We all grew up with the knowledge that women live longer than men, but the phenomenon is actually fairly new. Differences in mortality between men and women didn't occur until the 20th century.

At that point, says a new study led by University of Southern California Davis School of Gerontology researchers, lifespans of both men and women lengthened as diets improved and infectious diseases were controlled, but women harvested the benefits at a much faster rate.

Life expectancy for Americans hit a new high in 2012. Females born in 2012 can expect to live 81.2 years, and males can expect to live 76.4 years. The difference of 4.8 years remains unchanged from 2011.

Why the gap between longevity for men and women during the 20th century? The data points to heart disease as the culprit in the gap between male and female lifespans.

"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 USC professor Eileen Crimmins.

The study, conducted by USC professors William F. Kieschnick and Caleb Finch, and University of Wisconsin-Madison researcher Hiram Beltrán-Sánchez, studied the lifespans of people born between 1800 and 1935 in 13 developed countries.

The team focused on cardiovascular disease, stroke, cancer, flu and pneumonia, and smoking-related deaths. Only adults over the age of 40 were considered to minimize the effects of war and accidents.

They found that during most of the 19th century, mortality rates were about the same between the two sexes But death rates for women born after 1880 decreased 70 percent faster than those of males. Even when smoking-related illnesses were considered — smoking accounted for only 30 percent of the differences in mortality — cardiovascular disease was still the cause of the majority of excess deaths in adult men.

According to Finch, the study raised questions as to whether or not men, especially those in middle and early old age, face different heart disease risks due to inborn biological risks or other factors.

"Further study could include analysis of diet and exercise activity differences between countries, deeper examination of genetics and biological vulnerability between sexes at the cell level, and the relationship of these findings to brain health at later ages," Finch said.