In many families, the succession of girls or boys intrigues as much as it defies the laws of probabilities. However, which looks like a statistical anomaly, however, begins to deliver its secrets. By looking closely with biological mechanisms, maternal age and even certain genetic signatures, researchers discover that the same sex births obey more subtle than expected logics. Behind the apparent equal opportunities hides a much more oriented system than it seems.
Researchers from the Harvard Th Chan School of Public Health wanted to confront this theoretical model with a much more complex reality. By analyzing the data of more than 58,000 American mothers having given birth to 146,000 children between 1956 and 2015, they found that families with three children of the same sex had about 60% probability of having an identical fourth. The results, published in Science Advances, deviate from the classic binomial distribution model, suggesting that each family has a clean probability profile, as if each piece launched had a subtle bias.
When the same sex births betray a biological logic
The American study revealed that the mother's age at the time of her first pregnancy changes the chances of having exclusively male or female descendants. A woman who started to give birth after 28 years has 13% probability in addition to having only children of the same sex compared to those who have become mothers before 23 years. This difference, although modest on an individual scale, takes on a significant magnitude at the population.
Researchers evoke several organic tracks to explain this phenomenon. One of them concerns the evolution of the menstrual cycle with age. A shorter follicular phase would promote sperm carrying the Y chromosome, while a more acidic vaginal pH would promote the survival of carriers of chromosome X. In other words, aging could influence the probabilities according to the physiological specificities specific to each woman. These observations show that the female body may act as a much more active selective filter than supposed.
Towards a new understanding of the factors that influence the distribution of the sexes
The genes are not outdone in this new reading of the same sex births. The study led by Jorge Chavarro highlighted two specific areas of the maternal genome. The NSUN6 gene would be associated with the exclusive birth of girls, while the TSHZ1 gene would concern that of boys only. These discoveries are still without precise functional explanation, but they mark a turning point in research. They suggest that certain genetic markers, present from birth in the mother, could tilt the balance in favor of a sex.
However, science still recognizes its limits. The father's role was not integrated into this analysis, the available data relating only to mothers. The potential interactions between male and female factors are therefore to be explored. Siwen Wang, the first author of the study, stresses that fully understanding the determinants of sex involves a joint study of parental contributions. In the meantime, this work redraw the contours of a process long perceived as random, revealing that within families, the dice could be faked long before being thrown.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
