It's well known that women live longer than men do, but this wasn't always the case: A new study finds that differences between men and women's life expectancies began to emerge in the late 1800s.

For the study, researchers analyzed information from people born between 1800 and 1935 in 13 developed countries.

They found that over this time period, death rates decreased among both men and women. But starting in 1880, death rates decreased much faster among women, leading to differences in mortality rates between the sexes.

The findings show that, although a greater life expectancy for women is seen as normal today, it is actually "a relatively new demographic phenomenon that emerged among people born in the late 19th century," the researchers concluded.

For example, among people born before 1840, death rates were about the same for men and women of a given age. But for people born between 1880 and 1899, death rates for men ages 50 to 70 were 1.5 times greater than those for women of the same age.

Among people born after 1900, the death rate of 50- to 70-year-old men was double that of women of the same age, according to the study. [8 Tips for Healthy Aging]

Cardiovascular disease was the main cause of the higher death rates among men, the researchers said. Heart disease and stroke accounted for more than 40 percent of the increase in male mortality rates versus female mortality rates between 1880 and 1919, the researchers noted.

Biologically, men may be more vulnerable to cardiovascular disease, but this susceptibility was seen only after deaths from other causes, such as infections, started to decline, the researchers said.

Body fat (also called adiposity) tends to be distributed differently over men's bodies compared with women's, and "their differing patterns of adiposity could make men more vulnerable to the increasing weight that resulted from changes in diet and activity," the researchers said in their study, published this week in the journal Proceedings of the National Academy of Sciences.

Future studies could investigate other differences between the sexes, including genetic dissimilarities, that may play a role in the increased risk of death from heart disease in men, the researchers said.

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