Women live longer than men these days because they don't have as many babies as they used to, a new study suggests.
Researchers at Uppsala University in Sweden looked at the demographic records of 140,600 parents, from the early 1800s through to the 20th century.
While lifespans for both men and women improved over this time, women gained much more -- and the rise correlated with a halving of the reproduction rate, from 8.5 births per woman to 4.2.
By the early 1900s, women in the study were living four years longer than men -- 100 years earlier their lives were two years shorter.
Women in the database who gave birth to 15 or more children lived six years shorter than women with a single child.
Lifespans for men, on the other hand, were unaffected by the number of children they sired.
"As fertility decreased over time, female lifespan increased, while male lifespan remained largely stable, supporting the theory that differential costs of reproduction in the two sexes result in the shifting patterns of sex differences in lifespan across human populations," the researchers noted in journal Scientific Reports.
While women need to put in about nine months' effort to produce a child, for men it's closer to nine minutes, give or take. Having several children takes a toll on a woman's body that it doesn't on men.
"As more and more countries throughout the world go through the demographic transition, the overall sex differences in lifespan may increase," says study leader Elisabeth Bolund.
The data was taken from the Utah Population Database, one of the world's largest and most comprehensive collections of genealogical records. The state's birth rates were very high in the 19th century, probably as a result of its large Mormon population.
New Zealand men born in the last few years can expect to live 79.5 years, and women, 83.2. A calculator for finding out your life expectancy, based on how old you already are, is available on the Statistics NZ website.