Although the number of people with dementia continues to increase, the rate of growth has declined by 13 percent in each of the past three decades. The brain disorder currently affects nearly 50 million people worldwide and nearly 6 million in the United States. The new finding, reported by Harvard researchers in the journal Neurology, suggests that the number of people developing dementia in coming years may be less than expected. Nonetheless, that number — known as the prevalence of dementia — is expected to triple in the next 30 years, growing to more than 150 million people worldwide, due in large part to increases in life expectancy and population size. Dementia, which involves deterioration in memory, thinking and behavior beyond what is considered a normal part of aging, includes but is not limited to Alzheimer’s disease, which accounts for 60 to 70 percent of dementia cases. The researchers cited a “somewhat stronger” decline in the rate of growth — referred to as the incidence rate — among men than women (24 percent vs. 8 percent). They projected that, if the trend continues, it is possible that up to 60 million fewer people than expected would develop dementia worldwide by 2040. The researchers did not determine underlying causes of the decline in incidence, but they did note that improvements in lifestyle overall — as well as better control of blood pressure and cardiovascular issues — may have contributed to the decline. Their research was based on data from seven long-term studies, involving 49,202 people 65 and older from six countries in Europe and North America, including the United States. But the database included only people of European ancestry, and other research has found stable or increasing rates of dementia diagnoses in other ethnic and geographic regions.