Harvard study finds men who mature where women far out number men, making mating less competitive, live longer than others

Aug. 9, 2010 – Senior citizens seem to always be interested in studies about longevity. Who is not curious about how long they will live? There is a new factor for senior citizen men to calculate – men who matured in an environment where men far outnumber women live, on average, three months less than those who matured among more females and less competition for a mate.

Working hard for love does shorten your life by at least three months, says a new study from Harvard Medical School. But it can get worse - the steeper the gender ratio (also known as the operational sex ratio), the sharper the decline in lifespan.

"At first blush, a quarter of a year may not seem like much, but it is comparable to the effects of, say, taking a daily aspirin, or engaging in moderate exercise," says Nicholas Christakis, senior author on the study and professor of medicine and medical sociology at Harvard Medical School as well as professor of sociology at Harvard University's Faculty of Arts and Sciences.

"A 65-year-old man is typically expected to live another 15.4 years. Removing three months from this block of time is significant."

An association between gender ratios and longevity had been established through studies of animals before, but never in humans. To search for a link in people, Christakis collaborated with researchers from the Chinese University of Hong Kong, the University of Wisconsin and Northwestern University. The researchers looked at two distinct datasets.

First, they examined information from the Wisconsin Longitudinal Study, a long-term project involving individuals who graduated from Wisconsin high schools in 1957. The researchers calculated the gender ratios of each high-school graduating class, then ascertained how long the graduates went on to live.

After adjusting for a multitude of factors, they discovered that, 50 years later, men from classes with an excess of boys did not live as long as men whose classes were gender-balanced.

By one measurement, mortality for a 65-year-old who had experienced a steeper sex ratio decades earlier as a teenager was 1.6 percent higher than one who hadn't faced such stiff competition for female attention.

Much attention has been paid to the deleterious social effects of gender imbalances in countries such as China and India, where selective abortion, internal migration and other factors have in some areas resulted in men outnumbering women by up to twenty percent. Such an environment, already associated with a marked increase in violence and human trafficking, appears to shorten life as well.

The researchers have not investigated mechanisms that might account for this phenomenon, but Christakis suspects that it arises from a combination of social and biological factors. After all, finding a mate can be stressful, and stress as a contributor to health disorders has been well documented.

Harvard Medical School