When women get their claws out

It is because females become far more critical of the looks of potential rivals when they are at their most fertile, say researchers.

This use of 'indirect aggression' to denigrate potential rivals is designed to boost their chances of finding a 'good man'.

And the phase of cattiness can last as long as ten days each month, psychologists have found.

'When women are at their most fertile, they'll pay more attention to each other's appearance,' said Maryanne Fisher, of York University, Toronto.

'There can be more catty behaviour, there will be more gossiping, nit-picking and spreading of nasty stories.

'You might see two women in a pub and one might say to the other, "Oh my God, look at her, she's so ugly" or "your hair is such a mess". That is an example of a competitive strategy.'

It was that ruthless approach to romance that Sex and the City captured so brilliantly as it exposed the fictional love lives of four single women in New York.

The Canadian researchers discovered the catty storyline is more than fiction when they showed photographs of the faces of 35 women and 30 men to a group of mixed gender undergraduates.

The students were asked to rate how attractive they found each face on a scale from one to seven, with seven the most good-looking, reports the Royal Society journal Biology Letters.

The women were also asked at what point they were in their cycle. Women rated men as equally attractive regardless of how fertile they were, at 2.3. But there was a significant difference in how they rated women, with those at a low fertility stage giving an average score of 3.59, while the highly fertile gave scores of just 3.2. Men gave average ratings of 3.4 for women and 2.6 for men.

Miss Fisher said: 'One potential strategy for competition is competitor derogation. During periods of high oestrogen, competition, and hence derogation, increased as evidenced by lower ratings of female facial attractiveness.'

She says men who can provide for a woman and children can choose from 'an array of available women' who must make themselves 'desirable' in order to stand out from rivals. Miss Fisher now plans to look at other ways in which women seek to tarnish competitors, such as questioning their fidelity or ability to make a good mother.

Previous studies have shown that women tend to dress in a more provocative way when they are ovulating, but this was the first to look at the effects of the monthly cycle on their attitudes towards other women.