A recent study has deemed that the ratios between males and females can adversely affect human longevity.
Men who reach sexual maturity in a context in which they far outnumber women live, on average, three months less than men whose competition for a mate isn't as stiff.
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," said Nicholas Christakis, senior author on the study and professor of medicine and medical sociology at Harvard Medical School.
"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 and 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 had not faced such stiff competition for female attention.
Next, the research team compared Medicare claims data with census data for a complete national sample of more than 7 million men throughout the United States and arrived at similar results (for technical reasons, the study was unable to evaluate results for women who outnumbered men at sexual maturity).
These results are published in the August issue of the journal Demography.