Women who develop slight but detectable deficits in memory and mental acuity late in life tend to decline faster than men with mild impairment, researchers reported on Tuesday.

Some two-thirds of the five million Americans with Alzheimer’s disease are women, a disparity that is partly because women live longer. Researchers have searched in vain for decades to determine other reasons.

The authors of the new study, who presented their work at the Alzheimer’s Association International Conference in Washington, said their findings say nothing about possible causes for gender differences and have no immediate implications for treatment.

“All we can say at this point is that there appears to be a faster trajectory for women than men” toward dementia, said Dr. P. Murali Doraiswamy, a professor of psychiatry at the Duke Institute for Brain Sciences and the study’s senior author. Katherine Amy Lin, a student of Dr. Doraiswamy’s and a co-author, presented the study.

Previous research had found a steeper decline in women with mild deficits over a period of about a year. The new study extends that finding to up to eight years. “It’s a very interesting finding, but it’s also still early, so we’re limited in what conclusions we can draw,” said Dr. Edward D. Huey, a geriatric psychiatrist at Columbia University, who was not involved in the study. “I think of this as an excellent hypothesis generator; it’s something we need to investigate more deeply.”

In the study, the Duke researchers analyzed scores on standard cognitive tests taken by 398 men and women, most in their 70s, being followed as part of a large, continuing Alzheimer’s trial. The participants have been taking the cognitive tests — as well as other tests, like PET scans — on average for four years, and as long as eight years.

Controlling for factors that influence memory and mental acuity, like age, education, and genetic predisposition, the research team found that women’s scores slipped by an average of about two points a year, compared with one point for men.

The team also looked at a standard measure of life quality, rating how well people functioned socially: at home, at work and with family. That too slipped faster for women than men, and at about the same rate.

Even so, the researchers did not have strong enough evidence to conclude that, over the period of the study, more women developed full-blown dementia than men. “We will need larger numbers and more time to determine that,” Dr. Doraiswamy said. “But clearly it is very important to do.”

Scientists have tested a number of theories over the decades to explain gender disparities in dementia. It was thought for years that the female hormone estrogen, which declines with later age, was protective; but careful studies found otherwise. A gene variant, called ApoE4, linked to heightened risk of developing the disease does not appear to affect women any differently than men.

The new finding, if confirmed further, should spur additional work and also change how Alzheimer’s trials are devised.

“With any trial now, let’s say if you’re giving people a drug to see if it slows the rate of cognitive decline, you’re going to have to take account of this difference between men and women,” Dr. Huey said.