The findings are contrary to most other studies on the link between depression and mortality. Those studies have generally shown that depression increases the likelihood of death within a certain time period.
"This is totally counterintuitive to what you expect to see," said Dan G. Blazer, a Duke University professor of psychiatry and behavioral science. "We know that depression in younger populations is very clearly associated with mortality. It's not so clear in older populations."
The results might support the theory that mild depression is a survival mechanism, he said.
The Duke study, to be published in the May-June issue of the American Journal of Geriatric Psychiatry, is the first known examination of mild depression and death, Blazer said. Other studies looked only at people with severe depression.
The Duke study was based on a group that started with 2,401 women and 1,269 men, all older than 65. They were interviewed about their health at roughly three-year intervals from 1986 to 1997 and were separated into three categories - depressed, mildly depressed and not depressed - based on their answers to a 20-question test.
Blazer said 10.5 per cent of the women were considered mildly depressed.
The women with mild depression were, on average, 60 percent less likely than other women to die during any three-year period, Blazer said. Researchers took into account age, chronic illness and other factors in calculating the mortality rate.
The researchers found that depression had no influence on the mortality of men.
"We don't want to make too much out of this except that it's a very interesting finding," Blazer said.
Blazer said the study may support a theory advanced by University of Michigan psychiatrist Randolph M. Nesse that says mild depression may allow people to cope more easily with their problems and remove themselves from dangerous or harmful situations.
According to Nesse humans may need "low mood" or mild depression to deal with failure and disappointment. "People who don't have it waste their whole lives trying to do things they won't ever do," he said.
A psychiatrist not involved in Blazer's work, Dr. Richard Schulz of the University of Pittsburgh, questioned the findings, noting that previous research has shown that both mild depression and severe depression lead to increased mortality.
By William L. Holmes