Researchers from Wisconsin University in the USA also found that African American experienced 60 per cent more stressful events than white people during their lifetimes.
And a third study of almost 1,500 people by another group of researchers at the University of Wisconsin found "markedly worse" cognitive performance, based on tasks such as verbal learning, immediate memory and speed and flexibility of cognition, in people from the most disadvantages neighbourhoods.
The researchers said that each stressful experience was equivalent to approximately four years of cognitive aging in African Americans, compared with one-and-a-half years for whites. Additionally, they found that, on average, African Americans experience 60% more stressful events than non-Hispanic whites over the course of a lifetime.
The studies showed that the number of people with Alzheimer's and other dementias differed between races and also suggested that stress in early life and neighborhood disadvantage contribute to increased dementia risk. It comes as no surprise that white Americans faced less stressful events because African Americans are also subjected to racial discrimination.
The researchers found that people in the most disadvantaged neighborhoods performed significantly worse in every aspect of cognitive function that was tested; they also had disproportionately higher levels of an Alzheimer's biomarker. She said that even a change of school could be regarded as a stressful life event for some children.
A new group of studies into racial disparities among people with Alzheimer's disease suggests that social conditions, including the stress of poverty and racism, substantially raise the risks of dementia for African-Americans. Doug Brown, Director of Research for Alzheimer's Society, said that 70% of people living in care homes have dementia, so it is vital that staff have the right training to provide good quality dementia care.
"Studying the role of stress is complex".
"I think this is important because it contributes more information to a growing body of evidence that early life matters to brain health, and that maybe early life conditions partially explain the racial disparities we see in dementia risk", Gilsanz said. "For a racially diverse nation like the United States, and to address Alzheimer's and dementia on a global scale, these findings support the need for targeted interventions, whether preventative or service-driven, to help address the gaps we know exist - and for more research".