In two Indigenous communities living pre-industrial lifestyles in the Bolivian Amazon, dementia is remarkably uncommon. However, a new longitudinal study reveals why earlier estimates told only part of the story.

Among the Tsimané and Mosetén communities, dementia incidence, or the rate of new cases, didn’t dramatically differ from rates in U.S. and northern European populations, especially for individuals under 80 years old. Shorter survival after dementia develops appears to play a significant role in why dementia prevalence – the total proportion of people living with the disease at a given time – in the Tsimané and Mosetén populations is unusually low.

“The surprising finding was that there weren’t big differences in dementia incidence,” said lead author Margaret Gatz, a professor of psychology and gerontology at USC and a senior scientist at the USC Center for Economic and Social Research. “That challenges us to identify the risk factors for developing dementia in the Tsimané population.”