Study suggests biological sex in families may follow specific trends

A study published in Science Advances suggests that the biological sex of children in a family may not be a simple fifty-fifty chance, but rather influenced by a subtle familial tendency. Analyzing data from over 58,000 American women between 1956 and 2015, researchers found that certain families show a predisposition toward having more sons or more daughters.

Lead author Siwen Wang, a doctoral student at the Harvard T. H. Chan School of Public Health, noted that families may possess a slightly “weighted coin” regarding the sex of their offspring. For instance, a family with three daughters had approximately a 58 percent chance of having a fourth daughter, while a family with three sons had a 61 percent probability of having another son.

The research also highlighted the role of maternal age. Women over the age of 28 were found to be approximately 10 percent more likely to have siblings of the same sex compared to mothers under 23. Experts suggest this could be linked to hormonal changes, egg quality, or the uterine environment, though further research is needed to determine if paternal age plays a similar role.