STOCKHOLM, Sweden – For generations, the sex of a baby has been considered a coin flip—a simple 50/50 probability. A groundbreaking study published Friday, however, has overturned this long-held belief, revealing that the odds are systematically influenced by a combination of parental genes, maternal age, and the sex of a family’s previous children.

The research, detailed in the peer-reviewed journal Science Advances, suggests that for each family, the chances of having a boy or a girl are more akin to flipping a "weighted coin." This finding reshapes our fundamental understanding of human reproduction and could have significant implications for population demographics and genetic research.

"The 50/50 model is an oversimplification that doesn't hold up when you look at massive datasets," said Dr. Astrid Nielsen, a lead author of the study and a population geneticist at Sweden's Umeå University, which led the international research effort. "What we see is a complex interplay of factors that slightly but significantly tip the scales for each unique couple."

A 'Weighted Coin' for Each Family

The study analyzed anonymized birth records from Swedish national registries spanning several decades and involving millions of births. By linking this data with genetic information, the research team identified consistent statistical patterns that diverge from a simple random chance.

The core finding is that some families have a statistically significant predisposition to have more children of one sex. The researchers identified several key factors that contribute to this "weighting" of the biological coin.

One of the most significant factors is a parent's own genetic makeup. The study pinpointed specific, though currently unidentified, heritable genetic factors that appear to influence the sex ratio of offspring. This suggests that the tendency to have more boys or more girls can run in families, passed down through generations.

"We've long heard anecdotal stories of families with all boys or all girls," Dr. Nielsen explained. "Our work provides the first strong, large-scale evidence that there's a real biological basis for this phenomenon."

The Influence of Age and Sibling Order

Beyond heritable traits, the study confirmed a clear link between maternal age and the sex of a child. The data shows a slight but consistent trend for younger mothers to have a higher proportion of boys, with the probability of having a girl increasing as the mother gets older.

Furthermore, the researchers uncovered a pattern related to birth order, often referred to as the "sibling effect." The sex of previous children in a family can influence the sex of subsequent children, a phenomenon that has been debated by demographers for years. The new study provides robust statistical support for its existence, though the precise biological mechanism remains an area for future investigation.

Experts not involved in the study have called the findings a major step forward. Dr. Kenji Tanaka, a professor of reproductive biology at the University of Tokyo, described the research as "meticulously executed and profoundly important."

"This study moves us away from a purely probabilistic view to a more sophisticated biological one," said Dr. Tanaka. "It doesn't mean we can predict the sex of a specific baby with certainty, but it helps us understand the subtle forces that shape populations. It explains statistical anomalies observed globally that were previously unaccounted for."

The implications extend to evolutionary biology, suggesting that these subtle biases may confer an evolutionary advantage under certain environmental or social conditions. For now, the findings provide a compelling answer to a question parents have asked for centuries, confirming that when it comes to a baby's sex, it's not just luck—it's biology.