Mrs Bennet may have lamented having five daughters and no son in Pride and Prejudice, but it appears that Lizzy and her sisters would be unlikely to produce a similar set of children: research suggests having multiple offspring of the same sex does not run in the family.
In the largest study of its kind, researchers have found that whether a family is dominated by boys or girls – or has an equal mix – is simply down to 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.
The team say the new findings scotch – at least for humans – a long-held theory that the sex-ratio of offspring is not random and that biases are heritable.
“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.
Writing in the journal Proceedings of the Royal Society B, an international team of researchers report how they scrutinised records for all people born in Sweden in 1932 or later and had at least one child before 2014. Overall, 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 results reveal that there is no such link – a result that held even when the team looked only at first-born 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.
However, looking within a nuclear family of parents and children, the team found that families with just two children tended to have a boy and a girl – a split that occurred more often than expected by chance – while families with more children tended to have a skew towards either boys or girls.
This, the team said, appears to be down to parents calling time on having more children once they have the family makeup they desire, and continuing to have children until then: indeed further analysis showed that it was not possible to predict the sex of a child based on the sex of its older siblings alone – in other words, without knowing the family size.
The upshot, the team say, is that the sex ratio of offspring is random – rather like flipping a coin – and not heritable.
“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 team say the findings overturn a number of theories. One, called 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.
While Kuja-Halkola said there may be environmental factors that influence whether an individual will tend to have offspring of one sex, he said it was improbable since such factors may also be expected to be linked to other heritable traits, meaning sex ratio in offspring would show some signs of heritability.
However, Prof Stuart West of the University of Oxford, who was not involved in the research, said that was “a huge jump”. “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,” he said.
Prof Ben Sheldon, also of the University of Oxford, welcomed the study. “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.”
But, he said, that does not mean sex ratios were not inherited in other species. “ 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,” he said.