Having too few men in the neighborhood could affect when women choose to have children, new research suggests.
In affluent neighborhoods where women outnumber men, the women tend to put off childbearing, while those in poorer communities that lack men have children earlier, according to a study published Tuesday (Feb. 12) in the journal Biology Letters.
The findings are correlational, so there's no way to know whether or not the skewed sex ratio actually causes the differences in childbearing, and many other factors are known to guide the complex choice of whether and when to have children.
But the findings hint that sex ratio could subconsciously alter the mating strategies of people throughout society.
"Women from poorer neighborhoods may have a live-fast, die-young strategy," said study co-author Abby Chipman, a behavioral ecology doctoral candidate at the University of Portsmouth. "If they don't have any opportunity to have a partner that will invest in their offspring or their relationship, they may begin reproducing earlier."
By contrast, women from richer neighborhoods may respond to a man shortage by investing in their careers to provide more resources for their children on their own, delaying baby making longer, she said. (Essentially, these women have the luxury of waiting to have a baby.)
Previous research has shown other effects from unbalanced gender ratios. Researchers have tied a shortage of women to a glut of unmarried men, as well as an increase in crime rates, Chipman told LiveScience. And when women are scarce, men are likelier to commit to marriage.
But the impact of having too few men has been less well studied.
So Chipman and her colleagues looked at the gender ratios and economic status of tiny urban neighborhoods throughout England from 2006 to 2008. The researchers then found the average mother's age in those same areas. Excluding prisons, boarding schools and other single-gender locations, the ratio varied across neighborhoods from about seven men for every 10 women to 14 men for every 10 women. [Busted! 6 Gender Myths in the Bedroom and Beyond]
In poor neighborhoods where women outnumbered men, there were more children born to teens and women under age 29 compared to neighborhoods with more balanced sex ratios.
By contrast, in affluent neighborhoods, skewed gender ratios (fewer men) led to an increase in the birth rate for women over age 29.
In the modern metropolis, people aren't constrained to dating only those within a few blocks of them. Still, it's possible that simply seeing proportionally more female faces on the street could subtly influence people's mating strategies, Chipman told LiveScience.
Humans aren't unique in this regard.
"Sex ratio has been shown to affect evolution and behavior in a range of animals," Hope Klug, an evolutionary biologist at the University of Tennessee at Chattanooga, who was not involved in the study, wrote in an email. "The idea that sex ratio might interact with resource availability to influence reproduction in humans is an intriguing possibility."
Still, it may not be the sex ratios per se, but other factors that correlate with these ratios that directly influence people's mating strategies, she wrote.
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Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, Wired.com and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.