Stressful events in life, such as the death of a child, divorce or being fired, can age the brain by at least four years, USA researchers suggests.
The team examined a sample of 1,320 people who reported stressful experiences over their life time and underwent tests of thinking and memory.
A second study found that rates of dementia were higher among African Americans in states with a high infant mortality rate when compared with white counterparts, suggesting the long term impact on the brain of early events.
The tests examined several areas including four memory scores - immediate memory; verbal learning and memory; visual learning and memory; and story recall.
A stressful life experience can be losing a job, death of a child, divorce, or growing up with a parent who abused drugs or alcohol.
Another Wisconsin study showed that living in a disadvantaged neighborhood is associated with later decline in cognitive function and even the biomarkers linked to Alzheimer's disease, which is the most common form of dementia.
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Rates of stress were found to be 60 per cent higher among African Americans and these events predicted cognitive abilities even more than traditional risk factors such as age, education and genetics.
Four new studies found evidence suggesting that stress of poverty and racism are factors that can increase risk of dementia in African Americans. It comes as no surprise that white Americans faced less stressful events because African Americans are also subjected to racial discrimination. Dr Dean Hartley, of the US-based Alzheimer's Association, said: 'It is not only things like good schools, nutrition and exercise programmes [in wealthier areas], it is not having that daily stress that disadvantaged areas bring, like when you're going off to school wondering "will I eat today?", "do I have to worry about my little brother or sister?", or the stress of not having a job or not being able to put food on the table'.
Dr Doug Brown, director of research and development for Alzheimer's Society, said studying the role of stress was complex.
"The findings do indicate that more should be done to support people from disadvantaged communities that are more likely to experience stressful life events".
"It is hard to separate from other conditions such as anxiety and depression, which are also thought to contribute towards dementia risk". Researchers at Kaiser Permanente and the University of California, Irvine, found that racial disparities in the incidence of dementia that were previously found among people who are 65 years and older also appear in the very oldest demographic, people who are 90 or more.