TOUGH, confident females may be more likely to give birth to sons than women with less pushy personalities, researchers have found.

Scientists have established that eggs taken from female mammals have varying levels of testosterone and that those with the highest levels are more likely to develop into male embryos. Testosterone in women has been linked to more dominant character traits.

The discovery of the “queen bee” syndrome in mammals is the latest in a growing body of work that challenges the traditional view that a baby’s sex is determined by chance.

Valerie Grant, author of the study, said the sex of offspring might be due to more than just whether “male” Y chromosomes or “female” X chromosomes prevailed when sperm fertilised an egg.

Grant, a behavioural science lecturer at Auckland University in New Zealand, said her latest research, conducted on cows, suggested that a female mammal’s testosterone level might predispose her eggs to accept “male” sperm. She said: “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.”

Her theory may help to explain other studies showing a preponderance of male births at particular times.

During the war in Bosnia-Herzegovina between 1991-95 there was a slight increase in the proportion of newborn boys. Grant said this might have been caused by a surge in female testosterone levels during a time of chronic stress and the absence of men.

Further fuel for the theory is provided by numerous studies in animals which show that the proportions of male and female offspring born can vary according to the status and health of the mother and the environment in which she lives.

The theory rings true for Rachel Elnaugh, 41, an entrepreneur who has appeared as an expert on BBC2’s Dragon’s Den reality business show. She is the mother of four boys aged six months to 10 years. Her company, Red Letter Days, which sold activity gifts such as balloon rides, went into administration in 2005 but she is now working on a new project.

“Everyone always says it’s the men that dictate the sex of babies,” she said, “but I’m living proof that’s not the case.”

Another woman with a famously dominating personality who has given birth only to males is Victoria Beckham, wife of David, who has three sons but no daughters.

If the sex of a child were solely down to chance, the number of boys and girls being born should even out over time. But there are 105 boys born for every 100 girls, and at times the excess of boys is 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, so that she appears, like a queen bee, to dictate the sex of her offspring.

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

The team, whose research appears 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 in fluid surrounding immature eggs that developed into male embryos, the level of testosterone was on average twice as high.

OTHER THEORIES

Previous theories on whether a couple are more likely to have sons or daughters include:

Domineering deer. In 1984 a Nature magazine report suggested that dominant female deer bore more males

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