A new study involving more than 21,000 people across the country finds that dementia rates in people over age 65 fell from 11.6 percent in 2000 to 8.8 percent in 2012 -- a decline of 24 percent. CNN reports: The decline in dementia rates translates to about one million fewer Americans suffering from the condition, said John Haaga, director of behavioral and social research at the National Institute on Aging, part of the National Institutes of Health, which funded the new study. Dementia is a general term for a loss of memory or other mental abilities that's severe enough to interfere with daily life. Alzheimer's disease, which is believed to be caused by a buildup of plaques and tangles in the brain, is the most common type of dementia. Vascular dementia is the second most common type of dementia and occurs after a stroke. The study, which began in 1992, focuses on people over age 50, collecting data every two years. Researchers conduct detailed interviews with participants about their health, income, cognitive ability and life circumstances. The interviews also include physical tests, body measurements and blood and saliva samples. Although researchers can't definitively explain why dementia rates are decreasing, Langa said doctors may be doing a better job controlling high blood pressure and diabetes, which can both boost the risk of age-related memory problems. High blood pressure and diabetes both increase the risk of strokes, which kill brain cells, increasing the risk of vascular dementia. Authors of the study found that senior citizens today are better educated than even half a generation ago. The population studied in 2012 stayed in school 13 years, while the seniors studied in 2000 had about 12 years of education, according to the study. People who are better educated may have more intellectually stimulating jobs and hobbies that help exercise their brains, Langa said. The study has been published Monday in JAMA Internal Medicine.