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Study: Age gap between partners linked to baby's sex
September 24, 1997
Web posted at: 10:55 p.m. EDT (2255 GMT)NEW YORK (CNN) -- Men passionate about wanting a son might do well to find a younger woman for a partner, while women looking forward to raising a daughter might look for a younger man, according to a new British study.
The study, published Thursday in the journal Nature, found that for men who had children with women at least five years younger than themselves, firstborn male children outnumbered firstborn female children by 2-to-1.
When the tables were turned, women who had children with men younger than themselves showed the opposite -- firstborn girls outnumbered boys by 2-to-1.
Researchers cautioned that such results aren't guaranteed. And just why this pattern occurs remains a biological mystery, says study author John Manning, an evolutionary biologist at the University of Liverpool in England
Manning said he favors the possibility that somehow, women tend to have more miscarriages with fetuses of one sex or the other, depending on whether the father is older or younger than they are. But there's no evidence for that, he said.
When it come to explaining his result biologically, he said, "we're on very shaky ground."
The study involved a group of 301 British families. A bigger sample, covering England and Wales, showed further evidence of the trend.
Deepening the mystery, though, was the fact that the trend only occurred in firstborn children. The pattern was not found among second-born children.
Experts in fertility and demographics were cautious and said it's not clear whether the finding would apply to people in general.
"What we have is an observation without an explanation," said J. Richard Udry, a professor at the Carolina Population Center at the University of North Carolina at Chapel Hill. "It's provocative."
The British research joins a list of studies that suggest the ratio of newborn boys to girls can be influenced by things such as the ages of either parent, their social status, the mother's personality and societal stresses such as war.
This area of study has long been controversial, in part because it's been hard to demonstrate biological explanations.
Manning said his work was inspired by research showing that elite groups, such as American presidents, produce slightly more sons than other people do.
The evolutionary explanation would be that in a class-conscious society where rank is inherited through sons, it would benefit elite couples to have sons. For low-ranking couples, it would make sense to have daughters who could marry upward in status.
The researchers thought that the age difference between husband and wife might be related to social status. They reasoned that in societies with repeated marriages or polygamy, well-off men would be able to use their riches to attract wives who are younger than they are. So researchers expected couples where the man is older to have an excess of sons.
When it came to firstborns, couples in which the man was five to 17 years older than his female partner had 37 sons and 20 daughters. Couples in which the women were one to nine years older than their male partner had 14 sons and 29 daughters.
When researchers looked at marriage and birth records for England and Wales from 1911 to 1952, they found that the age gap between husbands and wives was similarly related to the ratio of newborn boys to girls.
The Associated Press contributed to this report.
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