Dementia rates in the United States may be falling, an influential study released Wednesday said, offering hope that some cases of the disease might be delayed or even preventable.

The findings, published in the New England Journal of Medicine, appeared to defy fears over an explosion in the disease among the country's aging population.

The new study was based on data from the Framingham Heart Study (FHS), whose participants were continuously monitored for the occurrence of cognitive decline and dementia since 1975.

Looking at four distinct periods in the late 1970s, late 1980s, 1990s and 2000s, researchers found that there was a progressive decline in incidence of dementia at a given age, with an average reduction of 20 percent per decade since the 1970s, when data was first collected.

The decline was most pronounced with a type of dementia caused by vascular diseases such as stroke.

The researchers noticed that the decline in dementia incidence was limited however to people with high-school education and above.

"Currently, there are no effective treatments to prevent or cure dementia," noted author Sudha Seshadri, professor of neurology at Boston University School of Medicine.

"Effective prevention could diminish in some measure the projected explosion in the number of persons affected with the disease in the next few decades."

FHS is considered a reliable source of data but the study's authors cautioned that the sample population was overwhelmingly of European ancestry and that further studies were needed to extend the findings to other populations.

Moreover, it does not mean that the number of people with dementia will decrease anytime soon because baby boomers are aging and people are living longer.

By 2025, the number of people aged 65 and older with Alzheimer's disease in the US -- Alzheimer's is a type of dementia -- is estimated to hit 7.1 million, a 40-percent increase