Other evolutionary theorists suggest that we do not even need something as specific as breasts. Absolutely any female who seems young, healthy and fertile will set us off - and there seems plenty of evidence for that, too. Then there are the amateur votes for anything from hands to earlobes.
But according to a new theory, the secret of sex appeal lies in the waist - or, to be precise, the waist- hip ratio calculated by dividing the waist measurement by the hip size. The smaller the waist in relation to the hip, the more desirable a woman is seen to be.
Professor Devendra Singh at the University of Texas believes that this may be the most powerful sexual trigger of all, and what strengthens her theory is the fact that this ratio has recently been recognised as a key indicator of health.
'The waist is one of the distinguishing human features, such as speech, making tools and a sense of humour,' says Professor Singh. 'No other primate has one. We developed it as a result of another unique feature - standing upright. We needed bigger buttock muscles for walking on two legs.'
The ideal ratio in healthy pre- menopausal women ranges between 0.67 and 0.8. In terms of the tape measure, this is produced by waists between 24in and 28in with 36in hips, and waists between 27in and 31in with 40in hips.
'Men and women lay down fat in different ways,' Professor Singh says. 'In childhood they are much the same but, come puberty, the sex hormones start directing it differently. Oestrogen, the hormone of female sexual characteristics, concentrates it on the buttocks and hips while the masculinising hormone testosterone encourages fat to form around the waist.' At the same time testosterone encourages fat to be burnt off the buttocks while oestrogen takes it off the abdomen.
These characteristically feminine fat stores are used in the last months of pregnancy and during breast-feeding. This is another reason why women who are seriously underweight often stop menstruating - they would not have the resources to support a pregnancy or a baby.
Research shows that women who fall within the 0.67 to 0.8 range are healthier. The chances of developing diabetes or having a heart attack have been found to depend more on the distribution of fat than on its volume, in addition to other factors.
A low waist-hip ratio is also linked to fertility. Not only is a wasp-waisted female likely to be healthy, she is also likely to be more fertile than her thicker-waisted sisters. Women with a low ratio, Professor Singh says, tend to start ovulating younger, and those with a high ratio find it more difficult to become pregnant and tend to have children later.
Although a high waist-hip ratio most commonly goes with being overweight, it can also be found in women of normal weight who have high testosterone levels - a condition that is also associated with being hairy, infertile and having a 'male' body shape.
The view that men are attracted to a variety of female physical attributes and that there is therefore no single ideal woman does not find favour with Professor Singh. 'Just in America over the past 50 years, ideas of beauty have varied a lot. A few years ago a popular idea was that the ideal shape was going to be androgynous,' she says. 'But between 1955 and 1987 the waist- hip ratio of Ms America contestants and Playboy playmates varied only between 0.68 and 0.71.' The young women may have been tall and slender, but they maintained female curves.
In a series of experiments, Professor Singh found that males had a clear reaction to different waist-hip ratios. In a survey of 106 men aged 18 to 22, the favourite was a female of average weight with the classic hour-glass figure. Not only were such women rated as young, sexy and healthy, they were also seen as ideal for childbearing.
The young men regarded the underweight women - defined as women of 5ft 5in weighing less than 90lb - as 'youthful' but not particularly attractive, especially for childbearing. And they viewed the overweight - women of 5ft 5in weighing more than 150lb - as unattractive, but more suitable as prospective mothers.
In Professor Singh's other surveys, men of all ages agreed with these findings - thus bearing out her theory of the waist-hip ratio. So perhaps, as well as fashion's dictates on uplift bras, we may one day see the return of the corset.
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