Boys with gender dysphoria already speak differently by the age of five, pronouncing words in a more girlish manner, a new study suggests.

Researchers in the US, looked at a group of 90 youngsters who had been brought to a gender clinic by parents because they were their children were behaving differently to others of their biological sex.

Some had begun playing with 'girls' toys, while others had wanted to wear dresses or be called by a female name. Around half had been diagnosed with gender indentity disorder, which is now known as gender dysphoria.

Previous studies have shown that gay adult males pronounce the 's' differently and researchers wanted to see if similar vocal traits could be picked up in childhood.

A group of volunteers were asked to rate on a scale of one to six whether the speech of the boys sounded more like a boy, or that of a girl.

They found that boys with gender dysphoria rated one step lower on the scale than those without a diagnosis.

"Even as young as five years of age, those boys (with the diagnosis) are rated as sounding less prototypically boy-like than the boys without the diagnosis, and they are rated as sounding more girl-like," said Ben Muson, professor of speech, language and hearing sciences at the University of Minnesota.