The results were even stronger in APOE ε4 carriers, who were 50 percent less likely to develop dementia if they held positive age beliefs.
Researchers found people with positive age beliefs had a 2.6 percent risk of developing dementia, while those with negative age beliefs had a 4.6 percent risk.
The odd thing is though, whilst it's been widely accepted that the e4 gene puts people more at risk of developing dementia, only 47% of APOR e4 carriers actually develop it.
Also striking: The apparent benefits of positivity were even greater among the subgroup of adults whose genes put them at greater risk of dementia.
Participants' beliefs about ageing were assessed using an "Attitude toward Ageing" scale, in which they were asked about the degree to which they agreed or disagreed with statements such as, "The older I get, the more useless I feel".
The study of more than 4000 over 60s found that in those with positive beliefs about ageing, the dementia risk was almost half that of those with negative beliefs.
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Levy and colleagues investigated whether culture-based age beliefs influence the risk of developing dementia among older people, including those who carry the high-risk gene variant.
Becca Levy, from the Yale School of Public Health, who led the research, believes it makes the case for introducing public health campaigns against ageism and negative age beliefs. In the entire group, having a positive attitude toward aging was associated with a 19 percent reduced risk of dementia; in the high-risk group, those with a positive attitude were 31 percent less likely to develop dementia. This isn't the first study to link age beliefs to dementia. By the age of 85 one in three individuals have dementia in Australia.
"Conversely, those who have more negative age stereotypes seem to have an exacerbated response to stress", she said.
Stress may be a key factor in the development of dementia, the Australian Broadcasting Corporation (ABC) reported. In a major review of research released in 2016, a team from the University of Toronto looked at a number of studies on stress and anxiety in humans and found that if it's continuous, that stress could wreak havoc on the brain. He recognised that it was a big study and that the sample has been well studied.
However, Professor Brodaty noted, the results should be interpreted with some caution because the diagnostic tool used to identify patients with dementia - a short telephone interview - was not the gold standard for diagnosing dementia. "Others have found that stress can be related to the development of dementia, so, our thinking is that it's possible that stress is part of the mechanism in what we're observing in this study". It is not clear, why the rest of the persons with this genetic makeup do not develop dementia.
Brodaty said: "We don't know yet whether changing beliefs will make a difference...if you could show that, then that would be interesting".