Subconscious may bias sex of babies

August 4, 2004

Mothers who expect lengthy lives tend to produce sons.

Women who thought they were going to live longer were more likely to have given birth to a son.

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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.

References

Johns S. E., et al. Proc. R. Soc. Lond. B., (2004).
Trivers R. L. & Willard D. E. Proc. R. Soc. Lond. B.179. 90 - 92, (1973). | ChemPort |
Gibson M. & Macy R. Proc. R. Soc. Lond. B.,270. S108 - S109 (2003). | PubMed | ISI |

http://www.nature.com/news/index.html

Optimists more likely to have boys

A woman's expectation of how long she will live appears to affect whether she has sons or daughters.

Dr Sarah Johns, a lecturer in biological anthropology at Kent University, found optimistic women are more likely to have boys.

Dr Johns quizzed 609 women who had recently become mothers.

She found that for every extra year a woman thought she was going to live, the odds of her first-born being a boy increased significantly.

Previous research has found that women in good physical health, and those who have comfortable living conditions have a tendency to give birth to boys.

Conversely, women living in harsh conditions tend to produce more girls.

Among the questions Dr Johns put to the new mothers was how long they expected to live.

Some of the women, who were mostly from lower-middle class and working class backgrounds, believed they would die as young as 40, while others believed they would still be alive at 130.

Dr Johns believes that an optimistic frame of mind may lead to physical changes in the body that make it more likely that a woman will conceive a male child. For instance, it may alter the level of sex hormones around the time of conception.

She told BBC Radio 4's Today programme it was important that a boy's mother was not only in good physical condition, but that she had a positive outlook and the dynamism required to give him the best possible start in life.

By and large, there was a fair chance that a daughter would go on to have children of her own, she said.

But Dr Johns said that, for a male, the odds were much more variable.

The chances of a son going to reproduce were dependent on whether he actually made it to adulthood at all, and on whether, once adult, he had sufficient qualities to make him an attractive proposition.

She said: "It is much more difficult to raise a son to adulthood. Male foetuses put much more strain on the mother's body, they are more difficult to give birth to, and they are much more likely to take risks.

"If you cannot invest in your son, making sure that he grows up to be successful and attractive to the opposite sex then it is likely he will not reproduce at all."

Dr Peter Bowen-Simpkins, of the Royal College of Obstetricians and Gynaecologists, said it was quite possible that the sex of a baby was decided by factors other than pure chance.

He said: "A lot more boys were born after the huge slaughter of men in the First World War.

"Somehow nature seemed to make up for what was lost."

The research is reported in the journal Biology Letters.

Source: BBC News Online, 04/08/2004

http://www.mentalhealth.org.uk/index.cfm?js=1&dom=1

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