more than 760,000 people who did not have dementia or a spouse with dementia.

Participants were matched according to age, sex, income and residential location before being followed over time to determine whether they subsequently developed dementia themselves.

Read more: What is mild cognitive impairment? And does it always lead to dementia? The analyses also accounted for participants’ medical conditions, health-care use and family size.

This meant the researchers could ask a more sophisticated question: will someone whose partner has dementia still be more likely to get a diagnosis themselves, even after accounting for the factors couples already share? The answer was yes.

Women whose partners developed dementia had a 74% higher relative risk of developing dementia.

For men, the increase was 69%.

These figures only tell part of the story though.

Relative risk describes how much more likely one group is to develop one condition compared to another condition.

It says little about the actual probability of developing that condition.

This is where absolute risk becomes important.

Over five years, around 6.5% of women whose spouse developed dementia were diagnosed themselves, compared with 3.7% of women whose spouse did not.

Among men, the corresponding figures were 9.2% and 5.8%.

Viewed another way, a spouse’s diagnosis was associated with roughly three additional dementia cases per 100 women, and just over three cases per 100 men, over five years.

More than shared habits These findings should not be interpreted as....