BioEd Online

Mothers who think they have longer to live are more likely to give birth to boys than girls, a survey of British women shows. The finding backs up the long-held theory that women may unwittingly be able to influence the sex of their unborn child.

Sarah Johns from the University of Kent asked 609 first-time mothers, who had already given birth, to guess when they thought they would die. By subtracting the mother's age, she then calculated the number of years each woman thought she had left to live. The results are reported in the Proceedings of the Royal Society¹.

As the number of perceived years left rose, so too did the chance that they had had a son. Every extra year on the clock increased the odds of producing a male by 1%.

The finding backs up a 30-year-old hypothesis² that suggests women can bias the sex of their unborn babies, to enhance the chances of their genes being passed on to future generations.

Boys need more looking after than girls, the theory says. So when food is scarce and resources are low, females preferentially give birth to girls because they are more likely to live through the hard times. But boys are able to produce more offspring, so when resources are plentiful, mothers should be more likely to give birth to boys, to maximise the number of potential grandchildren.

When women guess their age of death, they may unwittingly be assessing these factors, says Johns. "Perceived life expectancy may be the observable product of an evolved, subconscious psychological mechanism that assesses environmental and physical conditions," she says.

Cost analysis

"It's not something I've ever come across in practice," says midwife Sue Jarman from South Norwood Medical Centre, London, but she says the idea that unborn sons are biologically more costly makes sense. Male babies are more likely to spontaneously abort than female babies, so women may need to be on top form to carry a son to term.

Once born, males may also require more parental investment than females. Women tend to breast-feed sons longer than daughters, says Jarman.

The research accords with other human and animal studies. Mhairi Gibson of University College London showed that rural Ethiopian women with low levels of nutrition are more likely to give birth to girls³.

But the current study is one of the first to show the effect in a society where there is no obvious lack of resources. "There may be a physiological mechanism that influences psychological factors," says Gibson.

Environmental and physical factors may affect testosterone levels, which may make women more likely to produce sons, says Johns.