The odds of having a boy or a girl have traditionally been assumed to be completely random — roughly 50/50. But a new study published in the journal Science Advances shows that this is not always the case at the individual level. Harvard scientists led by Jorge Chavarro have found that some people may be biologically predisposed to having children of only one sex — either boys or girls. The deviation from equal chance is particularly pronounced in people who had their first child later in life.

The team analyzed data from more than 58,000 women who participated in the long-term Nurses’ Health Study, which was conducted between 1956 and 2015. About a third of the families in the sample had children of only one sex. In many of these families, the number of children exceeded two, significantly reducing the likelihood that this was the result of chance alone. This suggests that within a family there may be a “unique probability” of having a child of a particular sex, even though there is an overall balance between boys and girls at the population level.

Scientists suggest that age at first birth plays a role. As women age, changes occur in the body that can create conditions that favor sperm carrying a particular sex chromosome. For example, the more acidic vaginal environment typical of older women may increase the chances of conceiving a girl, as sperm with the X chromosome are more resistant. At the same time, the shortened ovulatory phase in older women may favor sperm with the Y chromosome, which would increase the chance of a boy.

The study also revealed two genetic variations associated with the birth of only boys or only girls, respectively, but the role of these genes is still unclear. They are not known to affect reproductive processes, which makes their action in this case mysterious.

While the findings are impressive, some scientists remain skeptical. Behavioral geneticist Brendan Zeich of the University of Queensland points out that a previous study of the entire Swedish population born after 1931 found no statistical tendency in individual families to have only boys or only girls. According to him, claims of genetic influences on the sex of children need confirmation in other populations. Chavarro himself shares the same opinion, emphasizing that 95% of the study participants were white American women, which necessitates a broader and more diverse study. It would also be important to include data on the age of fathers, since they are often in a similar age range to mothers and could also have an influence.

According to Chavarro, the current study is a “first draft” of a deeper biological explanation for why in some families children are always of the same sex. He believes that many new directions for research are yet to open. | BGNES