June 11, 2015

Women and dementia risk

A Herald story headlined “Women face greater dementia risk – study” has been nominated for Stat of the Week, I think a bit unfairly. Still, perhaps it’s worth clarifying the points made in the nomination.

People diagnosed with dementia are more likely to be women, and the story mentions three reasons. The first is overwhelmingly the most important from the viewpoint of population statistics: dementia is primarily a disease of old people, the majority of whom are women because women live longer.

In addition, and importantly from the viewpoint of individual health, women are more likely to have diagnosed dementia than men in  a given age range

European research has indicated that although at age 70, the prevalence of dementia is the same for men and women, it rapidly diverges in older age groups. By 85, women had a 40 per cent higher prevalence than men.

There could be many reasons for this. A recent research paper lists possibilities related to sex (differences in brain structure, impact of changes in hormones after menopause) and to gender (among current 85-year-olds, women tend to be less educated and less likely to have had intellectually demanding careers).

The third statistic mentioned in the Stat of the Week nomination was that “Women with Alzheimer’s disease (AD) pathology have a three-fold risk of being diagnosed with AD than men.”  This is from research looking at people’s brains.  Comparing people with similar amounts of apparent damage to their brains, women were more likely to be diagnosed with Alzheimer’s disease.

So, the differences in the summary statistics are because they are making different comparisons.

Statistical analysis of Alzheimer’s disease is complicated because the disease happens in the brain, where you can’t see. Definitive diagnosis and measurement of the biological disease process can only be done at autopsy. Practical clinical diagnosis is variable because dementia is a very late stage in the process, and different people take different amounts of neurological damage to get to that point.



Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar
    Martin Kealey

    Another plausible reason why dementia is higher among women than men in the 70+ age bracket might be that in men there’s a stronger correlation between things-that-cause-dementia and things-that-kill-you, so more of the men who would have dementia have already died. (I’m being a rank amateur here and considering obesity, which is a known risk factor for dementia. And in men excess weight goes around the waist, which is contraindicated for longevity, whereas in women it tends to go “all over”.)

    2 years ago

    • avatar
      Thomas Lumley

      Yes, it could also be a competing-risks explanation of that sort.

      2 years ago

  • avatar
    Nick Iversen

    Truth be told, I may have been a bit unfair on that article. I did read that article with a bias.

    I had just listened to and interview on RadioNZ with Professor June Andrews who has written a book on dementia. In the interview she said that by age 90 50% of women will have dementia and ALMOST AS MANY MEN.

    So I read the Herald article in that light and interpreted the ambiguous statement “women had a 40 per cent higher prevalence than men” as being solely due to the prevalence of women rather than the prevalence of the disease.

    The Herald says that the disease is more prevalent in women and a prfessor says they are about the same so who ya gonna believe? In my defence I did state that I wanted Thomas to help clear this up and he did.

    The RadioNZ interview http://www.radionz.co.nz/national/programmes/thiswayup/audio/201755456/dementia-care
    The book http://www.allenandunwin.com/default.aspx?page=94&book=9781781251713
    The Professor http://juneandrews.net/

    2 years ago