- Larger families often have all boys or all girls, rather than an approximately equal share of each.
- A new study shows that a child’s biological sex often links with factors like maternal age and the sexes of older siblings.
- Mothers with at least three children had children of a single sex more often than can be expected by chance.
Folks who are thinking of expanding their brood of boys or girls to get a baby of the opposite sex might face some disappointment.
Large families may be more prone to have either all boys or all girls, according to a new study from Harvard that says a child’s biological sex does not seem to come with 50/50 odds.
The study, which was published in the journal Science Advances, finds that birth sex is often linked to maternal age, certain genes and the sexes of older siblings. Nor is it the first to suggest such findings. European studies also suggest that families have their own weighted pattern of sex distribution for offspring.
Harvard T.H. Chan School of Public Health researchers looked at data from 58,000 nurses participating in the Nurses’ Health Study in the U.S. who between them had more than 146,000 pregnancies between 1956 and 2015. They only included nurses who had at least two singleton births, but excluded those with twins or other multiples. They also didn’t count pregnancies that ended in stillbirth or miscarriage. Women with a history of infertility treatments were not included. And because it’s possible the mother would stop having children after she had the mix of boys and girls she wanted, they left each woman’s final birth out of their analysis.
Some families clearly were more prone to having children who were either all boys or all girls, “akin to a weighted coin toss,” the study said. They also identified two genes they said amplified whether one was prone to boys or to girls.
Moms with three or more kids had all boys or all girls more often than would be expected if sex was just chance.
As The Washington Post put it, “The study suggests that sex at birth follows a weighted probability and that biological influences may sway the sex of the child.”
“If you’ve had two girls or three girls and you’re trying for a boy, you should know your odds are not 50/50. You’re more likely than not to have another girl,” Jorge Chavarro, professor of nutrition and epidemiology and senior author of the study, told the Post.
Factors impacting the odds
The research showed that women over age 28 when they first gave birth were a bit more apt to have all boys or all girls, which the researchers said could be due to biological changes mothers experience as they age. They also found two genes that appeared linked to having only boys or having only girls.
According to the study, in families with three boys, the probability of having another boy was 61%. In families with three girls, the probability of having another girl was 58%. They also identified heritable, demographic and reproductive factors that did not impact a baby’s sex.
Lead author Siwen Wang, a doctoral student in public health at Harvard, told NPR that “if you’re starting your family younger than 23 ... (there is) around a 40% chance” of having a family where all the children are either boys or girls.
The Post article said the “researchers also found that parents were more likely to have one boy and one girl than would be expected by chance, a pattern they believe reflects a tendency to stop having children once both sexes are represented.”
More study needed
The authors noted some study limitations. They didn’t have information about biological fathers. Ninety-five percent of the study participants were white women who lived in the U.S. They wrote that “since sex preferences and reproductive behaviors vary across cultures, religions or countries, the sex ratio distribution pattern observed in our study may not apply to other societies.” And since all the mothers were nurses, they can’t say there’s no predisposition “to having a unisex brood due to their occupational exposures.”
They also didn’t have a way to factor in half-siblings, they added.
The study concluded, “Future research is warranted to study the extent to which each of these factors explains the sex clustering within families. Until then, families desiring offspring of more than one sex who have already had two or three children of the same sex should be aware that when trying for their next one, they are probably doing a coin toss with a two-headed coin.”
David A. Haig, an evolutionary biologist at Harvard University not involved in the study, told the Post the study suggests the probability of a baby’s sex varies by family.
“Different families are flipping different coins with different biases,” Haig said. “It speaks to something very intuitive and personal, even if the underlying biology is complex.”