Science has long known that women have a greater life expectancy than men, but they still do not fully understand why. Across the world, women are expected to live five to 10 years longer than men, while 85 percent of those who have reached age 100 are women. But what accounts for this life expectancy gap, and what is it about gender differences that creates this disparity?

In a new study published in the Proceedings of the National Academy of Sciences, researchers from the University of California’s Davis School of Gerontology discovered this difference can be traced back to the turn of the 20th century. As vaccinations and antibiotics revolutionized disease prevention, and diets were improved along with other health practices, researchers found those born during the 1800s and 1900s had a longer life. However, when it came down to it, those living longest were always women.

For their study, researchers looked at individuals born between 1800 and 1935 in 13 developed nations, and examined their lifespan. What they found is that the major mortality difference among the sexes occurs in instances of heart disease, as more men succumb to the health problem than women.

“We were surprised at how the divergence in mortality between men and women, which originated as early as 1870, was concentrated in the 50 to 70 age range, and faded out sharply after age 80,” said Eileen Crimmins, professor of Gerontology, in a press release.

While examining death rates in individuals 40 and over, female mortality decreased a dramatic 70 percent quicker than male mortality rates in individuals born after 1880. Despite taking into consideration smoking-related illnesses, cardiovascular disease still accounted for this large gap between life expectancy; smoking was only found to cause 30 percent of mortality differences in individuals born after 1890.

Researchers are now looking into why this difference creates such a prominent gap, and what it is about female anatomy that makes women succumb to heart disease later in life. Caleb Finch, professor in the Neurobiology of Aging, says it can be anything from biological factors to differences in behavior that determines the age of onset of cardiovascular disease.

Several other studies have explored theories for this difference, proposing it does come down to biological characteristics unique to each sex. Tom Perls, a Boston University researcher who founded the New England Centenarian Study told Time that one possible reason women develop cardiovascular disease later is that women are more iron deficient than men. He says that because of menstruation, women have less iron in their bodies, and iron is often found to produce damaging free radicals that can age cells. Another theory, he proposes, involves the double X chromosomes women have that men don’t; by having two X chromosomes, women have a greater likelihood of gene variations that can protect cells from aging.

A study from March conducted by Harvard researchers also found a link between increased incidences of cardiovascular disease and greater levels of testosterone. They found that in subjects whose levels of testosterone were higher, “good” cholesterol in their bodies dropped. As a result, these men were more vulnerable to heart disease.

As of now, there is no definite answer as to why this difference exists. Researchers at the University of Southern California, however, hope to conduct further research to shed more light on the issue. “Further study should include analysis of diet and exercise activity differences between countries, deeper examination of genetics and biological vulnerability between sexes at all levels and the relationship of these findings to brain and health at later ages,” Finch said.

Understanding the reason for the difference between this gap may bring us one step closer to closing it.

Source: Yu Elaine, et al. Endocrine Society's 97th annual meeting. 2015.

Sanchez H, Finch C, Crimmins E, et al. Twentieth century surge of excess adult male mortality. Proceedings of the National Academy of Sciences. 2015.