A major new study by American scientists has challenged one of the basic assumptions about dementia. Researchers found that Black and Hispanic patients — who are significantly more likely to suffer from dementia — are far less likely to show the characteristic signs of Alzheimer’s disease on brain scans.

The study was published in the journal Alzheimer’s & Dementia.

Scientists analyzed data from 5,757 older patients with cognitive impairment or dementia from different regions of the United States. All participants underwent PET brain scans (positron emission tomography) to detect amyloid plaques — one of the main biological markers of Alzheimer’s disease. For many years, the accumulation of amyloid has been considered the central mechanism behind most cases of dementia.

However, the results were unexpected. Compared to other ethnic groups, the likelihood of detecting amyloid plaques was 28% lower in Black patients and 22% lower in Hispanic participants. At the same time, previous studies have shown that these groups experience dementia more frequently: the risk is approximately 54% higher among Black individuals and nearly 92% higher among Hispanic people compared to white Americans.

The authors believe this may point to something highly significant: dementia in different population groups may have different biological causes. In other words, memory and cognitive decline are not always driven by classic Alzheimer’s disease. Vascular disorders, chronic stress, social inequality, access to healthcare, diabetes, hypertension, and other factors may play a major role.

Researchers also warned about a practical issue. Many modern Alzheimer’s treatments today are prescribed only to patients with confirmed amyloid plaques. In addition, such patients are more likely to be included in clinical trials for new drugs. This means Black and Hispanic patients may automatically have less access to treatment and experimental therapies.

Another unexpected finding was linked to social inequality. People living in the most disadvantaged neighborhoods were 40% more likely to have positive PET scan results compared to residents of the most affluent areas. Scientists believe this is a strong indication of how deeply poverty and living conditions can affect brain health.

The authors emphasize that medicine has focused on dementia almost exclusively through the lens of Alzheimer’s disease for too long. The new findings suggest that this approach may be too narrow and could even worsen existing healthcare disparities. Researchers are calling for broader studies into the causes of dementia, especially across different ethnic and social groups.