From Marilyn Monroe’s curvacious wiggle to the catwalk strut of a supermodel, scientists have decoded the secrets of a woman’s walk — and have found that those swaying hips are not always intended to impress the opposite sex.

Love songs may rhapsodise “something in the way she moves”, but a sexy walk is not a sign that a woman is ready to become pregnant. In fact, a new study suggests that the way a woman walks changes during her monthly cycle, and that the most seductive wiggle occurs when she is least fertile. As such, a woman’s walk is just another of her feminine wiles, experts say, designed to put off unsuitable partners from a distance.

If she flaunts herself too openly at fertile times, she could be made pregnant by an unsuitable man, so women may have an evolutionary interest in sending out mixed messages, says Meghan Provost and her team, from Queen’s University, Ontario.

Previous research suggests that vital statistics, and particularly waist-hip ratio, are important when judging female attractiveness, with European men preferring those with a 0.7 ratio — a waist measurement that is 70 per cent of the hip circumference. Preferences vary according to culture, but Western icons of “hourglass-figure” beauty from Playboy centrefolds to the Venus de Milo typically take a ratio nearing 0.7 in their stride, irrespective of their body weight, experts suggest.

The waist-hip theory suggests that a woman with a 25-inch (65.5cm) waist and 36-inch hips could have just the right proportions to carry off a sexy swagger, while others may have to resort to a vertiginous pair of heels to achieve the same effect.

Other researchers suggest that there is more mystery to female attractiveness than a numbers game. For the latest study, Dr Provost and her team dressed female volunteers in suits adorned with light markers, as used in Hollywood special effects departments, along the joints and limbs.

This allowed them to film each woman as she walked and then analyse her gait. They also collected saliva samples to find out whether each woman was in the more or less fertile phase of her menstrual cycle.

The women who were ovulating walked with smaller hip movements and with their knees closer together, New Scientist magazine reported. When 40 men were shown the images of the women walking they rated those in the less fertile part of their cycle as having the sexiest walks.

Dr Provost, whose study is to be be published in the US journal Archives of Sexual Behaviour, said: “This was so surprising I replicated the study with two more independent groups of men to make sure.”

The results appear to contradict other research, which suggests that men respond more markedly towards women who are ovulating, and therefore at their most fertile.

Female lap dancers, for example, were found to earn more tips during their fertile period, a US study reported last month. But Dr Provost and her colleagues say that such studies are investigating different kinds of signal. Previous research looked at male responses to intimate signals such as smells and facial expressions made by fertile women.

That makes evolutionary sense, because it would benefit a woman to advertise her fertility only to those men she believes would make a suitable mate. In contrast, men can pick up on the attractiveness of a woman’s walk from long distance, and it can therefore act as an unwitting signal to less appealing males whom she might not want to choose.

Dr Provost said: “If women are trying to protect themselves from sexual assault at times of peak fertility, it would make sense for them to advertise attractiveness on a broad scale when they are not fertile.”

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