Looking at generations of family is a common way to assume the sex of your child. The four March sisters of Little Women can’t be a coincidence, right?
But a new study has found that families aren’t prone to birthing one sex over another — it’s all up to chance, according to research from the University of Queensland in Australia.
The analysis looked at extensive data of the entire population of Sweden from 1932 to 2000 and challenges other past theories that the sex of a child is inherited.
Sex often refers to biological labels assigned at birth based on genitalia (which can exclude intersex individuals), while gender identity refers to how someone wants to be identified as which can include multiple identities outside of “male” or “female”.
Individuals don’t have a generic predisposition to have children of a particular sex, said Dr. Brendan Zietsch, co-author of the new findings in a press release.
“The chances are more like 51 to 49 of having a boy, but the genes of the mother and father don’t play a role. These findings have crucial implications for biological and evolutionary theories of offspring sex ratios,” he explained.
The research data pool included every Swede born since 1932, which is around 3.5 million people and their 4.7 million children. Using this information, Zietsch and the other authors determined whether siblings tended to have children of the same sex.
Even though siblings share half their genetics, the study found that when siblings have their own families, they do not have children of the same sex — meaning that sex isn’t inherited, the researchers explained.
Environment was also a factor taking into consideration, as some theories have examined whether the climate the mother lives in has an impact on the sex of her children. That can’t be possible as siblings born and raised identical environments were not any more likely to have girls or boys, said researchers.
Past theories that have been scrutinized, including concepts like tall parents were more likely to have boys, or that attractive people were more likely to have girls have now been proven wrong, said Zietsch.
“It was also thought that parents’ hormone levels at the time of conception were important,” he said. “Our results rule out all these possibilities.”
Some reports from the past year from scientists in Japan found that climate change could impact the newborn sex ratio, with more boys likely to be born with rising temperatures. Another study from the same researches found that events like a major earthquake and added stress around that could impact gestation, as boys are more vulnerable in the womb.
Regardless, “a rethink of offspring sex ratio theory is necessary to properly understand why offspring sex ratios appear to vary, for example, across countries,” said Zietsch.