Why some families have all boys or all girls: New study says birth sex isn’t just luck

The study found that women who began having children after age 28 were more likely to have children of only one sex

The sex of a baby has long been thought to be a matter of chance—akin to flipping a coin. But a new study published in Science Advances suggests that for some families, the odds may be weighted, influenced by factors such as maternal age and certain genes. The findings offer a scientific explanation for why some families have all boys or all girls—and why the chance of having a child of a different sex may not be as even as commonly believed, the Washington Post reported.

A coin flip—but not for everyone

Researchers at the Harvard T.H. Chan School of Public Health analysed data from more than 146,000 pregnancies involving 58,000 US nurses between 1956 and 2015. They found that families with multiple children were more likely than expected to have all sons or all daughters, a pattern that wouldn’t arise from pure 50-50 odds. For example, families with three girls had a 58% chance of having a fourth girl; those with three boys had a 61% chance of adding another boy.

According to study author Jorge Chavarro, “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.”

Maternal age may play a role

The study found that women who began having children after age 28 were more likely to have children of only one sex. Chavarro suggested this could be due to biological changes as women age—such as increased vaginal acidity—which may affect the survival of Y chromosome-carrying sperm. While paternal factors may also contribute, the study did not include data on fathers, which was cited as a limitation.

The genetic factor—and its mystery

Researchers also discovered two genes associated with having only boys or only girls. Though the function of these genes in determining birth sex is unknown, their presence suggests there could be a hereditary component to the observed patterns. Still, outside experts urged caution. Geneticist Iain Mathieson of the University of Pennsylvania said the sample size used for the genetic analysis was limited, and further research would be needed to confirm the results.

Not just biology—family decisions matter, too

Interestingly, the study also found that parents were more likely to stop having children after having both a son and a daughter, which could skew broader birth sex data. To account for this, the researchers removed final births and still found the same pattern: birth sex odds were not evenly distributed.

What it means—and what it doesn’t

The findings help explain why some families, like the fictional ones in Malcolm in the Middle or Pride and Prejudice, have children of all one sex without it being statistically improbable. The researchers say more studies are needed to examine other possible factors, including nutrition, lifestyle, and exposure to environmental chemicals.

Some variables—like race, hair colour, body mass index, and blood type—were not associated with the birth sex pattern. However, because the study population was 95% White and made up entirely of nurses, researchers caution that results may not be fully generalizable.

As evolutionary biologist David Haig summed it up: “Different families are flipping different coins with different biases.” While the biology may be complex, the implications are both personal and profound.