A new study of competition among women suggests that they are 'bitchy' towards each other because they are evolutionarily programmed to be that way.

Published by the journal, Aggressive Behaviour, the study by psychologists at two Canadian universities looked at how groups of heterosexual women reacted to other women who were dressed conservatively and sexily.

Of 46 women between the ages of 19 and 23 studied, all but two were ruder and more judgemental about a sexily dressed woman - who wore a short skirt, high boots and a cleavage-showing top - than they were about the same woman wearing chinos and a plain T-shirt.

Speaking with the Globe and Mail, the report's co-author Tracy Vaillancourt said the women's bitchiness, according to a 'bitchy scale' she and her colleague had set, 'was so obvious.'

The women's reactions were filmed and then judged by a panel of 13 other women.

'They were saying thing like, "Oh, she’s dressed to have sex with her professor," or "Oh, her boobs are about to pop out." They were looking her up and down, and as soon as she leaves the room, they start laughing hysterically.'

She suggests that evolution is to blame.

Citing a 2002 study, the report's authors write: 'Females who make sex readily available compromise the power holding position of the group. It is therefore in the best interest of the group to punish those who violate this unspoken rule/convention.'

Or, as Vaillancourt explained to the newspaper: 'We can’t tolerate anyone giving the milk away for free.

'We are living in a modern context, but we are operating with an old brain. We have this instinctual response to people who defy social conventions in a way that threatens the group.'

The study's authors found that the results fitted with their - and probably many women's - predictions. That is, that women are 'particularly intolerant of sexy attractive peers.'

She even goes to suggest that 'it's women who suppress the sexuality of other women.'

(Read by Lee Hannon. Lee Hannon is a journalist at the China Daily Website.)

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(Agencies)