Thanks to advances in medical technology, people are living longer — which is good news. But it also means that more people are reaching the age when the risk for Alzheimer’s increases. This is one of the main reasons that the past several decades have seen a rise in cases of the disease.

But research now indicates that in some wealthy countries that trend is changing. Dr. Kenneth Langa of the University of Michigan reported that U.S. citizens age 60 and older today have a probability of developing dementia that is 44 percent lower than that of the same age group 30 years ago.

However, in poorer countries in South America and Africa, the opposite trend is occurring. Scientists in Colombia found that the projections for dementia cases there have been underestimated by as much as 50 percent.

Experts believe that the education and improved health care in developed nations has been effective in addressing modifiable risk factors for dementia, thus lowering the prevalence of the disease.