The intangible quality that makes a woman sexy has always mystified men.
But, according to research from America, there could be a simple biological explanation.
Psychologists at the University of Texas have discovered that women look and feel more attractive when they are at their most fertile.
Matthew Davis studied 27 women at different stages of their menstrual cycle and found that ovulating women made more effort to look attractive.
They show off more skin, wear tighter clothes and take more care and attention over their make-up.
The women, who were asked to keep a record of their feelings, also felt more passionate at this time – about halfway through the cycle and daydreamed about love more often than usual.
They felt less stressed than at other times of the month, had fewer headaches and could detect a more ‘positive mood’ about themselves.
The women also changed physically. Their waists shrank by about half an inch, giving them a more curvaceous and shapely silhouette, and their breasts were likely to be more symmetrical.
Mr Davis, whose findings are published in New Scientist magazine, said a woman’s body ‘is unconsciously advertising the fact that she wants to attract a mate when she is at her most fertile.’
He added: ‘She will be likely to seek more sexual opportunities, draw more attention to herself and thus get better quality partners when it counts the most.’
The study has not analysed the chemistry behind the changes, although it is known women produce the hormone LH when an egg is released from the ovaries.
Behaviourial psychologist Dr Craig Roberts, of Newcastle University, is not convinced men can pick up the signals sent out by women unless they already know her well.
‘These signals are probably more effect at co-ordinating sexual behaviour within long-term partnerships than in attracting a new man,’ he said.