We’re not the only ones who appear to have one sex run in the family. Everyone knows a family who has a boy … and a boy … and then another, all in a row.
However, a new study that examines the entire population of Sweden since 1932 says that the sex of offspring is purely down to chance.
“We found individuals don’t have an innate tendency to have offspring of one or the other gender — instead, the sex of their offspring is essentially random,” said Dr. Brendan Zietsch, a fellow at the University of Queensland’s School of Psychology and the lead author of the study, which published Tuesday in the journal Proceedings of the Royal Society B.
“If you have a lot of boys or girls in your family, it’s just a lucky coincidence,” Zietsch said.
Using information from Swedish birth registries, the researchers compared whether siblings tended to have offspring of the same sex. Their statistical analysis ruled out the possibility that characteristics of the parents influence the likelihood of having boys or girls.
“Because siblings share 50% of their genetics, if there was a genetic underpinning of offspring sex determination we would see an association between siblings with regard to offspring sex,” said Zietsch. “However, siblings did not tend to have offspring of the same sex — the probability of having, say, a girl, did not depend on whether one’s siblings had a girl or a boy.”
Zietsch said the enormous size and accuracy of the data they used — 4.7 million births — meant they were “very confident” of the findings: “We analyzed all Swedes born after 1932.”
Other research on the topic had used much smaller samples, which could have produced a false positive, said Zietsch.
“We can’t rule out the possibility that extreme environmental events, like famine, could affect offspring sex ratios. But we can say for sure that the variability of environments that Swedes born after 1932 experienced did not affect their having boys or girls,” Zietsch said.