The reason why women live longer than men has been explained

Scientists have long noted that women, on average, live longer than men. But while this fact is well-known, the reasons for this have long remained a mystery. Jokes aside, the explanation turned out to be somewhat unexpected, though perfectly logical.

An international team of researchers reports that this is largely, but not exclusively, due to the so-called "heterogametic sex theory."

Men are called the "heterogametic sex" because their sex chromosomes are not the same—one X and one Y. This puts them at a slight disadvantage compared to women, who have two X chromosomes.

Because of XY chromosomes, men are more susceptible to harmful genetic mutations and diseases, and ultimately die earlier on average.

Speaking to the Daily Mail, study author Fernando Colchero from the Max Planck Institute for Evolutionary Anthropology in Leipzig explained that the researchers found that there are factors closely linked to our evolutionary history that also contribute.

Since the 1740s, it has been known that women, on average, live longer than men – a trend observed in virtually all countries and throughout all historical periods.

According to Our World in Data, the average life expectancy globally is currently 73.8 years for women and 68.4 years for men.

While these figures vary from year to year, the average age of women remains consistently higher.

This trend can be observed beyond our species as well. According to an international group of experts, female mammals, baboons, and gorillas, typically live longer than males. However, the opposite is true for other classes of animals.

For example, in many birds, insects, and reptiles, males are the longer-lived sex. To determine the cause of this lifespan difference, researchers analyzed data from 528 mammal and 648 bird species in zoos worldwide.

The scientists found that, as in humans, 72% of mammals had a lifespan advantage for females, while 68% of birds had a lifespan advantage for males.

Researchers point to the heterogametic sex hypothesis as the primary explanation for this pattern in mammals. Studies show that the presence of two X chromosomes protects females from harmful mutations, giving them a survival advantage.

Unlike women, men do not have a second X chromosome to compensate for a single "harmful" "allele" (a version of a genetic sequence in a particular region of the chromosome).

"Basically, if you have two copies of the same genes, it's better than one," study author Dr. Johanna Sterk of the Max Planck Institute for Evolutionary Anthropology told the Daily Mail.

She also added that the Y chromosome often contains long, repetitive fragments of DNA, which can be harmful. Scientists acknowledge that there are other, less significant factors that explain why female mammals live longer than males.

First, they point to "sexual selection." In the animal kingdom, males have evolved distinctive characteristics that attract females, such as large body size, colorful plumage, or even "weapons" such as large horns.

While these evolutionary traits may increase reproductive success, researchers suggest they come at the cost of reduced lifespan.

A team of scientists believes this may be related to parental care. Researchers have found evidence that the sex that invests more in raising offspring tends to live longer.

Female mammals, who are typically the primary caregivers, have likely evolved to survive until their offspring become independent or reach sexual maturity.

Overall, the new study, published in the journal Science Advances, shows that sex differences in lifespan have deep roots in both genetics and evolutionary processes.

Differences between the sexes are not only a product of the environment, but also part of our evolutionary history and are likely to persist into the future.