Tomboys bred in the womb

The sex of a baby's twin may effect their development

The way a girl twin's brain develops may be in part influenced by the sex of her fellow twin.

Researchers have discovered that girls who grow inside the womb with their twin brothers may develop slightly more "masculine" brains than girls with twin sisters.

The scientists believe this may be because the girls with twin brothers are exposed to higher levels of the male sex hormone testosterone in the womb.

It is thought that exposure to sex hormones before birth influences the way brains develop.

One clear difference is that men's brains tend to have a dominant side when it comes to hearing words. Women register words more evenly in both sides of the brain.

Headphones

New Scientist magazine reports that Dr Celina Cohen, of Utrecht University in the Netherlands, and her colleagues tested 67 girls with twin brothers and 54 girls with non-identical twin sisters. All were 10 years old.

The girls put on headphones and heard a voice saying a different number simultaneously in each ear. They were then asked to recall one of the numbers.

Each ear is connected most strongly to the side of the brain opposite it. Girls with twin sisters recalled numbers from left and right sides fairly evenly.

But girls with twin brothers were more one-sided in their response, indicating that one side of the brain was more active in hearing the numbers.

Dr Cohen thinks the brains of girls with twin brothers are slightly more masculine, which supports the idea that these girls are exposed to higher levels of testosterone in the womb.

However, the difference is small and Dr Cohen plans further tests to confirm it.

She hopes to find out if these girls have other typically masculine traits in areas such as verbal fluency, spatial reasoning and even throwing balls.