Do you only have girls or boys? Here’s what a study reveals

No, having a girl or boy is not just a battery or a face design

It is a widespread idea: it is believed for a long time that the birth of a boy or a girl was based on pure biological possibilities. As if every pregnancy was a battery or a genetic face, with one on two possibilities of one sex or another. But a study published in Scientific progress In July 2025 he shocked this certainty. Analyzing the data of over 58,000 American women and 146,000 births in almost 60 years, Harvard researchers show that some families have a predisposition for having children of a sex.

A probability that varies from a family to family

Unlike the classic binomial model (that of the battery or face), births follow a so-called “beta-binding” model. Clearly: each family would like to have a boy or a girl. In families with three or more children, the brothers entirely made up of girls or boys are more frequent than expected by pure possibility. For example, after three boys, the probability of having a fourth climbing at 61 %.

Mother’s age plays a role

The study also revealed it the mother’s age at the time of the first child influences this trend. The more an older woman is in her first birth (over 28 years old), the more likely it is that she has brothers entirely consisting of boys or girls.

This correlation persists even when the last child is removed from the calculation, often influenced by the possible desire to “complete” brothers.

A genetic influence not to be excluded

Scientists pushed the analysis even further with a genetic study (GWAS). Have identified two maternal genetic variants associated only with descending women (gene Nsun6) or male (gene Tshz1). Although these results ask to be confirmed, they suggest that the mother could play a more active role than expected in the child’s sex.

What does it mean for parents

This study does not question the fact that, overall, the births of boys and girls are balanced in the population. But for each family, this distribution is not necessarily balanced. In other words, if you have already had two or three children of the same sex, There is a little more likely that the next is of the same sex. Not enough to predict the future with certainty, but enough to say that genetic fate is not as risky as we thought.

Beware in the face of hurry up interpretations

This study does not say that parents can predict or choose the sex of their future child.

As Numerama observes, The observed devices remain low on an individual scaleand the data concern only the mothers, without taking into account the factors related to the fathers. In addition, the cohort studied is mainly composed of a white and nursing profession, which limits the universality of the results.

Finally, we must also distinguish Biological bias and social behavior : In some cultures, having a child of a particular sex can influence the decision to expand the family or not.