A study involving twins suggests that if you have a sibling who develops dementia, that might not bode well for your life span.

That’s true even if you don’t go on to develop dementia yourself, according to a study from U.S. and Swedish researchers.

One investigator was surprised by the finding.

“We expected a different result. We expected that, in twins where one developed dementia and the other did not, the difference in life span would be just like we see in unrelated people,” said lead study author Jung Yun Jang.

She led the trial as part of her doctoral studies in the department of psychology at the University of Southern California (USC), in Los Angeles. The object of the study, funded by the National Institutes of Health, was to assess typical life span after a dementia diagnosis.

“One of the most frequently asked questions when a family member receives a diagnosis of dementia is: How much time do we have?” Jang noted in a USC news release.

The study involved 90 pairs of identical twins (who share all their genes) and 288 pairs of fraternal twins, all from a 40-year database out of Sweden known as the Swedish Twin Registry.

In all sets of twins used in in the new study, one twin had developed dementia while the other had not.

As has been seen in other studies, the average life span after a dementia diagnosis was about seven years, the researchers reported Dec. 11 in Alzheimer’s and Dementia.

Among this study’s twins, specifically:

  • Life span post-diagnosis was roughly similar for the identical twins where one is diagnosed with dementia

  • Among the fraternal twins, when one is diagnosed with dementia, the twin unaffected by dementia has a slightly shortened life span, compared to people who have no sibling with dementia

Why would simply having a sibling with dementia lower a person’s life span? The team say it’s not clear, although the shared environment siblings grow up in could play a role.

For example, if both developed unhealthy dietary or other habits in childhood, that could lead to a shared risk for heart disease decades later. Doctors already know that heart disease raises dementia risk, and for the sibling unaffected by dementia, heart disease could work to shorten life span there, too.

“We assumed the reason a person who has developed dementia has a shortened life expectancy is because the dementia leads to other medical conditions that affect mortality,” Jang said. “What we’re seeing instead is the increased risk of mortality is not due to just the dementia itself, but also a whole package of other influences that the person brings to their disease.”

Much of that “package of influences” begins early in life.

“What happens early in the life course is really important,” said study co-author Margaret Gatz, a professor of psychology, gerontology and preventive medicine at USC. “You may not be able to change that for yourself, but it does seem like the message to parents is, make sure your kid eats healthy, make sure your kid gets exercise, make sure your kid gets an education. You’re actually contributing to giving that kid a lower chance of developing dementia 75 years later.”

More information

Find out more about dementia at the Alzheimer’s Association.

SOURCE: University of Southern California, news release, Dec. 11, 2023