New research gives some biological clues to why women may be more likely than men to develop Alzheimer's disease and how this most common form of dementia varies by sex, The Associated Press reports.

At the Alzheimer's Association International Conference in Los Angeles, scientists offered evidence that the disease may spread differently in the brains of women than in men. Other researchers showed that several newly identified genes seem related to the disease risk by sex.

Two thirds of Alzheimer's cases in the U.S. are in women and "it's not just because we live longer," said Maria Carrillo, the association's chief science officer. There's also "a biological underpinning" for sex differences in the disease, she said.

Some previous studies suggest that women at any age are more likely than men to develop Alzheimer's. Scientists also know that a gene called APOE-4 seems to raise risk more for women than for men in certain age groups.

At the same time, women with the disease in its early stages may go undiagnosed because they tend to do better on verbal tests than men, which masks Alzheimer's damage.

The new studies add more evidence and potential explanations for suspected variations between how men and women develop the disease.

Vanderbilt University researchers found differences in how tau, a protein that forms tangles that destroy nerve cells, spreads in the brains of women compared to men. Using scans on 301 people with normal thinking skills and 161 others with mild impairment, they mapped where tau was deposited and correlated it with nerve networks — highways that brain signals follow.

They found that tau networks in women with mild impairment were more diffuse and spread out than in men, suggesting that more areas of the brain were affected.