Scientist shows why tough women likely to have sons

Tough, confident women may be more likely to give birth to sons than less assertive women, New Zealand research shows.

Valerie Grant, a behavioural science lecturer at Auckland University, said the sex of a woman's children might be due to more than just whether the first sperm to an egg passed on "male" Y or "female" X chromosomes, London's Sunday Times reported.

Other evolutionary psychology specialists said last year that foetuses of women in "masculine" professions such as engineering or accountancy, might encounter more testosterone in the womb, making them more likely to be male.

But Dr Grant's latest research, conducted on cows, suggested that a female mammal's testosterone level might predispose her eggs to accept "male" sperm.

"What seems to be happening is the outer layers of the egg are pre-programmed to match with either the X or the Y sperm," she said.

If the sex of a child were solely down to chance, the number of boys and girls being born would even out over time, but overall there are 105 boys born for every 100 girls, and at times the excess of boys is even higher.

While women have only a tenth the testosterone men have, some scientists believe variations can influence whether a woman conceives a boy or a girl.

To test the theory, the Auckland researchers extracted eggs from 80 slaughtered heifers and measured the testosterone in the fluid around them.

The team, whose study has been published in the Journal of Experimental Zoology, then fertilised 34 of the eggs and compared the sex of the resulting embryos with the earlier testosterone levels.

They found that the level of testosterone was on average twice as high in fluid surrounding immature eggs that developed into male embryos.

This is the latest challenge to views that a baby's sex is determined by chance.

Dr Grant's theory may help to explain other studies showing more boys are born in tough times, such as during the war in Bosnia-Herzegovina between 1991-95.

She said this might have been caused by a surge in female testosterone levels during a time of chronic stress and the absence of men.

Dr Grant has previously argued in a book, Maternal Personality, Evolution and Sex Ratio, that testosterone is the "biological underlay of dominance".

In the 1998 book, she canvassed evidence for the theory that mothers have control over the sex of their infants, and particularly that more dominant women were more likely to conceive boys. She suggested that a woman conceived an infant of the gender she was psychologically most suited to raise.

Researchers have been trying to understand for decades what predisposes the gender of babies, with a growing consensus that low levels of testosterone and high levels of another hormone, gonadotrophins, in either parent increases the chances of having a girl.

An early study reported in the scientific journal Nature in 1984 showed that dominant female deer bore more males.

- NZPA