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Why do some moms have more boys than girls [science.org]:

At first glance, the sex of your unborn child seems like it should be a coin toss: Most sperm carry either an X or a Y chromosome, giving the baby a roughly one-in-two chance of being biologically male or female. But some families are lopsided. Reality TV star Kris Jenner, for instance, has five daughters and one son; former Spice Girl Victoria Beckham has three sons and one daughter.

A study published today in Science Advances suggests that, far from a “coin toss,” some mothers may be biologically biased toward having children of one sex or another [doi.org], with the effect seemingly increasing with age of first pregnancy and the number of children they have.

“It’s not a coincidence that your sister has four girls and no boys,” says Jorge Chavarro, an epidemiologist at the Harvard T.H. Chan School of Public Health and one of the study’s lead authors. “There is probably an underlying biological basis for that, but we don’t fully know exactly what it is—yet.”

The work adds to growing hints that the odds of sex determination may sometimes be weighted, says University of Michigan geneticist Jianzhi Zhang, who has also researched the phenomenon. Still, other experts caution that the study looked at a small and geographically narrow subset of mothers. University of Queensland geneticist Brendan Zietsch, who found no signs of skewed sex ratios in his own 2020 study of 4.7 million Swedish births [royalsocietypublishing.org], calls the new study “spurious,” in part because of its sample size.

To conduct the study, Chavarro and his colleagues analyzed data from the Nurses’ Health Study [nurseshealthstudy.org], a decadeslong U.S. research program that primarily studies risk factors for chronic disease in women. The team looked at 58,007 women with two children or more, accounting for 146,064 live births from 1956 to 2015. They found that the sex ratios were skewed from what would be expected by random chance. This meant some women in the study may have had a biological tendency to have children of one sex or the other.

Regardless of the family’s final size, women who had their first child after the age of 28 also had 13% higher odds of having only sons or daughters, relative to women who had their first child before the age of 23.

The researchers then decided to look at the genomes of more than 7000 women from the nurse study. They found that women with all daughters tended to have specific variants of the NSUN6 gene on chromosome 10, whereas women with only sons tended to have specific variants of the TSHZ1 gene on chromosome 18. Neither of these genes is known to play any notable role in reproduction. NSUN6 is involved in protein formation [nih.gov], and TSHZ1 is thought to be involved in olfaction [nih.gov].

The findings parallel previous research by Zhang. In 2024, his team used data from the UK Biobank to discover one possibly weighted coin: a gene on chromosome 10 involved with sperm formation and fertilization [royalsocietypublishing.org]. A mutation at just one spot within the gene “is associated with a 10–percentage point increase in the probability of giving birth to a girl,” he says. However, he adds that his study and the new study should be taken with a grain of salt, as the participants of both were mostly of European ancestry and therefore may not fully represent global genetic diversity.

In addition, choosing to have children isn’t just biological—it’s deeply cultural, too. For instance, some families choose to stop having children only once they have babies of both sexes. The new study’s authors tried to account for these behaviors by removing the final child in their analysis. And yet, there were still deviations. Among the two-child families included in the study, nearly 53% had one boy and one girl, which is more frequent than expected by pure chance.

And as family sizes got bigger within the study, families with single-sex children became more common—possibly indicating that families keep trying until they have a child of the opposite sex. Families with three boys, for example, had a 61% chance of having another boy, whereas families with three girls had a 58% chance of having another girl.

University of Pennsylvania geneticist Iain Mathieson, who wasn’t involved with the new study, wasn’t surprised to see individual variation, because differing cultural norms and policies can influence how and why people have children. He agrees with Zhang that the study’s genetic claims need further testing and that the findings, as they stand, are a bit speculative. “I would be interested to see an estimate of the heritability,” Mathieson said.

One of the fundamental challenges with studying this phenomenon, Zhang adds, is that humans’ limited family sizes make it very difficult to detect genetic variants that influence offspring sex ratios. He believes future research will get past this bottleneck, thanks in part to the growth of biobanks’ genetic data sets. “As more biobanks are being established, more genetic variants influencing human sex ratio at birth may be discovered,” he says.

Journal Reference:
Is sex at birth a biological coin toss? Insights from a longitudinal and GWAS analysis, Science Advances (DOI: 10.1126/sciadv.adu7402 [doi.org])
Just a moment..., (DOI: https://royalsocietypublishing.org/doi/10.1098/rspb.2019.2849 [doi.org])
Just a moment..., (DOI: https://royalsocietypublishing.org/doi/10.1098/rspb.2024.1876?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed [doi.org])
Error: Not a DOI, (DOI: /doi/full/10.1126/science.279.5356.1459b [doi.org])

Original Submission