Some people may be predisposed to having children of just one sex.

That’s the conclusion of new findings published July 18 in Science Advances. While the chances of having a male or female child across whole populations may resemble a coin flip, for many individuals — especially those who have their first child later in life — the odds might be skewed toward one outcome.

The probability of conceiving and birthing a male or female child is theoretically about 50-50. This is because at least initially, equal proportions of sperm cells carry X and Y sex chromosomes, which are a major — though certainly not the only — factor determining sexual development in humans.

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But reproductive epidemiologist Jorge Chavarro and his colleagues were interested in the frequent cases of people having multiple children of a single sex. 

The team compiled data from the Nurse’s Health Study, an ongoing series of epidemiological studies analyzing the pregnancies and births of over 58,000 people from 1956 to 2015. Around a third of families had siblings all of the same sex. Of those, more than expected had three, four or five kids — assuming a standard coin flip probability of male or female children.

Each individual family may have a “unique probability” of having babies of a specific sex, says Chavarro, of the Harvard T.H. Chan School of Public Health in Boston. However, that probability varies family to family across the whole population, says Chavarro. So it mostly balances out to a 50-50 chance at that larger scale.

Families susceptible to birthing one sex may continue having children until they eventually have a child of the other sex. Some families may also cease having children after two if they have a male and a female. But when the analyses accounted for this, the team still found clustering of sexes within families, particularly for people who were older at the birth of their first child.

Higher maternal age may increase the likelihood of single-sex sibling sets due to biological factors related to reproductive aging. For example, the vaginal environment may become a bit more acidic with age during the reproductive years. And that may favor sperm carrying an X chromosome, the team says. They’re slightly larger than Y sperm and may have more buffering chemicals for surviving the acidic surroundings. Another factor could be the phase of the menstrual cycle that prepares an egg for release from the ovary. It shortens with age. That may create conditions in the cervical mucus or oviduct fluid that favor the survival of sperm carrying a Y chromosome.

The ultimate impact on the skew toward one sex or another may vary depending on what biological factors are most dominant for that individual as they age.

Chavarro and his colleagues also analyzed genetic data from a subset of study participants and found two gene variants associated with having children of just one sex — one for all male and one for all female. It’s unclear what the genes do, but they aren’t known to be associated with reproductive traits, so their influence is mysterious for now.

Nicola Barban, a demographer at the University of Bologna in Italy, says the research provides “valuable insights,” but more work is needed. “This research underscores that investigating biological factors alone is insufficient to fully explain reproductive patterns.”

But Brendan Zietsch, a behavioral geneticist at the University of Queensland in Brisbane, Australia, isn’t convinced by the findings. “We previously showed in a much larger sample, comprising the entire Swedish population born after 1931, that there is no tendency for individual families to have [just] boys or girls.”

Zietsch says studies claiming genetic associations with offspring sex ratios need replication in another sample. Chavarro would like to see this approach replicated in another population. Ninety-five percent of the study’s participants were white and predominantly from the United States. Paternal information could also be crucial; it’s possible the partner’s paternal age is causing the skew and not maternal age, since partners tend to be fairly close in age.

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The study is the “first draft of biological explanation,” Chavarro says, with many avenues ready to explore.