Your mother may have had four girls and no boys, but does that mean you are destined to only have daughters? A recent study says no. Researchers have found that whether a family is strictly made up of boys or girls or has an equal mix is simply a matter of chance.


“If you have a lot of boys in your family, or a lot of girls, it’s just a lucky coincidence,” said Dr Brendan Zietsch, co-author of the research from the University of Queensland.

These new finding debunk the long held theory that the sex ratio of siblings is not random, rather it is based on genetics.

“In some other animals, such as wasps, sex ratio is clearly not random, and scientists thought human offspring sex ratio may be subject to similar evolutionary forces,” said Zietsch. But, he added, the idea had problems. “Most theories were about the type of sperm men make, but no one had a good idea of a biological mechanism that would create tendencies for having more boys or girls,” he said.

An international team of researchers reported in the journal Proceedings of the Royal Society B, how they looked into records for all people born in Sweden in 1932 or later and had at least one child before 2014. The study included more than 6.7 million individuals.

The team then carried out an analysis to explore whether first cousins tended to be of the same sex. The findings revealed that there was no link, even when researchers only based their evidence on firstborn children. 

“Siblings are genetically similar. Therefore, if offspring sex ratio is heritable – ie influenced by genetic differences– siblings should have similar offspring sex ratios,” said Zietsch.

The team found that within a family unit, families with only two children tended to have a boy and a girl. This split occurred more often than could be attributed to chance. Families who had more than two children did tend to have more of one sex than the other. 

The researchers deduced that this occurred due to the parents choosing to add onto their families in an attempt to achieve the balance they desired by continuing  to have children until they reached their goal. 

The team says that the sex ratio of offspring is completly random and not inheritable. 

“To be honest it is a bit surprising,” said Ralf Kuja-Halkola, a co-author of the research from the Karolinksa Institute in Sweden, noting that almost all other complex traits in humans show some degree of heritability.

The researchers found that the findings overturn a number of theories. Fisher’s principle, suggests the approximately 1:1 sex ratio in a population is maintained by an equilibrium effect operating through natural selection. If the sex ratio skews one way, children of individuals with a predisposition to have offspring of the rarer sex will be more in demand, and hence have more children themselves, such that the imbalance becomes redressed.

Kuja-Halkola recognizes there may be environmental factors that influence whether an individual will tend to have offspring of one sex, but feels it is improbable based on links to other inheritable traits. 

Although not involved in the research, Professor Stuart West of the University of Oxford, said, “It is interesting that offspring and parent sex ratio don’t correlate, but that could be because nothing is going on with the sex ratio – as the [authors] suggest– or because something is going on, such as sex ratio being adjusted in response to any of the things not measured.”

Professor Ben Sheldon, also of the University of Oxford, was not involved in the study but welcomed the results. He said, “The authors show here that there is really no hint of heritable variation in the sex ratio in their dataset, and this seems a really robust finding.  We know that there is very good evidence for heritable variation in the sex ratio in some animals – though these are typically living in quite different situations from humans.”

—Jennifer Swartvagher

Featured photo: Juliane Liebermann on Unsplash


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