The number of new dementia cases being diagnosed in the UK has fallen by 20% since the 1990s, a study has found. Researchers found this is largely due to a drop in rates among men – it is now estimated 74,000 men and 135,000 women are diagnosed with dementia every year.

The findings of the study, published in Nature Communications, are in stark contrast with current rhetoric around dementia. Last year, Prime Minister David Cameron warned that the number of people with dementia is expected to double in the next 30 years. The Alzheimer's Society reports that more than one million people in the UK will be living with dementia by 2025, which will increase to two million by 2051.

Researchers led by Carol Brayne from Cambridge university carried out a series of interviews with more than 7,000 people aged 65 and above between 1989 and 1994 focusing on three representative areas. They then returned two years later to calculate the number of people diagnosed with dementia over this time period, using a standardised diagnostic test.

Between 2008 and 2011, the researchers carried out the same interviews on another 5,000 people living in the same three areas. They used the same diagnostic test to – again – work out the number of new dementia cases.

Definitive evidence dementia across populations is changing

Findings showed that the number of new cases had fallen by 20% between the two time periods. They said the results should be taken as evidence that dementia cases are not growing as fast as thought among our ageing population: "Influential reports in the media and for government continue to promote future scenarios of huge increases of people with dementia in global societies," they wrote.

"Undoubtedly this is correct for some areas of the globe. But our study confirms the finding of reduction of not only age-specific prevalence but also incidence in the UK. This provides definitive evidence that dementia in whole populations is changing."

They scientists said the drop in new dementia cases was "driven by a reduction in men across all ages above 65". However, the underlying reasons for this are not known, they add.

James Pickett, head of research at the Alzheimer's Society, said the results were "encouraging", but warned there will still be more than 200,000 new cases of dementia every year. "That's still an enormous number of people who require better information and health and social care support," he said.

"The study indicates two thirds of new cases of dementia will be in women – this is in part due to the fact that women live longer, but it also appears that women are at a higher risk of developing dementia. Over the past 20 years the most significant change appears to have been a reduction in the rates of dementia amongst men."

"Since this study began, there have been substantial improvements in our understanding of dementia and many people are now being diagnosed at an earlier stage of the condition. It's possible, therefore, that not all of these people would be identified using the methods of this study, leading to an underestimate of people with dementia."