"Studying the role of stress is complex".
The team found that African Americans experienced 60 per cent more stressful events than white people during their lifetimes.
Researchers say experiencing anxiety, fear and stress is normal when it's occasional and temporary.
"These studies were done with United States data, but they add weight to the global body of evidence around disadvantage and dementia risk, which is an issue governments around the world grapple with, and one that requires coordinated action", said Alzheimer's Association chief scientist Maria Carrillo.
Stressful life experiences such as divorce, death of a child or job loss can age the brain by several years, a study claims.
This study, from the University of Wisconsin School of Medicine, found that African Americans were more at risk of stress in life than other ethnic groups. The effects may also be the outcome of cascading effects such as when disruptive events influence a person's early schooling, which then limits his achievements later in life.
Dr Doug Brown, the director of research at the Alzheimer's Society, said: "We know that prolonged stress can have an impact on our health, so it's no surprise that this study indicates stressful life events may also affect our memory and thinking abilities later in life".
"In addition to research on Alzheimer's risk factors and biology, the Alzheimer's Association is particularly interested in increasing understanding of stigma and concern related to Alzheimer's and other dementias in diverse communities", she adds.
Over the years, researchers have theorized that blacks are more susceptible to Alzheimer's owing to genetics and higher rates of obesity, diabetes, hypertension and cardiovascular disease. Other types of conflicts researchers looked into were educational difficulties, interpersonal conflicts, financial insecurity, legal and justice system issues, serious health events and psycho-social and/or physical trauma. It's always been known that stress is associated with social disadvantage, and in the United States and other countries, members of minority groups often suffer disproportionately from those disadvantages. However, when race was taken into consideration, African Americans were found to experience the most stressful events throughout their life - and it was these experiences that were linked to poorer memory and thinking skills in older age. "Our findings suggest that differences in early life conditions may contribute to racial inequalities in dementia rate, and they point to growing evidence that early life conditions contribute to dementia risk in late life".