Feisty females are more likely to give birth to boys than girls
Scientists had previously thought sex selection was merely a matter of chance
If you're an expectant mother who likes to stand up for yourself, you could soon be cuddling a newborn son.
Feisty women are more likely to give birth to boys than girls, according to scientists.
High levels of testosterone in the womb - apparently evident in more aggressive women - provide a much better environment for the sperm coded to produce boys, they claim.
But fiery females whose hearts are set on a daughter shouldn't throw out those pretty pink baby dresses just yet.
So far the correlation has been proved only in tests on animals.
The scientists who carried out the research, however, expect the same outcome will be seen in humans.
And those girls with an aggressive streak won't be the only ones expecting boys.
Testosterone levels rise in women under stress so those feeling anxious could also be in line for sons, according to the scientists. At the most basic level, it is the type of sperm which decides whether a child is a boy or a girl.
If the sperm which fertilises the egg contains an X chromosome, a girl will be born; but if it is a Y chromosome, a boy is the result.
However the environment within a woman's womb affects the likelihood of different types of sperm making it to the egg first.
This study appears to show that - at least in cows - a higher than average amount of testosterone in the womb makes it easier for Y-bearing sperm than X sperm.
Testosterone levels in women are linked to traits of maternal dominance and aggression.
To prove the link, scientists in New Zealand extracted the follicles from cow ovaries, which contain eggs. The follicles were tested for levels of testosterone, and then the eggs were fertilised in the lab.
The sexes of the resulting embryos were ascertained and then compared with testosterone levels in the follicles from which they came.
Team leader Dr Valerie Grant, of Auckland University, said: 'Results showed that follicular testosterone levels were significantly higher for subsequently male embryos.'
She said the fact that testosterone levels rose in women after experiencing chronic stress could mean that they were more likely to have boys after a nerve-racking experience.
Scientists had previously thought sex selection was a matter of chance based simply on which sperm reached the egg first.
But this could not explain how some animals had managed to change the ratio of male and females born as they had evolved.
