Socially active people who were not easily stressed had a 50 per cent lower risk of developing dementia compared with men and women who were isolated and prone to distress, they reported in the journal Neurology.

"In the past, studies have shown that chronic distress can affect parts of the brain, such as the hippocampus, possibly leading to dementia," Hui-Xin Wang of the Karolinska Institute in Sweden, who led the study, said in a statement.

"But our findings suggest that having a calm and outgoing personality in combination with a socially active lifestyle may decrease the risk of developing dementia even further."

An estimated 24 million people worldwide have the memory loss, problems with orientation and other symptoms that signal Alzheimer's disease and other forms of dementia.

Researchers believe the number of people with dementia may quadruple by 2040, underlining the importance of better understanding the condition.

The Swedish study involved 506 elderly people who did not have dementia when first examined. The volunteers were given questionnaires about their personality traits and lifestyles and then tracked for six years.

Over that time, 144 people developed dementia with more socially active and less stressed men and women 50 per cent less likely to be diagnosed with the condition.