The real reason for Girl Power?

Researchers have discovered that the hormone oestrogen plays as big a part in a woman's desire to win as testosterone does for men.

Steven Stanton of the University of Michigan and Oliver Schultheiss of Friedrich-Alexander University in Germany set out to see if women's hormones fluctuate in the same way as men's have been found to in pressured situations.

They tested women to discover who was the most power-motivated based on a story they were asked to write.

Then, 49 of the women deemed to be the most power-hungry were pitted against one another in a computer game.

In highly-motivated women, levels of oestradiol (a form of oestrogen) shot up when they won and stayed high for at least 24 hours. However, when they lost, levels of the hormone dropped.

The researchers concluded that the more oestrogen a women has, the more ambitious she will be, with the effects particularly pronounced in single women.

"Our findings perfectly parallel what we have observed for power motivation and testosterone in men. In women, oestrogen appears to be the critical hormone for power motivation," said Mr Schultheiss.

Spice Girl Geri Halliwell popularised the term "Girl Power" in the late 1990s and it was added to the Oxford English dictionary in 2001. The definition is "a self-reliant attitude among girls and young women manifested in ambition, assertiveness and individualism".