Women outlive men, by something of a long shot: In the United States, women have a life expectancy of about 80, compared to around 75 for men.

This holds true regardless of where women live, how much money they make and many other factors. It’s even true for most other mammals.

“It’s a very robust phenomenon all over the world, totally conserved in sickness, during famines, during epidemics, even during times of starvation,” said Dr. Dena Dubal, a professor of neurology at the University of California, San Francisco.

But the reasons women live longer are complicated and less established — and the fact that they are outliving men doesn’t necessarily mean they are living better. Women tend to have shorter health spans (the number of healthy years a person lives) than men, said Bérénice Benayoun, an associate professor at the U.S.C. Leonard Davis School of Gerontology. Women are generally more physically frail than men in old age; they’re also more vulnerable, particularly after menopause, to developing cardiovascular issues and Alzheimer’s disease, in part because age itself is a risk factor for those conditions, Dr. Benayoun said.

Scientists are trying to uncover the reasons men and women age differently in the hopes of extending life span and health span for both.

“If we can understand what makes one sex more resilient or vulnerable, then we have new pathways, new molecular understanding, for new therapeutics that could help one or both sexes also be resilient,” Dr. Dubal said.