Castrated men live longer
In the industrialised world, women live at least five years longer, on average, than men.
Scientists have attributed that difference to everything from healthier habits to hardier cells.
Now, a new study that analyses the longevity of eunuchs, or castrated men, suggests that testosterone may play a part in shortening men's lives.
The idea that testosterone, the male sex hormone, affects lifespan isn't new.
Neutered dogs and other animals that have had their sources of testosterone removed often live longer than their intact counterparts.
But studies on the connection between castration and longevity in humans are harder to come by, and the results have been inconclusive.
A 1969 study of institutionalised patients in Kansas found that castrated men lived an average of 14 years longer than other men in the same facility, but a 1993 study of Italian castrati (singers castrated as boys to preserve their high voices) found nothing unusual about their longevity.
Almost five years ago, biologist Kyung-Jin Min of Inha University in Incheon, Korea, found himself considering this lack of data while watching a Korean TV drama about eunuchs.
Min began to wonder if Korea's rich historical records could shed light on the link between castration and longevity in humans.
Until the late 19th century, Korean rulers employed eunuchs to serve the royal court.
These eunuchs were allowed to marry and adopt castrated boys as their sons.
The Yang-Se-Gye-Bo, a genealogical record of the eunuch families, has survived, and it documents the birth and death dates and other personal details of 385 eunuchs who lived between the mid-16th century and the mid-19th century.
Min and colleagues from the National Institute of Korean History and Korea University began to pore over the Yang-Se-Gye-Bo.
After painstakingly comparing it with other historical records, the team was able to identify and verify lifespans for 81 of the listed eunuchs.
To rule out the effects of cushy conditions on longevity, they compared the eunuchs' lifespans to those of uncastrated men of similar social status living at the same time.
The eunuchs outlived their uncastrated contemporaries by 14 to 19 years, the researchers report online today in Current Biology.
The eunuch group also boasted three centenarians among the 81 verified lifespans - an unusual number considering that the current incidence of centenarians is just 1 in 3500 in Japan and 1 in 4400 in the United States.
"I thought there were errors in our data and checked everything again," Min said.
"I was quite surprised by the big difference in longevity and the number of centenarians."
The study doesn't directly explain why the eunuchs lived so much longer, but it provides the strongest evidence yet that testosterone - the key difference between the eunuchs and their peers in this study and a proxy for the difference between women and men - plays a role, says Steven Austad, a biogerontologist at the University of Texas Health Science Center, San Antonio, who was not involved in Min's study.
"This is the most thorough, well-controlled study of its kind," Austad said.
"The sex difference in aging and longevity is an almost unexplored area, and this study highlights that testosterone is part of the issue."
Identifying all the factors that contribute to the difference in longevity between men and women could help researchers find ways to temper their effects, Austad said.
That, in turn, could help men live longer - without losing any body parts in the process.