Women tend to live longer than men. But it wasn’t always that way, a study said.

Adults born in the late 19th century were the first to see men dying at much higher rates than women. So the gender difference in life expectancy is fairly recent, and probably isn’t based on anatomy alone, researchers said.

In medical and public health circles, women’s long lives relative to men’s have often been considered “a given,” said Hiram Beltran-Sanchez, a demographer at the University of Wisconsin in Madison and the lead author of the study published in the Proceedings of the National Academy of Sciences.

In general, men and women born in the 19th century had roughly similar life expectancies. But once antibiotics, safer water and more nutritious food became available, the odds of dying in any given year between ages 40 and 90 fell by 0.29 percent for women, compared with 0.17 percent for men.

That means for people born between 1900 and 1935, men were two to three times more likely to die in their 50s and 60s than were their female counterparts, the researchers wrote. This gender imbalance is caused primarily by cardiovascular disease and smoking-related deaths, Beltran-Sanchez said.